The AM Forum
March 29, 2024, 05:59:50 AM *
Welcome, Guest. Please login or register.

Login with username, password and session length
 
   Home   Help Calendar Links Staff List Gallery Login Register  
Pages: [1] 2   Go Down
  Print  
Author Topic: Dangerous Technical Error - QST Magazine  (Read 11860 times)
0 Members and 1 Guest are viewing this topic.
ka4koe
Contributing
Member
*
Offline Offline

Posts: 1157


It's alive. IT'S ALIVE!!!


« on: July 24, 2014, 07:48:29 AM »

I was reading over the August 2014  QST, "The Doctor Is In", Page 55, and found this dangerous advice with respect to powering an amplifier at 240V:

"Thus, in my opinion, the ideal configuration is to run the linear from a dedicated (not powering anything else, and with its own pair of breakers in the service box) 240V line that is as short as possible."

Emailed the Editor yesterday:

"I read with interest the column with respect to powering an amateur radio amplifier from a residential 240V, 1 phase, 3 wire power source. The column editor stated that two (2) breakers should be installed in the home electrical panel to feed each ungrounded, current-carrying conductor from the amplifier.

From an electrical safety standpoint and the provisions of the National Electric Code, NFPA 70, this advice is dangerous.

Reference NFPA 70, Article 240.15. To summarize; an overcurrent condition shall open all ungrounded conductors via the overcurrent protection device (circuit breaker). Two (2) single pole breakers can be used but a connecting bar SHALL be installed that connects both handles so that a fault will cause both breaker handles to activate.

The danger is that faults will often only trip on one leg of the circuit corresponding to the portion of the waveform at that time.

If a ham installs two single pole breakers that are not connected, then only one side may trip, leaving one leg energized. This presents a definite electrocution hazard as the ham may assume that all parts of the 240V circuit are deenergized.

Solution: Install a two (2) pole breaker of the appropriate amperage and AIC rating."

This is not the first time I've seen dangerous wiring advice appear in this magazine. An article appeared several years ago with respect to emergency power and electrical panel transfer switches that made the hair on the back of my neck stand up.
Logged

I'm outta control, plain and simple. Now I have a broadcast transmitter.
Chuck...K1KW
Contributing
Member
*
Offline Offline

Posts: 167


« Reply #1 on: July 24, 2014, 10:23:07 AM »

Correct you are!

I've seen this done before and it is very dangerous for the reason you state. 

The author may have meant a 2 pole breaker but he didn't say it.


Chuck  K1KW
Logged

73, Chuck...K1KW
KD6VXI
Contributing
Member
*
Offline Offline

Posts: 2648


Making AM GREAT Again!


« Reply #2 on: July 24, 2014, 11:12:15 AM »

You aren't even allowed to use a breaker without a tie bar if you have more than one current carrying conductor in the sheath.

That's 2011 NEC. .

--Shane
KD6VXI
Logged
AB2EZ
Member

Offline Offline

Posts: 1722


"Season's Greetings" looks okay to me...


« Reply #3 on: July 24, 2014, 11:39:55 AM »

Agreed about the issue that Philip brought to the attention of the QST editor.

There is something about electrical codes that has always puzzled me.

The codes say: "You must do "XYZ" or "You can't do XYZ".

Sometimes, it would be self evident... to any reasonable person who might be doing electrical work... as to why the rule must be followed. I.e. it would be obvious as to what dangers are avoided by following the rule.

However, there are some electrical code rules for which the potential consequences of violating the rule are not apparent... even to licensed people who do electrical work for a living. They know those rules, and (of course) they follow them... but they don't know what dangerous consequences would result from not following those rules.

For those rules, if the cost and effort required to follow the rule is high enough, there is a strong motivation to implement a non- compliant "solution"... that appears to be okay... but is actually creating a dangerous situation.

Is there a companion reference book that explains... for each rule in the code... what the "rainy day scenario" is that will be protected against, only if the rule is followed?

I know of recently adopted NEC rules that some states or counties or municipalities have decided not to adopt/enforce (e.g. arc fault interrupters). A trend toward making NEC rules discretionary will probably lead to a proliferation of dangerous workarounds of rules that are deemed inconvenient or too expensive to follow; and where the associated rainy day scenario is not understood.


