Let's Suppose

[lauraj]

One of my students came up with this question and it baffled me...

Single phase, 15A, 120V circuit. Let's say we've calculated total voltage drop to not exceed 3%.

There are 2 circuit conductors, ungrounded and neutral. Assuming each wire was calculated separately:

a. Could we pull a #8 ungrd. conductor with a #14 neutral? Also according to 250.122, EGC would have to be a #8.
b. Could we pull a #14 ungrd. conductor with a #8 neutral? EGC could be a #14 in this case according to 250.122.

Both of these scenarios seem to be thwarting the system, but I can find anything in the code that would prevent it.

Help!

 

[infinity]

Both (a) and (b) are code compliant.

 

[winnie]

I wouldn't quite call it 'thwarting the system'. It is a more expensive install than the 'proper' one (well, it uses more pounds of copper for the same performance), and it makes some of the rules regarding sizes of conductor not work as desired.

A #8 conductor has 4x the cross sectional area (and thus 1/4 the resistance) of a #14. The resistance of the #14 leg is 80% of the resistance of the circuit, the #8 leg is only 20% of the total resistance.

If we replace this design with two #10 conductors, then each conductor only has 40% of the resistance of the original #14. The total resistance of this #10 + #10 circuit would be 64% of that of the #14 + #8 circuit, with approximately the same mass of copper.

-Jon

 

[rlundsrud]

distance

distance

What are you using for your distance to do your voltage drop calculations? It seems to me that only one of the scenarios is necessary, and the distance would dictate which.

Bob

 

[lauraj]

I considered it "thwarting the system" because in case (a) you're required to pull a larger EGC than in case (b). That is the aspect I am interested in more.

I made up the numbers for this scenario. I don't think it's a good installation but if you had an issue with conduit size it would save some space by pulling a smaller EGC.

 

[petersonra]

I considered it "thwarting the system" because in case (a) you're required to pull a larger EGC than in case (b). That is the aspect I am interested in more.

I made up the numbers for this scenario. I don't think it's a good installation but if you had an issue with conduit size it would save some space by pulling a smaller EGC.

if you had an issue with conduit size and it is metallic conduit just skip the wire type egc entirely.

I do not consider it thwarting the system or cheating, especially in the case of the case of up sizing conductors requiring one to up size the EGC. What if you went with a #12 on a 2 foot run just because you happened to be out of #14 wire? should you have to up size the EGC?

if you run a #14 for a 1A load 1000 feet it is code to run a #14 EGC, but for the exact same install it is not code to make it #12 with a #14 EGC.

it is code to run the same circuit with no wire type EGC at all if (for instance) it is EMT, but if you put a wire type EGC inside the EMT you can't run a #14 EGC with #12 circuit conductors.