I need some clarification on upsizing conductors. Upsizing a #12 to a #8 for a receptacles that is 300' away , one would have to downsize the conductor at the receptacle. Now, is it permissible to to run #8 for voltage drop to a jbox and tie in #12 to several receps in the area?
Upsizing conductors for voltage drop
- Thread starter pitkas
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- Massachusetts
I agree with Smart $ on both points and here is a code reference for the size increase.
Out of curiosity how did you come up with 8 AWG?
Out of curiosity how did you come up with 8 AWG?
Nope. 10 awg MC cable contains 10 awg green grounding conductor so complies with 250.122 (B).
For bigger size wires you could use type AC cable not to have problems with this code section.
- Location
- New Jersey
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- Journeyman Electrician
Yup, for #10 AWG a cable will work, for #8 AWG it won't.
edlee
Senior Member
- Location
- Western Massachusetts
No code issue I'm aware of. It's fused at 15a or 20a, right, so the #12 is protected. I've done that in the past for some long outdoor circuits.
I have always wondered about the EGC being sized accordingly for voltage drop. My question is if #10 EGC is good for up to 60 amp OCPD then if you up size your phase conductors from lets say number 12s to number 8s like his guy did, how come you need to have a number 8 ground? I guess if the reason is fault current on a ground fault and the EGC needs to be able to carry the current then why is number 10 acceptable to use on a 60 amp circuit. Hope I am getting my question across clear enough. But I always thought that proportionally on this code means not the same size. That is what the word proportion means....
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- Placerville, CA, USA
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- Retired PV System Designer
During the time the fault is being cleared (waiting for the breaker to trip) you will have a voltage divider effect. If the two wire resistances are comparable, the voltage will be no more than half the line voltage.
If the EGC is small relative to the hot, the voltage on exposed metal will be closer to line voltage.
But perhaps more significant, if the run is long enough that VD is a concern with wire which can just handle the required amps, then the bolted fault current will be a lot lower, delaying the breaker trip.
Take the extreme case of the VD with minimum wire size being 25%. That means that fault current would be roughly 4 times the normal load current. Not enough to cause an instant trip of the breaker?
Now resize the circuit wires for a 2% VD. The line to neutral fault current will now be 50x normal current.
If you do not also resize the EGC, then line to ground fault current will be only about 8x normal current.
So to maintain fast trip on a fault (and remember that exposed metal is at a dangerous voltage the whole time) you have to increase the EGC size too.
In a really extreme case of VD (1000 foot run perhaps) you might even find that a line to ground fault would not even trip the breaker at all. Just as you would see if you had only a local earth connection without a fault clearing path.
PS: If for some reason a reduced size EGC was originally permitted, then the proportional increase would still leave you with a reduced EGC after adjustment, but larger than the original.
Tapatalk!
If the EGC is small relative to the hot, the voltage on exposed metal will be closer to line voltage.
But perhaps more significant, if the run is long enough that VD is a concern with wire which can just handle the required amps, then the bolted fault current will be a lot lower, delaying the breaker trip.
Take the extreme case of the VD with minimum wire size being 25%. That means that fault current would be roughly 4 times the normal load current. Not enough to cause an instant trip of the breaker?
Now resize the circuit wires for a 2% VD. The line to neutral fault current will now be 50x normal current.
If you do not also resize the EGC, then line to ground fault current will be only about 8x normal current.
So to maintain fast trip on a fault (and remember that exposed metal is at a dangerous voltage the whole time) you have to increase the EGC size too.
In a really extreme case of VD (1000 foot run perhaps) you might even find that a line to ground fault would not even trip the breaker at all. Just as you would see if you had only a local earth connection without a fault clearing path.
PS: If for some reason a reduced size EGC was originally permitted, then the proportional increase would still leave you with a reduced EGC after adjustment, but larger than the original.
Tapatalk!
Last edited:
kwired
Electron manager
- Location
- NE Nebraska
That is mostly the main idea here, though there are occasions where sometimes it just doesn't quite make any sense, like the 60 amp feeder circuit without any ampacity adjustment being able to be protected by a 10 AWG, but a 20 amp circuit with ampacity adjustments resulting in a 8AWG conductor must use a 8AWG EGC, even if a relatively short length of run.
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