The AM Forum

Merry Xmas

Need a memory Jog OM's....

Installing a separate 50 amp service to the transmitter Desk here, refresh my memory if i recall on separate systems neutral remains above ground and the fourth wire is panel ground return...correct...it is that simple i think the Nat Builders code requires the service to be installed that way...it's been awhile...


Any help is Much Obliged...

TNX
73 Jack.

You've got it right.The neutral and ground are only bonded together at the main panel.Have a Merry Christmas.Joe WB2FQD.

Jack,

   Another thing to consider for a remote panel is the wire size from the primary box. I'm not sure of the NEC requirements, but if you will be drawing large loads (> 15 amps) from 120v outlets (on one side of the 220v), then that wire length between boxes becomes more of a factor. Maybe pick 4-3G wire instead of 6-3G for up to 55' wire run between boxes. This will be overkill, since the wire is sized for a larger box, but the voltage drop will be less. Of course if your main load is 220v, then this is less of a factor.
   A few years ago I bought a house with a remote panel in the garage. I quickly found that when I keyed my amplifier (CE 600L Linear) that the 120 volts dropped to about 108v. I later discovered that at the other end of the house, i just blew out several CFL lights that I just bought to save money. It turned out that whenever I turned on my amplifier full tilt, the voltage would dip at the amplifier and peak elsewhere in the house. The power Neutral had a high impedance back to the pole pig outside. Looking up the pole, sure enough, the bare 3/8" aluminum Neutral (220v CT) was busted off and hanging.
   My point to you is that when you are done with your remote panel, take a 1500 watt portable heater and plug into your outlet fed from the remote panel, and turn on. Then back at that panel make sure the 220v is still balanced such that you have near equal voltage for the panel neutral spreader to each 220v wire.

Good Luck!
Jim
WD5JKO

Joe

The rule is clear enough... but I have a somewhat different question than what the rule is.

My question is the following:

What is the 'rainy day scenario" (i.e. what is an example of something that could "go wrong") that would create a hazard if one did not follow the rule (bonding neutral to ground only in the main panel).

I can invent a rainy day scenario that isn't very likely to be the one that led to this rule... but here it is anyway to illustrate what I am looking for...

Hypothetical rainy day scenario

There is a GFI in the main panel on the line that feeds to sub panel. If you connect the neutral to the ground in the sub-panel, then there will be an unbalance of the hot and neutral currents on the cable between the sub panel and the main panel... which would trip the GFI in the main panel.

Again, I doubt if the above is the "rainy day scenario" that the NEC folks had in mind when they created this rule.

Happy holidays
Stu

One of the few 'memorized' formulas from high screwel electrical shop is

cm= L I k/ e

this is the 'sizing' requirement in circular mills for a particular voltage drop.
 
               L is the one way run length
               I is the load current in Amperes
               e is the permitted voltage drop
               k is a constant relating the diferences in conductivity of vasrious materials. for Cu, 12.9 and 
               for Al,  21.2

To give a feel for the numbers, 12 AWG is 6,530 cm. Yah can get the closest  standard size from the handbook.

klc

I think the ground in one panel rule is to prevent ground loop current.

Could be wrong. Been wrong before.

73
KC4KFC

My recollection is the lecture that the ground should never be allowed to carry any of the neutral load current, because it is a safety ground and must be at ground potential.  If it carries some of the neutral current, as in the case of ground to neutral bond in a subpanel, then it is no longer at ground potential, and safety may be compromised.

Excellent fellas...that right there comes to mind...neutral remains above...

I am Very much Obliged...and Maybe this time by doing things the right way ...I can avoid that ground spike problem I get during lighting storms...never cut corners...on HV Supply...

Thank you all and a Very Merry Xmas...

73 Jack.

Even more important code compliance issue:  Never ever leave any knockout holes uncovered!  



Merry Christmas!

Ahhhh...

Snake 'n Bake.

snake bite

In light of these wiring code and safety comments, what does one do with a piece of equipment that runs on 240V and contains both 120V and 240V loads internally so that the chassis ground is also the neutral? It is not possible (economical) to rewire the equipment

For now, I have a 4-conductor 10 guage cable going to it.
1 - (L1)120V
2 - GND (breaker box)
3 - neutral - is grounded in the breaker box
4- (L2)120V

The issue is more complicated since the equipment is in a building fed by a 100A breaker from the main panel, and inside the main panel, neutral is grounded and the main panel has a ground rod.

In the building is a panel with a 100A disconnect. The building has a ground rod. The 3-wire power running to the building is L1, N, L2. No additional GND from the main breaker box.

I think it (the equipment) is safe as long as the chassis ground stays grounded, and is made more safe by the neutral being connected to GND at the breaker box.

A diagram is attached because I have not described it well.

Jack

Thank you! Your explanation clears this up for me.

The "rainy day scenario" would occur in a situation where the sub-panel were far enough away from the main panel (and the earth ground rod) so that the cable between the sub-panel and the main panel had a significant amount of resistance (e.g., 0.25 ohms). Current flowing over the ground wire (in parallel with the neutral wire) between the sub-panel and the main panel (because of not following the code) would raise all of the ground connections of all of the circuits connected to the sub panel to a voltage above true earth ground.

