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[Jeff Bankston]

Interesting. In a 125/250V, 3 pole, 4 wire, single-phase system, as commonly encountered in audio (connecting a generator to a spider box via a 50A Cali plug), my interpretation of NEC table 400.5(A) allows for 55 A in 6 AWG up to an ambient temperature of 30 deg C. We can use column B as only two of the conductors are "current carrying". The third (neutral) conductor carries only the out-of-balance component. This makes physical sense so long as we are not concerned with harmonic current in the neutral. 

For higher ambient temperature they tell us to derate in accordance with Table 310.16, using the column corresponding to the cord's rated temperature (or, I suppose, the lowest temperature rated connection made to that cord). The trouble is that the lowest column in the table 310.16 is for 60 deg C and the closest piece of SOOW I have lying around has 40 deg C stamped on it. What to do now?

640.42(E) (Audio Signal Processing, Amplification, and Reproduction Equipment), refers us to 520.5, 520.10 (Theaters...), and 525.3 (Carnivals...), which mostly just refer us back to Article 640 and say nothing special about ampacity. So it appears that article 400 applies to us. Sorry to flog this dead horse but is this everyone's understanding?

I never had an issue before with audio use or on construction sites when I was a commercial electrician(I'm retired now). Construction sites connect a 6/4 cable to a 50 amp 2 pole breaker in on a generator or permanent service panel and daisy chaining many spider boxes together so we can power the temporary lights and power tools. All temp power requires a permit and a LA building and safety inspector comes once and makes sure the temp power cords and boxes in good condition. My question was the non UL listed 6/4. I went to home depot earlier and all the bulk cable has UL stamped on it. I called one of the electrical supply houses and they have bulk cable that is UL listed and non UL listed and thats all they could tell me. There are electrical suppliers on the web and ebay selling name brand 6/4 and 4/4 that is both UL and non UL and the price is roughly the same for either. I will buy the UL listed. I'm going to continue to use 6/4 for my 25' and 50' runs and 4/4 for the run to my FOH mixer.  As for current carry yes a hot leg is current carrying and the neutral and ground are not.

[TJ (Tom) Cornish]

As for current carry yes a hot leg is current carrying and the neutral and ground are not.

This is not correct for non-resistive loads such as lighting dimmers and power amps.  The neutral counts as a current-carrying conductor.

[Jeff Bankston]

This is not correct for non-resistive loads such as lighting dimmers and power amps.  The neutral counts as a current-carrying conductor.

We were  never aware of that in the construction industry but we also never had anything to do with PA equipment or stage lights. We also had to wire according to the blue prints and were not allowed to make changes with written permission from    the electrical engineer and a written change order. How does that affect house wiring for home stereo systems and my old QSC  series 3 power amps ?

[Jeff Bankston]

I just got off the fone to the electrical dept at Los Angeles Building and Safety. They want to see UL printed on or molded into the outer jacket of SOOW cable no matter if its bulk with no ends or with user installed ends or with ends from the factory. The guy I talked to was quite "shocked" that there is non UL SOOW being sold.

[Frank Koenig]

This is not correct for non-resistive loads such as lighting dimmers and power amps.  The neutral counts as a current-carrying conductor.

Hi TJ, do you have a code reference for that? I have not been able to find it. Thanks. --Frank

[Stephen Swaffer]

310.15.B.5 (c)

Interestingly, if you are supplied by a 4 wire Y connected 3 phase, the neutral always counts [paragraph (b)], so unless you are using a distro exclusively on a single phase/split phase system, you need to count the neutral.

[Jeff Bankston]

310.15.B.5 (c)

Interestingly, if you are supplied by a 4 wire Y connected 3 phase, the neutral always counts [paragraph (b)], so unless you are using a distro exclusively on a single phase/split phase system, you need to count the neutral.

We are talking single phase not 3 phase. hot - hot - neutral - ground = 4 wires. 120V / 120V. 3 phase is hot - hot - hot - neutral - ground = 5 wires. 120V / 120V / 120V.

[Mark Cadwallader]

310.15.B.5 (c)

Interestingly, if you are supplied by a 4 wire Y connected 3 phase, the neutral always counts [paragraph (b)], so unless you are using a distro exclusively on a single phase/split phase system, you need to count the neutral.

Stephen, does the phrase "being supplied by" refer to the main service connection (to the building) to which the 50A split phase distro is connected via the Cali connector? 

[Frank Koenig]

310.15.B.5 (c)

Interestingly, if you are supplied by a 4 wire Y connected 3 phase, the neutral always counts [paragraph (b)], so unless you are using a distro exclusively on a single phase/split phase system, you need to count the neutral.

Yes, I noticed that. 310.15(B)(4) informs us that in the case of (b) two legs and a neutral of a 3-phase wye-connected system or (c) all three legs and the neutral of a 3-phase wye-connected system that supplies non-linear loads, the neutral counts. But it looks like they let us off the hook for single-phase (a).

I think I can design a "load from hell", however, that will burn the neutral in a single-phase system, but it sounds like it hasn't been a problem from NFPA's point of view. Try 884 uF across one leg and 7.95 mH across the other. Each will pull 40 A at 120 V. But the currents will add in-phase in the neutral to give 80 A, a bit much for the 6 AWG wire And it's not even a non-linear load, just a linear load with a sucky power factor.

Seriously, I would like to know where we stand on the 6 AWG 50 A single-phase question.

--Frank

[TJ (Tom) Cornish]

Yes, I noticed that. 310.15(B)(4) informs us that in the case of (b) two legs and a neutral of a 3-phase wye-connected system or (c) all three legs and the neutral of a 3-phase wye-connected system that supplies non-linear loads, the neutral counts. But it looks like they let us off the hook for single-phase (a).

I think I can design a "load from hell", however, that will burn the neutral in a single-phase system, but it sounds like it hasn't been a problem from NFPA's point of view. Try 884 uF across one leg and 7.95 mH across the other. Each will pull 40 A at 120 V. But the currents will add in-phase in the neutral to give 80 A, a bit much for the 6 AWG wire And it's not even a non-linear load, just a linear load with a sucky power factor.

Seriously, I would like to know where we stand on the 6 AWG 50 A single-phase question.

--Frank

Answering your question and Jeff's from earlier, for residential wiring, #6 NM wire can handle 55 amps and so is fine for a 50A load.  This is due to a much thinner jacket than SOOW cord, so it sheds heat better.  It is also fed almost exclusively from a split phase service, so other than Frank's extreme cruddy power factor situation, it's unlikely that the neutral current will be significant, and therefore is not counted as a current-carrying conductor in most cases.

Shifting over to SOOW and live sound reinforcement, the missing link in most folks' minds is that while a Cali-fed spider box is a two-phase device, when used in a commercial building, it's fed from a 3-phase service, even though only 2 phases are used.  This significantly increases the neutral current compared to the same load fed from a split-phase residential service.

This is largely academic in real life, however.  6/4 cord is indeed used for 50A cables, and it would take an observant inspector to catch it.  That said, I think this is a foolish compromise.  Amplifier power is heavily dependent on voltage drop.  We spend thousands of dollars on larger amplifiers and dragging more speakers to gigs, only to give up a significant fraction of that power to undersized power distribution.  This makes no sense.  What's unlikely to cause a fire (the main goal of the NEC which is published by the National Fire Protection Association) isn't what's optimal.

Friends that care about good sound quality don't let friends use 6/4 cord for 50A services.

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