Do switch legs count as current carrying conductors

Status
Not open for further replies.
M

MiElectrician

Member
Location
mi
In the building we're working on there is a lighting control panel that most of the lighting goes to and then a switch leg is pulled from there to the area it is needed. We also have to pull the main circuit to feed the emergency lights in that room. So in one area we will have circuit 1 that feeds switch legs 1, 3, 5, and 7. Being that all together these conductors will never draw more than 16 amps in total I feel like they should only count as one ccc. Am I wrong, it's there code specific to this?

Thanks
 
chris kennedy said:
310.15(B)(3) makes no mention of how much current is carried, they all count.
Click to expand...


Note 1 of the Table in 2014

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.
Click to expand...
 
4 switch legs, all coming from the same circuit, in one conduit... yeah, I can see where you'd think it should be counted as one CCC, but I dont think there is an exception for it. Derating isnt going to be a problem with #12 wire until you get past 9 CCC (table 310.15(B)(2)(a)) in one raceway anyway... or are you saying you have 10CCC from the 4 switchlegs and their neutrals from one circuit plus the hot and neutral for the e-lights of another? If so, use MWBCs to reduce the # of neutrals, thus reducing the # of CCCs as well as conduit fill and materials. or run another conduit. afaik those are your only two code compliant choices here.

fwiw I agree that 4 switchlegs originating from one circuit should count as 1 CCC.... 4 #12s running at 20% load ea generates less heat than one at 80% does.
 
MiElectrician said:
In the building we're working on there is a lighting control panel that most of the lighting goes to and then a switch leg is pulled from there to the area it is needed. We also have to pull the main circuit to feed the emergency lights in that room. So in one area we will have circuit 1 that feeds switch legs 1, 3, 5, and 7. Being that all together these conductors will never draw more than 16 amps in total I feel like they should only count as one ccc. Am I wrong, it's there code specific to this?

Thanks
Click to expand...

The tables do apply, but keep in mind that the ampacity of a given switch leg is limited to the draw of the fixtures on it, so if the unswitched feeder isn't in the same conduit, you are probably OK with the ampacity adjustments.
 
Strathead said:
The tables do apply, but keep in mind that the ampacity of a given switch leg is limited to the draw of the fixtures on it, so if the unswitched feeder isn't in the same conduit, you are probably OK with the ampacity adjustments.
Click to expand...

You have to go by the OCPD, not the draw of each fixture.
 
Thanks for all the replies. I talked to my foreman about it and he said it doesn't matter how much they're drawing and they have to be derated for the ocpd.
We have 1" pipe with 15 #10s. The pulls are nightmares. No mwbc allowed in this building.
 
iwire said:
You have to go by the OCPD, not the draw of each fixture.
Click to expand...

I was going to ask you where it says that, then I per chance looked at 240 real quickly. 240.4 is the answer. Just another example of the arbitrary nature of the code, protecting ourselves from ourselves.
 
Strathead said:
I was going to ask you where it says that, then I per chance looked at 240 real quickly. 240.4 is the answer. Just another example of the arbitrary nature of the code, protecting ourselves from ourselves.
Click to expand...

If we could go by the draw of the fixture we could run 18 AWG from a switch to the fixture. It is really not that arbitrary. :)
 
iwire said:
If we could go by the draw of the fixture we could run 18 AWG from a switch to the fixture. It is really not that arbitrary. :)
Click to expand...

No you couldn't because there is still table 310.16 that limits 20A circuits to minimum #12 branch, but the arbitrary nature is that you CAN actually run #18 to the fixture itself.
 
Strathead said:
No you couldn't because there is still table 310.16 that limits 20A circuits to minimum #12 branch,
Click to expand...

Table 310.16 does no such thing. 210.4(D) is what limits us to 12 AWG for some types of 20 amp circuits.

but the arbitrary nature is that you CAN actually run #18 to the fixture itself.
Click to expand...

Not as part of the branch circuit which is what we are talking about. Fixture wires are treated differently and have length limitations on them.
 
Derating is for heat build-up, correct? Then why would one conduit with, say, 10CCC all originating from one 20A circuit, max 16A load, need the same derating as another identical conduit with 10CCC, originating from 5 20A breakers, giving an 80A max continuous load? There isnt even close to the same potential for heat build up.
 
JFletcher said:
Derating is for heat build-up, correct? Then why would one conduit with, say, 10CCC all originating from one 20A circuit, max 16A load, need the same derating as another identical conduit with 10CCC, originating from 5 20A breakers, giving an 80A max continuous load? There isnt even close to the same potential for heat build up.
Click to expand...

I agree, yet the code does not address that. Because of this better planing of the circuit routing may save you money.

I am pretty sure moderator George Stolz has put proposals in regarding this and they have not been accepted.
 
JFletcher said:
Derating is for heat build-up, correct? Then why would one conduit with, say, 10CCC all originating from one 20A circuit, max 16A load, need the same derating as another identical conduit with 10CCC, originating from 5 20A breakers, giving an 80A max continuous load? There isnt even close to the same potential for heat build up.
Click to expand...

I've been arguing that for years because distributing the load accros 4,5, or 10 switch legs all on one circuit will not cause excessive heat but as Bob stated the NEC is silent on this.
 
Correct me if I am wrong here. Say you have the feed to 5 switch legs and the 5 switch legs in the same conduit and let's say the load is 15 amps. Let's also say that the 5 switch legs are carrying 3 amps each so we have 15 amps travelling on the 5 switched wires and 15 amps on the feed-- does that not create a lot of heat?
 
Dennis Alwon said:
Correct me if I am wrong here. Say you have the feed to 5 switch legs and the 5 switch legs in the same conduit and let's say the load is 15 amps. Let's also say that the 5 switch legs are carrying 3 amps each so we have 15 amps travelling on the 5 switched wires and 15 amps on the feed-- does that not create a lot of heat?
Click to expand...

IMO, the 15A feed would create heat, but assuming we are talking #12 because in my world #14 doesn't really exist for 120V, 6 current carrying conductors (actually 7 potential because of the neutral) is allowed for #12 at 15 amps, so not too much. However, I bet 50 #12's carrying 3 amps a piece doesn't generate a lot of heat.
 
Dennis Alwon said:
Say you have the feed to 5 switch legs and the 5 switch legs in the same conduit and let's say the load is 15 amps. Let's also say that the 5 switch legs are carrying 3 amps each so we have 15 amps travelling on the 5 switched wires and 15 amps on the feed-- does that not create a lot of heat?
Click to expand...
The point is that (5) 3 amp wires create a lot less heat than (1) 15 amp wire.

To see this, suppose for ease of calculation the resistance of each wire is 1 ohm. Power equals current squared times resistance, P = I^2 * R. So the 15 amp wire is dissipating 15^2 = 225 Watts along its length. While each 3 amp wire is dissipating 3^2 = 9 Watts along its length; the five 3 amp wires total only 45 Watts, or 1/5 of the single 15 amp wire.

The choice of 1 ohm resistance was arbitrary; for any given wire length (resistance), the (5) 3 amp wires will dissipate only 20% of the heat that a single 15 amp wire will dissipate (at least to first order).

Yet the NEC procedure for derating will count the (5) 3 amp wires as contributing 5 times the single 15 amp wire.

Cheers, Wayne
 
Status
Not open for further replies.
Top