Example: 50 amps of neutral current flowing over the (improperly connected) ground wire (in parallel with the neutral wire) between the sub panel and the main panel would raise the "ground" potential of the sub panel to 50 amps x 0.25 ohms = 12.5 volts relative to true earth ground.

If someone had a separate ground rod bringing a separate ground wire into the shack (e.g. tied to the main safety ground, as per code)... then the ground wires coming from the sub-panel would be at 12.5 volts relative to this separate ground.

If any of the equipment connected to the sub-panel were to (briefly) draw a lot more current, then this potential different between sub panel ground and earth ground would be a lot larger.

Thanks again!

Stu

Pat: Is the 80 ft. of conduit metal or PVC.If its metal make sure the conduit is bonded to the main and sub panels with bonding bushings which makes the conduit the ground return to the main Not the best for every rainy day solution,but its the easiest way out.Joe WB2FQD

Joe, I regret the buried conduit is PVC.

Patrick ... as I understand your diagram, you have the neutral and equipment ground tied together and grounded in the "equipment" (far right side ) ....this is not proper grounding practice and is dangerous ....you need to keep the equipment neutral isolated from ground ... the worst case fault scenario is if you lost ground at the main load center and all the return current went to your common bond in your 'equipment' ... since this is not fused, a potential fire hazard ! ....73 ...John

10 gauge wire on a 50 amp circuit? You are begging for trouble

That's the way the Gates BC1-T is wired up.  They have both hots coming into the transmitter, with the neutral grounded to the transmitter cabinet.  No separate ground, so neutral is bonded to ground  at the transmitter cabinet.  I insulated the neutral from ground in the transmitter, and grounded the cabinet to the shack's earth ground.  My shack is about 50 ft. from the house, and I use PVC conduit.  The cable is heavy duty direct burial stuff normally used for underground runs from the pole pig to the house (given to me by a friend who worked at the power company), but I use the conduit to protect it from future damage from any of my frequent ditch-digging projects.  I use the intended ground/neutral lead as neutral only, and run a separate wire, outside the conduit, buried like a ground radial, to bond the shack ground to the mains ground, and allow the neutral to float free as it comes into the shack.  I have found enough voltage difference between neutral and earth ground to make visible sparks when I short the two at the shack entrance with a clip lead.  With the two shorted together, there is a noticeable increase in hum in my audio equipment.

I suspect the Gates is designed to be located within a few feet of the mains entrance to the transmitter building, but it would seem to me to be a violation of code.  But I have noticed that electric stoves also use a common ground/neutral wire.

Is there anything in the electrical code that says that using the same wire for neutral and ground is OK for 240 volt circuits?  Actually, this would seem to me to be the preferred way to do it, if everything in the house ran off 240 volts and unbalanced 120 volts were never pulled directly off one side of the line.  There would be no need for a separate ground and neutral wire, but only a safety ground capable of carrying the same current as the hot wires.

The standard unbalanced 120 volt circuit, which I  suspect exists as a legacy of the early 20th century, when electricity was used for lighting and little else, causes a multitude of the hum, noise and safety problems associated with electric service to-day.

Our old electric stove, which we replaced with propane (much nicer for cooking), had a 120v outlet for plugging in small appliances.  Wasn't that a direct violation of the code?  I now feed my shack with the 40-amp circuit that was originally used for the electric stove, and so far that has always been adequate for all my transmitters and power tools.

See previous thread on this topic (http://amfone.net/Amforum/index.php?topic=17945.0)

I am saving up for Todd Electric to come out and check my box and grounding and install a new 220 and a double 120 @ 20 amp line just for radio stuff. I'm worried about my service entrance "ground", power company says its my problem.

Maybe I explained it wrong. I am using 10GA wire in a 30A breaker. I might have hijacked this thread, it was originally about Jack's adding a 50A service to his desk. Which I think is cool. 240V/50A at the desk..

BFS,

In the schematic picture, I meant to show that the neut and GND are bonded together in the shack breaker box near the 30A breaker there.

I do not understand how any current could flow between the equipment's ground and the power neutral since they are connected together at both ends of the same power cable that runs from the shack breaker box's 30A breaker to the equipment cabinet.

I'm interested in understanding the fire hazard. - the loss of ground at the main load center, 80 FT away at the house. You are saying the gound-to-neutral current would then come through the neutral of the 80FT of underground cable and through the un-fused neutral & ground wires in the equipment cord, to meet at the equipment chassis. This would set the power cord ablaze? If that is so, Ok I see that could happen. I have to think about this.

It is an old HB transmitter and was built like that. When I got it, it had two 120VAC non-grounding two-blade plugs on it and that is how it was used. The builder had two 120V outlets there and one was one side of the 240V and the other was the other side. The only original ground was a ground rod from the cabinet -and of course the neutrals going back to his breaker box.

At the cold end of every relay coil, transformer primary, variac, blower, and the like, is a wire going to the chassis. This is so entrenched into the rig that it is even done in the several power supply chassis for low voltage rectifier heater transformers. There are possibly about 35 of these points. It was built in the 1950's, maybe this was not a consideration then. He might have figured to wire it like the electric stove!