Stu
Logged

Stewart ("Stu") Personick. Pictured: (from The New Yorker) "Season's Greetings" looks OK to me. Let's run it by the legal department
K4RT
Contributing
Member
*
Offline Offline

Posts: 520



« Reply #4 on: July 24, 2014, 12:53:34 PM »

I suspect that the underlying rationale for some sections of the electrical code might date back to tragic events decades ago, the details of which are not recorded in any of the materials associated with modern code sections.

Having now read the column, the author is not recommending anything. I think the concern noted is much to do about nothing.
Logged
ka4koe
Contributing
Member
*
Offline Offline

Posts: 1157


It's alive. IT'S ALIVE!!!


« Reply #5 on: July 24, 2014, 01:46:50 PM »

The NFPA codes are "minimum standards" and open to interpretation of the Authority Having Jurisdiction (AHJ), aka "fire marshal" or other official. That being said, you are really hosed if you run across an official with a "god complex" who is dead WRONG. Nitpicky stuff can slide.

I find that one is well served by obtaining a copy of the corresponding handbook. The handbooks go into detail as to WHY the rules are in effect. In terms of apparent conflicts, usually the most stringent requirement rules. However, the code is a liquid document because it is not perfect: this is why a new edition comes out at 3 year cycles.

In terms of breaker sizing, wiring rules, ground/arc fault protection, etc., most of the rules remain unchanged from year to year. Significant changes are often initiated by deaths, lawsuits, etc.

I've butted heads with state level inspectors and only extracted a favorable ruling after inquiries by a state senator (who just happened to be an architect).

Mike Holt's NEC website is an excellent resource:

http://www.mikeholt.com/
Logged

I'm outta control, plain and simple. Now I have a broadcast transmitter.
KA0HCP
Contributing
Member
*
Offline Offline

Posts: 1188



« Reply #6 on: July 24, 2014, 02:33:47 PM »

Ho, hum.

Nit picking wording (the endless annoyance of those who interpret NEC code).

The QST article was quite accurate and got the point across that breakers are needed in both lines of a 240VAC installation.  It is not intended to be a complete tutorial for household electrical installation.

There are equally many statements about electronic components, design etc. that could be endlessly caveated.

Electricians and the NEC code are worse than theologians arguing about angels on the head of a pin. 

Calm down and don't get so twisted up over general articles and fine detail.  Instead recommend people hire a qualified electrician or suggest good "How To" manuals.  Please.
Logged

New callsign KA0HCP, ex-KB4QAA.  Relocated to Kansas in April 2019.
W3NE
Contributing
Member
*
Offline Offline

Posts: 139


« Reply #7 on: July 24, 2014, 02:51:21 PM »

I'm glad someone called the editor on this. I'm presently restoring an SB200, which not only has individual breakers in each AC line, but locates them ahead of the amp's main power switch. In that configuration it is possible for the the neutral to be interrupted when the amp is wired for 120 V operation. Thank goodness for 3-wire power cords, separate equipment ground, and personnel safety procedures previously advised.

On a related topic, the location of the ground terminal on a 3-wire AC receptacle, from inception of that type of connector, was below the line contacts. In a recent hospital visit, as well as observations of new installations elsewhere during the past few years, it appears receptacles are now being installed with the ground terminal above the line contacts. That means formerly "standard" plugs have to be inserted "upside-down" from their intended design. Regardless of any argument that might favor grounding the appliance first on insertion of its power plug, it seems ridiculous, after decades of having the ground below the power contacts, to later reverse that widely accepted convention, resulting in some devices (a Kill-A-Watt meter, for example) to be unusable, not to mention the awkward reverse bend on every line cord (especially those with a molded 90 degree plug), the design of which does not seem to have been changed by manufacturers or affected by Underwriters' requirements to match the new receptacle position. Is this a new code requirement, and if so, what was the official rationale that justified such the resulting debacle?

Bob-NE
Logged
KD6VXI
Contributing
Member
*
Offline Offline

Posts: 2648


Making AM GREAT Again!


« Reply #8 on: July 24, 2014, 03:27:25 PM »

Inverted plugs are used to signify isolated grounds in industrial or medical facilities. .

