Current=Carrying Conductors

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augie47

augie47

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Asked by a student:
Scenario a: 5 switches, each an individual switch leg (10 conductors) in a 3/4 EMT from J box to switches. Is derating required ?
Scenario b: 9 switches, one supply, each switch to individual 1 amp load. Also, 3/4" EMT to j box.
Derating required ?
 
I would say the questions are not clear, Gus. What is the relationship amongst the switches? If all switches are "on," will there be current in all the wires within the conduit? If so, then derating is required. But if the switches are arranged such that if there is current in one wire there can't be current in a second wire, then perhaps derating is not required. The simple answer boils down to how many wires can carry current at the same time, and whether the switches would physically prevent any two specific wires from both carrying current simultaneously.
 
I was of the same opinion, but giving the student credit, in scenario b, the total load in the conduit would not exceed 20 amps since all the conductors are on that one circuit.
 
Oddly, the Code does not take into consideration the level of current in each of the wires (except when deciding whether to count a neutral). If it carries any current (including a switch leg for a small load or a receptacle circuit with nothing plugged in) then it is counted as a CCC for derating.
The logic that all the wires are on one limited branch circuit makes just as much sense as the neutral exception or the traveler pair exception, but I do not think that the code recognizes it!
 
I am definitely not as knowledgeable as a couple of the people who answered. That said, for those who answered no, especially Smart$ and Charlie, what about appendix B load diversity. It used to require "engineering supervision", but they took that line out and moved it to the appendix. But also, and again, there may be something I am missing but...

The code section states, allowable ampacity of the conductor. If the conductor feeds a known load, and that load is lower than the allowable ampacity, then what code section says a breaker or fuse has to limit that current on the supply end? By that, question 2 would be perfectly allowable even without using the load diversity table, IMO.

What am I missing?
 
This question gets asked often and if I am not mistaken it relates indirectly to a 3 way switch install where we know that not all wires of the 3 way will carry current. In the 2014 the following passage was added and I see that as an answer to the 3 way but it does not give us guidance to a group of switch legs as the op states.

note under Table 310.15(B)(3)(a) said:
1Number of conductors is the total number of conductors in the raceway
or cable, including spare conductors. The count shall be adjusted
in accordance with 310.15(B)(5) and (6). The count shall not include
conductors that are connected to electrical components but that cannot
be simultaneously energized.
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GoldDigger said:
Probably not very much.
It looks like we may be converging on agreement that all of the conductors have to be counted for derating but that the derated ampacity may not be as limiting as it first appears if we have all known loads.
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While conductors simply have to have a derated ampacity not less than the load they supply, 240.4 also requires that the conductor be protected by OCPD suitably rated for its derated ampacity (paraphrased).

The point I'm trying to make is that even if you stuffed a conduit with switch legs from the same 20A circuit, as soon as the conductor derated ampacity drops to 15A or below, there's a 240.4 violation.

I'm not saying it is technically sound requirement. It is what it is. :huh:
 
augie47 said:
I was of the same opinion, but giving the student credit, in scenario b, the total load in the conduit would not exceed 20 amps since all the conductors are on that one circuit.
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As I recall in one of the recent code cycles there was a proposal to permit that, but it was rejected.
 
The following was proposed for the 2011 code. It appears to me that the CMP did not really read the substantiation. The submitter is correct, it is much like not counting the neutral as a current carrying conductor.
If you have a single hot and a single switch leg with a 10 amp load, the amount of heat produced 200r watts (where r is the resistance of the conductors). If you put the same 10 amp load on 10 switch legs and each of the switch legs has a 1 amp load the heat in the conduit is 110r watts. This assumes that the conductors are all of the same size for both cases.

6-64 Log #2312 NEC-P06 Final Action: Reject
(310.15(B)(2)(a), Exception No. 6)
_______________________________________________________________
Submitter: Mike Green, Municipality of Anchorage
Recommendation: Add Exception No. 6 to read as follows:
Exception No. 6: Where the load is divided among multiple conductors of the same circuit and phase supplying different loads, they shall be permitted to be counted as one conductor for derating purposes where all of the conductors are
identified by circuit.
Substantiation: This proposal is based on the same principal that allows the neutral of single phase multi-wire branch circuits to be ignored when counting current carrying conductors. Since the heat produced in a conductor is the square of the current, the total heat will always be reduced when the current is divided among multiple conductors. Requiring the conductors to be identified by circuit is necessary to verify the proper application of the exception.
Luminaires have become so efficient that dozens of them can be supplied by a single 20 amp branch circuit and new energy codes will require more appropriate control of individual areas resulting in control panels with many switch legs from the same circuit. This exception will reduce the unnecessary oversizing of conductors in raceways connected to those panels and to enclosures with multiple switches.
Panel Meeting Action: Reject
Panel Statement: The panel does not agree with the submitter’s substantiation. There was no evidence of any need or problem addressed by the substantiation. Conductors connected to the same circuit and supplying different loads are, in fact, current-carrying conductors and need to be counted when applying 310.15(B)(2)(a).
Number Eligible to Vote: 11
Ballot Results: Affirmative: 11
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don_resqcapt19 said:
The following was proposed for the 2011 code. It appears to me that the CMP did not really read the substantiation. The submitter is correct, it is much like not counting the neutral as a current carrying conductor.
If you have a single hot and a single switch leg with a 10 amp load, the amount of heat produced 200r watts (where r is the resistance of the conductors). If you put the same 10 amp load on 10 switch legs and each of the switch legs has a 1 amp load the heat in the conduit is 110r watts. This assumes that the conductors are all of the same size for both cases.
Click to expand...
One of the problems I see with that proposal is that it does not state e.g. the "switch legs" of the same circuit are in the same raceway/cable.
 
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