Table 310.15(B)(3)(a) note

[jimport]

The note below the table says to count all conductors in the raceway to determine ampacity adjustments. The note says even spares are counted. (5) and (6) later on say that some neutrals are not counted and ignore the grounding conductors. I can understand the neutral, but not conductors that will not be carrying any current. If this is due to heat conduction the total number of conductors will limit the available free area to provide cooling. Seems like the note could be more clearly written.

 

[don_resqcapt19]

The note below the table says to count all conductors in the raceway to determine ampacity adjustments. The note says even spares are counted. (5) and (6) later on say that some neutrals are not counted and ignore the grounding conductors. I can understand the neutral, but not conductors that will not be carrying any current. If this is due to heat conduction the total number of conductors will limit the available free area to provide cooling. Seems like the note could be more clearly written.

Spares are required to be counted because they could become current carrying conductors if they are placed into service. That would have an effect on the ampacity of the current carrying conductors that were in use at the original installation if the spares were not counted for the ampacity adjustment.

 

[ramsy]

Where conductors are concerned, code is using the term "spare" as active wiring, either switched or available for future expansion. No other definition of "spare" is provided to change the context seen in 314.27(C), or Table 310.15(B)(3)(a) Note 1.

Don's explanation would make a good commentary for the NEC Handbook. If you don't remove the spares, someone else will use them when you turn your back.

 

[jimport]

Good point about the spares, but it is still not all conductors even with the spares counted. Maybe the title of the table should be CCC or potentially CCC. This would automatically drop the grounding conductors that are already excluded.

 

[Carultch]

Good point about the spares, but it is still not all conductors even with the spares counted. Maybe the title of the table should be CCC or potentially CCC. This would automatically drop the grounding conductors that are already excluded.

The NEC previously used to use the term "current-carrying conductors" for this section. As of 2014, the NEC switched this term to "number of conductors" with a footnote to provide further details. The footnote needs to be there, to confirm that you are required to count spare conductors as if they are active at the time of installation. It also advises that when you have mutually exclusive conductors that cannot carry current simultaneously, you don't need to count them all. You only count the maximum possible quantity that could be carrying current at any given time. For example: the traveler conductors of a three-way switching circuit, as they are routed between the switches.

The EGC is never counted, since it doesn't carry current under ordinary circumstances.

The neutral is conditional on whether it gets counted or not.

Summary of when it gets counted:
1. Mandatory part of the return path, such as phase-to-neutral 2-wire circuits, and phase-to-phase 3-wire circuits from a WYE system that feed a group of single phase-to-neutral loads.
2. Harmonic-intensive non-linear loads

Summary of when it doesn't get counted:
1. When it is possible for balanced loads to yield no current on the neutral, even when the load is unbalanced. No matter what the degree of imbalance, the heat generated among 4 wires never exceeds the heat generated among 3 wires carrying their maximum possible current equally. Same logic applies to split-phase systems, for 3 wires and 2 wires respectively.
2. When it is only used for instrumentation purposes, like voltage and phase measurement, and therefore carries negligible amps. Likely milliamps.

 

[wwhitney]

So if the active conductors in use in the conduit are on a 120V/240V system, say, and the "spares" are 6 black conductors and 3 white conductors, are we allowed to only count 6 spares? Is there language supporting that, or just common sense?

Cheers, Wayne

 

[Carultch]

So if the active conductors in use in the conduit are on a 120V/240V system, say, and the "spares" are 6 black conductors and 3 white conductors, are we allowed to only count 6 spares? Is there language supporting that, or just common sense?

Cheers, Wayne

That brings up a good point. The answer is that there is no code rule that anticipates this issue.

You might pull spares with the intent that they are used in an application where only 6 out of 9 conductors would need to be counted. Then, someone in the future who uses the spare conductors, uses them, and uses them in an application where all 9 conductors need to be counted. Conservatively, you'd assume they all would have to count, if there is any ambiguity about what someone might do with them in the future. However, if it is clear how the conductors are intended to be used in the future, such as a duplicate of your first circuit, or clear labeling to address this issue, I could see it being permitted to only count the conductors that would count as CCC's if everything were built today.

One place you might see this, is if you start with a split phase service, where the neutrals would not need to count. You pull 3 reds, 3 blacks, and 3 whites, with one of them being for your active circuit, and the remaining two sets being spare. Then, someone upgrades to a 3-phase circuit. Your reds and blacks used to be 180 degrees out of phase, and the neutral qualified as a non-CCC. Then, after the service upgrade, the reds and blacks are 120 degrees out of phase and all the neutrals in these 3-wire circuits, now need to count as CCC's.

 

[infinity]

That brings up a good point. The answer is that there is no code rule that anticipates this issue.

That's why the entire concept of counting spares in the first place is dumb.