Different colored plugs have different meanings in different areas of practice as well.

Those upside down plugs are as designed.

We also put them in rental homes to aid in the identification of half hot plugs......  Where one receptacle is always hot and one is switched.  Leasing agents requested this.

--Shane
KD6VXI
Logged
AB2EZ
Member

Offline Offline

Posts: 1722


"Season's Greetings" looks okay to me...


« Reply #9 on: July 24, 2014, 03:29:24 PM »

In the reference that Philip provided, there is a Q and A section.

I think that the answers provided to some of the questions illustrate my concerns about rules that are not accompanied by sufficient explanations of the underlying safety problems that the rules are intended to avoid.

I found one question particularly interesting because it was in the context of a very high power... 480VAC (each phase to neutral), 3 phase + neutral, 4000 amperes per phase... industrial installation.

The question was about the use of multiple, parallel conductors to provide the connection between the utility and the industrial facility for each phase (plus neutral).

Apparently, the code states that the parallel conductors have to be of equal length.

The questioner asked whether the lengths of the parallel conductors in each phase bundle had to be exactly equal. The questioner also asked if the length of each phase bundle had to be the same.

Answers provided included (paraphrasing somewhat):

"It is physically impossible for two conductors to be of the same length"
and "The individual phase bundles (of parallel conductors) don't have to be the same length... only the parallel conductors within each separate phase bundle".

No answer was provided regarding how much length difference would be acceptable (which was what the original poster wanted to know), what would go wrong if the parallel conductors were not approximately the same length, and how one might determine what the acceptable length difference is (in a given situation). No information was provided to substantiate the assertion that the three phase bundles didn't have to be the same length (which is probably true in almost all situations), but that the parallel conductors in each phase bundle have to be (approximately) the same length.

Stu

Logged

Stewart ("Stu") Personick. Pictured: (from The New Yorker) "Season's Greetings" looks OK to me. Let's run it by the legal department
WD5JKO
Member

Offline Offline

Posts: 1996


WD5JKO


« Reply #10 on: July 24, 2014, 04:34:30 PM »

Once when I was very young, and worry-free, got 220V from two extension cords with each plugged into a different rooms 120v outlet. This fed a big HB rig at the time running 1 KW DC input. If one plug or the other were to be unplugged, the prongs would still be hot. I told people to NEVER unplug that cord.  Huh

So how many violations would that be in the NEC? Each cord had its own circuit breaker too..

I am much more careful these days.

Jim
Wd5JKO
Logged
W4EWH
Member

Offline Offline

Posts: 833



« Reply #11 on: July 24, 2014, 06:36:54 PM »

Once when I was very young, and worry-free, got 220V from two extension cords with each plugged into a different rooms 120v outlet. This fed a big HB rig at the time running 1 KW DC input. If one plug or the other were to be unplugged, the prongs would still be hot. I told people to NEVER unplug that cord.  Huh

So how many violations would that be in the NEC? Each cord had its own circuit breaker too..

When I rented an apartment near my college, I found that the ancient (knob and tube) wiring had been fitted with fuse adapters that limited fuses to 15 amps. I bought a lamp socket (the kind that's used for temporary lighting on job sites, with two wires and a rubber housing for the Edison base) and wired it around the fuse, with a 20 amp fuse in it.

My air conditioner worked fine when nobody else could get one that wouldn't blow the fuse.

Bill, W1AC
Logged

Life's too short for plastic radios.  Wallow in the hollow! - KD1SH
ka4koe
Contributing
Member
*
Offline Offline

Posts: 1157


It's alive. IT'S ALIVE!!!


« Reply #12 on: July 24, 2014, 08:43:32 PM »

The only explanation I've heard (and very unsatisfactory in my opinion) vis a vis the ground terminal on top was that if a metal object (say a yard stick) happened to fall over it would contact the ground terminal and not the phase-neutral simultaneously. Yeah, right. There is no sound technical reason to install receptacles in this fashion. Typically hospital critical power receptacles are red, if memory serves....but its been awhile.
Logged

I'm outta control, plain and simple. Now I have a broadcast transmitter.
ka4koe
Contributing
Member
*
Offline Offline

Posts: 1157


It's alive. IT'S ALIVE!!!


