Dangerous Technical Error - QST Magazine

[AB2EZ]

Philip

Hi!

Let us agree that we disagree on the items discussed below.

Stu

Your comment about the importance of field cancellation makes sense to me. However, doesn't the fact that the parallel (currentee 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.

[ka4koe]

I edited my last response and did not see your follow up here prior to posting this.

I've been a "boots on the ground" EE for a long time and seen a lot of things that only served to make me shake my head. I encourage you to visit the Mike Holt web site. One thing I've learned is that the true craftsman electricians who care about their work can always teach one a thing or two in the practical application of this field if one is willing to respect their opinion and listen. Also, no one knows it all and there is always room for improvement and development.

Get a voltage drop spreadsheet off Mike Holt's page and plug in some numbers and see what you come up with. If you don't have it message me offline and I'll email it to you.

ADDENDUM TO THIS COMMENT:

If you are running high voltage/current feeds at near the rated ampacity, the tiny bits of resistance difference apparently can make a difference. For small stuff, I wouldn't sweat it.

[W3RSW]

Interesting thread.
Learning or relearning a lot paticularly about acceptable practice variances.

About hot or over capacity wiring; my gas co. at compressor and pressure lowering stations with self generated power (off the grid with 2000hp gen sets, etc.) and probably all high voltage ( 440 and up) plants routinely run infra-red camera scans of all junction boxes and breakers under load.

99% of anything hot that's found is due to loose connectors and faulty breakers.  I have never seen a hot wire by its self, very far removed from a faulty junction. Seems pretty much that all wiring in these stations has pretty large safety factors in sizing, dress, insulation and run.

[ka4koe]

I just wonder how we got from the issue of circuit breakers to paralleled feeders. The other reason given for not paralleling too many conductors is heat dissipation. If you have too many wires (pronounced "wars") in a raceway, it will inhibit dissipation. This is why there are derating factors. When using the referenced table, there is some debate with regards to counting the neutral. A perfectly balanced system will have very little neutral current. I don't parallel conductors in the same conduit in my practice in an event.

[W3RSW]

Oh we humans tend to wander in our discussions. One good thought  leads to another.

Back to the breakers, I picked 240 off the 50 amp breaker that goes out to the barn. Ran about three feet below that to the 240 outlet which feeds my HB twin 813's by 572B's rig.
The rig has a twin 20 amp breaker.  Guess I'd better put a connector bar in that trip.  And come to think of it check to see if the 50 amp breaker inside the service box has a connector bar or is DPST with only one handle.

I'll bet just about all hams have a lash up that doesn't meet some code or other, or someone's interpretation of such.  Too bad interpretation ends up in attorney hands with wiring practice still all over the place waiting for the next litigation.

[Opcom]

well the litigation is not as bad as a death but close.