Some of 'em run on one side of the 240 and some from the other. The reason I used a 4-wire power cable is so the neutral would have twice the current carrying capability (half the voltage drop), in an attempt to keep it closer to GND and avoid having the cabinet show any voltage.

If I un-bond the ground and neutral at the shack box, there could be current flow from neutral through the TX power cable, to the cabinet, then back to the breaker box GND. But I avoided this by having the breaker box's neutral be grounded in the box. Should I check the current in that strap?



Don, how many neutral points did you have to un-ground in the Gates?

The electrical inspector (who had not seen this transmitter) told me I had to put in a ground rod at the main breaker box, at the emergency generator, and at the shack's breaker box where the 240V comes in. He said these provide the grounds. He did not tell me to add a 4th wire for GND from the main breaker box 80FT to the shack's box. But you did add this wire? So maybe my transmitter is cheating by using the chassis as the neutral and ground. I have not tried the spark test but I never measure any voltage between the cabinet and the breaker box GND (where the ground rod is conected)

Where do you think your sparks and hum are coming from when you connect the ground wire? Due to this do you leave it un-connected?

Any 240VAC circuit in todays code that uses neutral needs a fourth wire safety ground. 240 electric heat and electric water heater do not need the fourth lead because the third was chassis ground. Electric stove needs the fourth wire.
No return current should ever flow on safety ground.

There used to be a code exception for ranges and dryers that allowed you to use a three prong plug and combine the neutral and ground. They eliminated that from the newer codes and grandfathered in the old stuff. Any new stuff has to have the four conductor wire and four prong plug.

Ranges used a combo of 120 volt and 240 volt in the different heat ranges and dryers used 240 for the heater and 120 volt for the motor and controls.

Even crazier, the way the Gates BC-1H switches to 1/2 power is one lead of the 240V plate transformer primary is lifted from the 240 volt main and instead grounded to the cabinet..

Lose the cabinet ground and yer *toast*.

In commercial installations, this prolly wasn't an issue since the transmitter cabinet had to be grounded to the antenna system somewhere, but in ham use, BE CAREFUL!

One reason that the neutral or center tap of our 240 volt services is supposed to be solidly grounded is in the event of a primary to secondary fault of the pole pig. Without that ground, everything would be lifted to 14 KV inside the residence.

As I recall there was just one point where the neutral was grounded directly to the cabinet.  I disconnected the wire connecting it to ground and just left the neutral floating, then grounded the cabinet to the earth/rf/safety ground.

When I short ground and neutral in the station, I assume the voltage difference is coming from voltage drop in the neutral wire, from unbalanced load between the two 120v legs.  That way the ground stays at earth potential, while the neutral is treated exactly the same way as the hot, and may not be exactly at earth potential, but near to it.  That sparking indicates that the ground wire is pulling part of the a.c. load when it is shorted to neutral.


That exactly the way my BC1-T did it till I ungrounded the neutral from the cabinet.  Now the low power tap switches one side of the plate xfmr from hot to neutral, leaving 120v on the transformer, but neither side of the 120v is connected to the cabinet.


That makes perfect sense.  Maybe we would have fewer problems if we ran all our appliances off 240v as they do in many other countries, and used the midtap on the pole pig secondary solely as a safety ground and ground reference to balanced power circuits, without any current-carrying neutral and unbalanced 120v line.

As explained in the other thread, I am now running all my low-power radio stuff, receiver, audio equipment, everything in the station except the transmitters themselves, through isolation transformers using a balanced 120v secondary (midtap to earth ground).

Don, Interesting problem. When you connect both neutral and safety ground they share any return current based on reisitance of each. The only way you could isolate them would be to float all of the primary neutral connections and connect them to the neutral. Then hope there isn't hum.

Yes, it's like driving a square peg into a round hole.  I just let all my neutrals float free, treating them exactly the same way as I would a hot wire, with separate earth ground on all equipment.  I believe that's that's exactly what the NEC says to do.  Any current on the earth or safety ground will set up a potential difference between any two units connected to that  line, and that is what causes ground loops.

You theoretically shouldn't  have any ground  loop  hum if the chassis on everything in the shack is exactly at zero potential.  But even then any closed ground loop would be equivalent to one large turn of wire, and would pick up hum and noise induced from nearby magnetic disturbances caused by transformers, motors and random electrical transients.

The "star" grounding system to a single point should be less prone to cause hum pickup than daisy-chaining all units to one continuous ground lead.

But sometimes,  the very measures that eliminate or reduce induced a.c. hum in the audio stuff will actually introduce or increase tunable hum on rf signals heard in the receiver, or cause rfi from the transmitter to affect other stuff, and vice versa.

Well my question was about installing a 100 amp sub panel over at the other workshop desk wall for the 813 machine, what I did prior was just run a rather long length of cable back to the main panel, I want to stop doing that...and i did...

The idea is when i leave the station i want to throw a switch and completely disconnect the xmitter from any service.. all grounds and all antenna...after discharging the main HV supply...completely isolate the machine...

good idea