« Reply #13 on: July 24, 2014, 08:51:39 PM »

In the reference that Philip provided, there is a Q and A section.

The question was about the use of multiple, parallel conductors to provide the connection between the utility and the industrial facility for each phase (plus neutral).

Apparently, the code states that the parallel conductors have to be of equal length.

The questioner asked whether the lengths of the parallel conductors in each phase bundle had to be exactly equal. The questioner also asked if the length of each phase bundle had to be the same.

Answers provided included (paraphrasing somewhat):

"It is physically impossible for two conductors to be of the same length"
and "The individual phase bundles (of parallel conductors) don't have to be the same length... only the parallel conductors within each separate phase bundle".

No answer was provided regarding how much length difference would be acceptable (which was what the original poster wanted to know), what would go wrong if the parallel conductors were not approximately the same length, and how one might determine what the acceptable length difference is (in a given situation). No information was provided to substantiate the assertion that the three phase bundles didn't have to be the same length (which is probably true in almost all situations), but that the parallel conductors in each phase bundle have to be (approximately) the same length.

Stu



Exact is a very dangerous word. Carried to the extreme, its impossible to meet. The rationale is to achieve phase balance so that the vector sum of the three phase currents will balance out in the neutral. We don't want the neutral to carry more than its rated current. The intent of what you mentioned, I believe, was to make the impedance of the phase conductors as close together in value as possible. At 60 Hz using multiple runs of 500 KCMIL per phase, the difference of a few inches, or a few feet, within reason, makes no difference. Since the various currents on each phase are never the same in practice given the loads and demand factors, any minor differences in impedance for correctly installed feeders (ie what the NEC calls "workmanlike" ---- try to define that one) will have neglible impact overall. The differences are lost in the "noise".

Not being particular is a good way to get sued if one's license is on the line. You are expected to exercise a "reasonable" standard of care in producing designs by your state boards.
Logged

I'm outta control, plain and simple. Now I have a broadcast transmitter.
AB2EZ
Member

Offline Offline

Posts: 1722


"Season's Greetings" looks okay to me...


« Reply #14 on: July 24, 2014, 09:26:15 PM »

Philip

I don't think that the issue is what you have pointed to.


I believe that the issue is as follows:

Suppose you have 8 conductors, in parallel, that are (ideally) supposed to equally share 4000 amperes. I.e. you size the conductors to be able to carry 500 amperes each... without generating unacceptable amounts of heat per unit length.

To simplify this analysis, let's assume that the 8 conductors are insulated from each other.

Now let's assume, as a worst case rainy day scenario, that one of the 8 conductors is only 0.9 x as long as the others. The other seven conductors are equal in length.

The shorter conductor will have 0.9 x as much resistance as the other 7 conductors.

For any given end-to-end voltage drop, the shorter conductor will carry 1/0.9 x as much current as each of the other 7 conductors.

Saying this same thing in a different way: the 7 longer conductors will each carry 0.9 x as much current as the shorter conductor.

So, instead of having each of the 8 conductors carry 1/8th of the total current, the shorter conductor will carry 1/[1+(7 x 0.9)] x the total current = [1/7.3] x 4000 amperes = 548 amperes. I.e. The shorter conductor will carry about 10% more current than it would with an equal 8-way split. Each of the other 7 conductors will carry 548 x 0.9 amperes = 493 amperes.

Therefore, the shorter conductor would be overloaded (vs nominal) by 10%

The above is just an example to illustrate what problem arises if the conductors in a parallel group are not of equal length.

Stu
Logged

Stewart ("Stu") Personick. Pictured: (from The New Yorker) "Season's Greetings" looks OK to me. Let's run it by the legal department
Opcom
Patrick J. / KD5OEI
Contributing
Member
*
Offline Offline

Posts: 8308



WWW
« Reply #15 on: July 25, 2014, 12:16:16 AM »


...the ham may assume that all parts of the 240V circuit are deenergized..."
(emphasis mine)

And there is the real issue, more than whether the original writer stated his intent correctly. People assuming things about powerful equipment. Suppose the equipment's control system runs on the side of the line that tripped out. The equipment would appear dead and ye olde ass-u-me-r might get dead.

