NEC 310.4(A). Why is the line drawn at 1/0?

[Timtrewyn]

I am working with an existing industrial installation that has been operating with a feeder using two sets of #2 CU, each set installed in 1 1/4" EMT under emergency conditions. The load is less than 60 Amps and is not going to grow. That was not known at the time of installation. I am aware the 2002 Handbook says "there does not appear to be any practical need to parallel conductors smaller than 1/0 AWG." The AHJ is now catching up with inspecting these emergency installations and is citing a violation of 310.4(A). What is the hazard?

 

[iwire]

The smaller the conductor the more likely one of the conductors will take on more than it's share of current due to differences in impedance between the conductors.

But the hazard really is irrelevant, the section says what it says and there is no exception for 'emergency installations'

 

[roger]

If the load is under 60 amps why not just do away with one set and size the OCP for the load?

Roger

 

[Dennis Alwon]

In your situation there is no hazard but the rule is there to cover all the different scenarios. I would comply exactly as Roger suggested. Why bother with parallel if the load is 60 amps unless voltage drop is an issue.

 

[Timtrewyn]

In your situation there is no hazard but the rule is there to cover all the different scenarios. I would comply exactly as Roger suggested. Why bother with parallel if the load is 60 amps unless voltage drop is an issue.

I thank all of you for your replies. The Handbook language gives me the sense that the Committee understood that the risks went up as conductor size went down, but that they drew the line at a point where it seemed no one would have a practical or economic reason to parallel conductors smaller than 1/0 copper. The handbook makes the same kind of remark with conductors larger than 1000MCM. No physical research results are cited to support the 1/0 minimum.

Why bother? Voltage drop is not the issue. I agree that if safety now or in the future is the issue, the Code is the Code and money cannot be the obstacle. Tragedy is always more expensive. But if safety is not the issue, as I sincerely think in this case, then approaching the AHJ for an exception is reasonable. It may not be approved because the AHJ is wary of setting precedents or other reasons. But am I being financially responsible if I do not ask, making a case based on more general principles of conditions of maintenance and supervision, qualified personnel, 24 hour supervision, industrial location, ampacity of a single set, amount and stability of the load, use of a plaque, etc? I am posting to get your views on when an exception might be sought and on what basis. I may sound cheap, but such are the times, and the cost of changing out a 22kA breaker and pulling out a set of conductors still gets my client's attention. Thanks again for your time.

 

[broadgage]

I do not believe that there is any significant hazard, this would be permitted in the UK and in many other places.
It is however a clear violation of the NEC and unless the AHJ agrees otherwise should therefore be corrected.

 

[George Stolz]

The AHJ is giving you the perfect opportunity to make money on a change order. Times are hard, charge accordingly.

There are so many times where we play the role of inspector and have to try to convince skeptical customers that a violation is a hazard. I try not to be in the habit of taking on the role of skeptical customer - it's a gift, make the most of it.

 

[skeshesh]

The AHJ is giving you the perfect opportunity to make money on a change order. Times are hard, charge accordingly.

:lol:

Ok so current unbalance as well as practical considerations (smaller conductors have ampacities that are pretty close to each other so there's much less physical limitations during the install) have been brought up, so it's time for a guess: Given the two considerations above and the exceptions for control/instrumentation wiring and high frequency systems that are included in 310.4, it seems that most practical scenarios are covered. I think that means they had a safety issue in mind. Consider this: if smaller parallel conductors were allowed it's more likely that such installations would happen in residential and light commercial, and therefore more likely for a DIY owner or untrained handyman to perform additions or alterations a few years down the line and do something like tap/extend one of the paralleled conductors to feed other loads (extra light, recept, what have you).

Again, just a guess FWIW.

 

[nollij]

Another issue to consider is that when a short circuit will occur, it will occur on one conductor of the parallel set. The OCPD may not protect the single conductor despite the parallel ampacity of the conductors. This article prevents people from doing silly things like using parallel #10AWG on a 480V, 60AT breaker where during a fault the conductors can melt before a breaker may trip.

With the higher gauges, there is more wiggle room where the breaker will still adequately protect individual conductors from melting when used in parallel sets.

#1/0AWG seems like a fair compromise. However, analysis should still be done to ensure that the OCPD protects the individual conductors for melting even for conductor sizes greater than #1/0AWG.

 

[petersonra]

This seems like such an easy thing to fix. just disconnect the extra set of conductors and be done with it.

Having said that, is it possible that at one time the extra set of conductors came from a different power source? Might explain why it was done.

I have never understood why the prohibition on paralleling conductors made any sense in this kind of case, as long as the conductors are adequately protected. It eliminates the potential to use a ring type connection to reduce voltage drop.

 

[rbalex]

Even for an "emergency" [590.3(C)] the installation would have been non-compliant [590.2(A)] and, even if it were permitted otherwise, it would have had to have been removed immediately [590.3(D)]

Assuming the installation were otherwise consistent with 310.14, the current divider is inversely proportional to the conductor lengths and even relatively small differences in length can make a significant difference. Smaller conductors are less forgiving as iwire noted. With respect to #1/0 - you gotta draw the line somewhere.