voltage drop
- Thread starter domnic
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kingpb
Senior Member
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- SE USA as far as you can go
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- Engineer, Registered
Are you wanting to put them in parallel; i.e. #12/#10 together on each phase?
Theoretically yes, more copper lower VD. However, the NEC does not allow anything smaller than #1/0 AWG to be paralleled.
Carultch
Senior Member
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- Massachusetts
In series, yes. In parallel, no.
The NEC doesn't allow you to parallel anything smaller than 1/0. And if you do, the parallel conductors have to be as identical as practical. Length, size, type, conditions of use, conduit, etc. Usually we're talking 300A and greater when you might want to parallel conductors.
In series, yes you can run a short segment of #12 at each end and splice it to a #10 for the long distance, provided that #12 has sufficient ampacity for the intended use. One reason you might want to do this, is if the equipment terminations cannot accept larger than #12 (which is rare, as usually #12 and #10 are within the size range of the same lug), and you have to have the larger size to curtail voltage drop.
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You only have to run a 10 equipment grounding conductor along the portion of the circuit that has #10 awg. Is that what you meant?
Carultch
Senior Member
- Location
- Massachusetts
NEC2014 specifies that any time the ungrounded conductors are larger than the "minimum size that has sufficient ampacity for the intended installation", you have to proportionally increase the size of the EGC by the ratio of cross sectional area. For small circuits, 30A and less, this usually means that the EGC size equals the ungrounded conductor size. So consider if length weren't a governing factor, what is the minimum size that satisfies terminations, conditions of use, and the overcurrent protection device rating. That is "the minimum size that has sufficient ampacity for the intended installation".
If you run a larger wire for an unnecessary reason, like having left over wire from the previous job, you'd still have to increase the EGC size proportionally. NEC2017 will specify for voltage drop only, meaning that a scenario like this will no longer require it.
Yes. Wherever #10 is used in the circuit that #12 is the minimum required size. For example, if one line is #12 and one #10 completely from source to motor, the EGC is required to be #10 from source to motor.
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- Massachusetts
No, not under the current code.
Under the current code you would have to upsize the EGC.
JFletcher
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- Williamsburg, VA
I interpreted the question as using a #10 on one leg and a #12 on the other. Why this would be done intentionally or even hypothetically is beyond me. I could see it being done by accident, tho that wouldnt lead to the question op asked
Well if wire size for voltage drop calculated out to be 11AWG, running one leg in #12 and the other in #10 averages out to running #11, right?
Carultch
Senior Member
- Location
- Massachusetts
Well if wire size for voltage drop calculated out to be 11AWG, running one leg in #12 and the other in #10 averages out to running #11, right?
Well, close, but not exactly 11 AWG. Because the AWG scale is not a linear function of the cross sectional area, or even of the resistances. But in concept you are correct that its performance will be somewhere between the two gauge sizes that you run for both halves of the circuit.
It is probably most accurate to add up the resistances and divide by two to get the equivalent combined KCMIL of the series assembly. So the inverse of the KCMIL. I get an equivalent 8.02 kcmil when combining #10 and #12 in series, which is less than the Wikipedia value for #11 AWG (8.23 kcmil).
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JFletcher
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- Williamsburg, VA
Well if wire size for voltage drop calculated out to be 11AWG, running one leg in #12 and the other in #10 averages out to running #11, right?
heh heh
I think the local supply house carries #11, right next to the bin of 24.7A breakers. :lol:
Carultch
Senior Member
- Location
- Massachusetts
Do they also sell NEMA3.5R enclosures and aught and a half wire?
Carultch
Senior Member
- Location
- Massachusetts
I do imagine that being allowed to run different sizes for different phases/polarities of an electrical system would open up a crazy "can of worms" when it comes to applying the concept to three phase systems. But at the same time, I don't see anything in the NEC that would directly prohibit it. You are allowed to run a smaller neutral where it carries less current by the nature of the loads. And in some cases, you may need a larger neutral due to other kinds of loads.
So consider a typical 120V circuit with a #12 and #10 pair of current carrying conductors. If you make the #10 wire the ungrounded one, you'd likely have to have a #10 EGC, and correspondingly a #10 grounded conductor because it has to be at least the same size as the EGC. However, if you run a #10 grounded conductor, it would seem that you could run a #12 ungrounded conductor and a #12 EGC (because EGC sizing is based on the OCPD and the size of the ungrounded conductors). Why would you want to do this, other than as a work-around of an error, I cannot imagine.
If anyone does know why this wouldn't be allowed, please contribute. It doesn't feel like it should be correct to do this.
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It's the 25.3A breakers that are really useful! They are always sold out in my HD and at my supply house.
kwired
Electron manager
- Location
- NE Nebraska
Would improve the voltage drop seen by the motor. Assuming same length on each conductor the amount of drop across the 12 would be higher then across the 10, but the net drop would be less then it would be with 2-12's.
Used to be common to see a reduced size service conductor on high leg of some high leg delta systems where there is limited load connected to the high leg, and if using fused disconnects with properly sized fuse on that leg I see no problem with it. If you OCPD is a breaker - chances are not that great of having a different trip setting on one pole though. Sort of on the same topic with that.
wwhitney
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