Simple additions good for home made and store-bought designs would be:
1. a dual (ganged) breaker on the front panel for use as a disconnect or main power switch. The house breaker ought be bigger so the equipment one trips first.

2. LED or other high reliability indicators, one on each mains line.

3. a plug and receptacle. no excuse for permanently wiring in a ham amp, they are not that big.
Logged

Radio Candelstein - Flagship Station of the NRK Radio Network.
wa3dsp
Member

Offline Offline

Posts: 295


WWW
« Reply #16 on: July 25, 2014, 12:43:50 AM »

Years ago both sides of 120V circuits were often fused. The situation that often comes to mind is the double fused plug that Heath and others used on their equipment. Back then plugs were not polarized and grounding did not exist. So it was a 50/50 chance that you would have a fuse on the right side of the line if you only used one fuse. It probably was a better idea to fuse both sides in that case although there was no good answer.

The Heath SB200 used dual independent primaries that could be used in series or parallel and each winding had a circuit breaker in series with it. In (I guess) a cost savings move they just left the breakers in both sides of the line when it was used on 120V. It would have been more complicated to do it the right way. I rewired my SB200 permanent for 120V and I use a single 10A fuse that has never blown. I see no reason to run an SB200 on 240V when most of us have multiple 20A 120V circuits in our modern shacks.

It is amazing that we made it through those times of no polarized plugs, no grounding, and no GFI's. We did get our share of shocks and the equipment biting you when you touched it. I have not experienced any of that in a long time. Whether it is politically correct or not every vintage piece of equipment I work on gets a three wire cord and fuse in the hot lead unless of course it is AC/DC, then it gets a polarized plug.

As for the positioning of the ground up or down on a 120V receptacle there is no code standard for that. You can do it either way. I always do ground down but I have seen it both ways. It would make sense to do it one way or the other throughout a location though. Sometimes it is more convenient one way or the other because of wire positioning in the box. Remember - Black to Brass and save you ass!

 
Logged
WD5JKO
Member

Offline Offline

Posts: 1996


WD5JKO


« Reply #17 on: July 25, 2014, 07:52:32 AM »

Therefore, the shorter conductor would be overloaded (vs nominal) by 10%

   Wires properly sized for the nominal load can easily handle a 10% overload forever, and perhaps be just fine for surges to 50% or more. In the example given, with 8 wires, the shorter wire has the 10% overload. This wire over several thermal time constants will warm up more then the other 7, and the copper resistance will rise somewhat more, and as a consequence the 10% overload of that conductor will be reduced.

   With the 3KW RF amplifiers I service, these run off 3 phase 208v delta (no neutral). At outputs of 1KW or less the per phase ampere balance is offset by 10% or more phase to phase. As the amplifier is run up towards full power, the phase balance gets close to perfect. I can see this effect looking at the converted DC which is largely unfiltered. The lumpy DC after full wave rectification shows the asymmetry which goes away with higher power. I suppose the bridge diodes have varying turn on thresholds where a lower phase current might be due to a higher diode threshold voltage. Then over time the diodes heat up and the threshold drops 2 mv / degree Celsius. This causes the currents to better balance.

Jim
Wd5JKO
Logged
AB2EZ
Member

Offline Offline

Posts: 1722


"Season's Greetings" looks okay to me...


« Reply #18 on: July 25, 2014, 10:12:50 AM »

Philip

No matter how small the resistance per unit length is:

The shorter wire will be carrying more current than the longer wires.

If the shorter wire is 10% shorter, then it will carry 548 amperes instead of the 500 amperes associated with an equal 8 way split

Stu
Logged

Stewart ("Stu") Personick. Pictured: (from The New Yorker) "Season's Greetings" looks OK to me. Let's run it by the legal department
ka4koe
Contributing
Member
*
Offline Offline

Posts: 1157


It's alive. IT'S ALIVE!!!


« Reply #19 on: July 25, 2014, 10:21:08 AM »

But, you're dealing with a three phase AC system. The other currents will be delayed in their peaks at 120 and 240 degrees respectively.

Also, the values of R are so vanishingly small in relation to the actual loads, that the difference in voltage drop is negligible.

for a 15 KW water heater at 120V, current is 125A, resistance of this load is 0.96 ohms. For this discussion, assume the R of the full length feeder is 0.00001 ohm. The shorter feeder would be 0.000009 ohm. For all intents and purposes, this is a good bonded connection. You won't see any differences in the real word.

You're limited to 3% voltage drop in any event for long feeder runs.



Logged

I'm outta control, plain and simple. Now I have a broadcast transmitter.
AB2EZ
Member

Offline Offline

Posts: 1722


"Season's Greetings" looks okay to me...


« Reply #20 on: July 25, 2014, 10:31:50 AM »

But, you're dealing with a three phase AC system. The other currents will be delayed in their peaks at 120 and 240 degrees respectively.

Philip

Hi!

I don't believe that the above is relevant to the problem at hand.

The issue is differences in the lengths of the individual, electrically parallel conductors, which collectively share the total current of just one of the 3 phases.


Stu
Logged

Stewart ("Stu") Personick. Pictured: (from The New Yorker) "Season's Greetings" looks OK to me. Let's run it by the legal department
ka4koe
Contributing
Member
*
Offline Offline

Posts: 1157


It's alive. IT'S ALIVE!!!


« Reply #21 on: July 25, 2014, 11:05:58 AM »

Still won't make any appreciable difference. Phase conductors (of like phase) cannot be run together in a dedicated conduit bundle in order to divide up the current. It doesn't work that way. Each parallel feeder will have A, B, C, and a N. If you do it this way you won't get field cancellation in each run and appreciable current will flow in other parts of the system, ie the conduit will warm up.

Also you also run into issues encountered by having to derate feeders per code after you exceed a certain quantity of conductors. So, it really doesn't pay to run a lot of wires in one raceway/conduit as you have to kick down the ampacity so much.

So, for a 1200 ampere feeder, I'd run 4 - 300 ampere parallel feeders, for instance......

Each 3" RGS conduit will have a 350 KCMIL conductor in it for each phase, ie 4 wires, plus ground if downstream from the main service disconnect.

Running parallel feeder conductors in the manner you describe is a big no-no code wise and from a safety standpoint. I busted an inmate contractor down in Brunswick, GA for this very issue. I told the General Contractor/Guard (who was carrying a rifle).....whoever did this work needs to be put in jail.....DAMN. True story.

Bottom line is the situation is purely academic IMHO as it would get ripped out after I wrote up an Order of Condemnation for the work.
Logged

I'm outta control, plain and simple. Now I have a broadcast transmitter.
AB2EZ
Member

Offline Offline

Posts: 1722


"Season's Greetings" looks okay to me...


« Reply #22 on: July 25, 2014, 11:30:57 AM »


Still won't make any appreciable difference. Phase conductors (of like phase) cannot be run together in a dedicated conduit bundle in order to divide up the current. It doesn't work that way. Each parallel feeder will have A, B, C, and a N. If you do it this way you won't get field cancellation in each run and appreciable current will flow in other parts of the system, ie the conduit will warm up.

Also you also run into issues encountered by having to derate feeders per code after you exceed a certain quantity of conductors. So, it really doesn't pay to run a lot of wires in one raceway/conduit as you have to kick down the ampacity so much.

So, for a 1200 ampere feeder, I'd run 4 - 300 ampere parallel feeders, for instance......

Each 3" RGS conduit will have a 350 KCMIL conductor in it for each phase, ie 4 wires, plus ground if downstream from the main service disconnect.

Running parallel feeder conductors in the manner you describe is a big no-no code wise and from a safety standpoint. I busted an inmate contractor down in Brunswick, GA for this very issue. I told the General Contractor/Guard (who was carrying a rifle).....whoever did this work needs to be put in jail.....DAMN. True story.

Bottom line is the situation is purely academic IMHO as it would get ripped out after I wrote up an Order of Condemnation for the work.


Your comment about the importance of field cancellation makes sense to me. However, doesn't the fact that the parallel (current sharing) conductors for each phase (whether 2 or 4 or more) would be in sepatate 4-conductor (3 phases + neutral) cables... in separate conduits... increase the likelihood that the conductors sharing the current of each single phase would be of unequal lengths?

Isn't the reason for the de-rating: the unequal current sharing effect that I identified in my earlier posts?

Is it purely academic for the case of 4 cables (each with 4 conductors: 1 per phase + neutral)... sharing the current that you mentioned above?

Stu
Logged

Stewart ("Stu") Personick. Pictured: (from The New Yorker) "Season's Greetings" looks OK to me. Let's run it by the legal department
VE3AJM
Member

Offline Offline

Posts: 378



« Reply #23 on: July 25, 2014, 11:50:12 AM »

Ho, hum.

Nit picking wording (the endless annoyance of those who interpret NEC code).

The QST article was quite accurate and got the point across that breakers are needed in both lines of a 240VAC installation.  It is not intended to be a complete tutorial for household electrical installation.

There are equally many statements about electronic components, design etc. that could be endlessly caveated.

Electricians and the NEC code are worse than theologians arguing about angels on the head of a pin.  

Calm down and don't get so twisted up over general articles and fine detail.  Instead recommend people hire a qualified electrician or suggest good "How To" manuals.  Please.

I agree. I haven't read the QST article in question, but from the information provided here, if one would be confused by what was quoted to have been written, as an electrician I would say, don't attempt to do any electrical work period. These are basic safety/code principles. Familiarize yourself with the basics. QST articles are not meant to be your guide here.

If the layman were to go into a Lowes or Home Depot or hardware store and ask for a 240v breaker of a given current rating, they wouldn't be handing you 2 single pole 120v breakers to put in each leg to do the job. Hey, even GOOGLE can be your friend if you want it to in this matter. Or, just have a look at your home breaker panel for the 240v breakers used for your a/c, electric hot water tank, electric stove etc. All these tangential posts would confuse the heck out of the guy attempting this more than the QST article info quoted here.

Al VE3AJM





Logged
ka4koe
Contributing
Member
*
Offline Offline

Posts: 1157


It's alive. IT'S ALIVE!!!


« Reply #24 on: July 25, 2014, 01:02:39 PM »

Quote

Your comment about the importance of field cancellation makes sense to me. However, doesn't the fact that the parallel (current sharing) conductors for each phase (whether 2 or 4 or more) would be in sepatate 4-conductor (3 phases + neutral) cables... in separate conduits... increase the likelihood that the conductors sharing the current of each single phase would be of unequal lengths?

Isn't the reason for the de-rating: the unequal current sharing effect that I identified in my earlier posts?

Is it purely academic for the case of 4 cables (each with 4 conductors: 1 per phase + neutral)... sharing the current that you mentioned above?

Stu


The first question is one of those you can't really answer. It all depends on WHO is installing the feeders. If you have a cracker-jack ole time electrician who takes pride in his work, then its very likely the install will be neat and have beautifully made conduit bends, etc., and the wires/conductors will be very close in length. If they are longer or shorter by a few feet due to conduit bends, it really won't make any significant difference in terms of voltage drop or other criteria.

Additionally, an install with the length issues you suggest (eg. feeder 10% shorter) would very likely exhibit other "heads up" issues with respect to the overall quality of the job. The electrician would surely leave work in place with other significant issues. Bottom line: you're discussing means and methods.

The second issue of sharing the current, OTOH, is misleading because the other phases are lagging behind (assuming we're discussing "A" phase) at either 120 or 240 degrees at some non-peak value. The peak of "A" phase at 100A for instance will mean that the other two phases are not at peak and at some other value.

In any event, differences in feeder conductor lengths in multi-wire bundles won't matter that much as the resistances are extremely small in the first place.

Reference NFPA 70 (2011), Table B.310.15(B)(2)(11) for multiconductor runs over 3. Derating factors are as follows:

4-6: 80%
7-9: 70%
10-24: 70% with diversity
25-42: 60% with diversity
43-85: 50% with diversity

High conductor counts are also discouraged as the likelihood of insulation/wire damage increases during the pulling process.
Logged

I'm outta control, plain and simple. Now I have a broadcast transmitter.
Pages: [1] 2   Go Up
  Print  
 
Jump to:  

AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
 AMfone © 2001-2015
Powered by SMF 1.1.21 | SMF © 2015, Simple Machines
Page created in 0.078 seconds with 19 queries.