I have a 120 volt circuit that because of voltage drop needs to be a #8awg conductor. It will be on a 20 amp breaker. We had planned on using mc cable but the standard # 8 mc cable only has a #10 equipment ground. The question I have is would it be permissible to use #6 mc cable which has a # 8 equipment ground.
Equipment Grounding Conductor Sizing For Voltage Drop
- Thread starter ramIII
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ceb58
Senior Member
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- Raeford, NC
Voltage drop has nothing to do with the EGC. Table 250.122 for EGC's is based on the OCD. For 20 amp OCD it is #12 Cu.
You can use the #6 if that's what's needed for voltage drop as long as the 20 amp breaker is rated to accept #6 wire.
- Location
- Connecticut
- Occupation
- Engineer
No, if the #12 ungrounded conductors are increased in size to #6, 250.122(B) says that the EGC must be increased in size proportionally. That would make the #12 EGC a #6 EGC.
From my vantage point when applying 250.122(B), if you use the #8 or #6 on the 20 amp circuit you still have a violation. On a 20 amp circuit the EGC is sized the same as the phase conductors so anytime you increase the phase conductor your are going to proportionally raise the EGC the same amount.
One solution might be to install a cable with an extra full size conductor and make the spare conductor your EGC.
One solution might be to install a cable with an extra full size conductor and make the spare conductor your EGC.
I can get the correct mc although I have to order it. That's probably what I will do. Just from an an electrical standpoint shouldn't the breaker still open on a fault as long as the equipment grounding conductor is sized equal to the voltage drop requirements even though the current carrying conductors are oversized for the voltage drop.
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
I think it most likely that a breaker would trip under the circumstances you describe. True, but irrelevant. The code still requires the proportional increase in the EGC, as the others have described.
I have done a little more research and understand what the code says but do not totally understand the reasoning. For instance if I run a piece of 8/2 with number 10 ground and I put it on a 20 or 30 amp breaker I am in violation but if I install it on a 40 amp breaker I am ok. Is there a calculation where this makes sense.
- Location
- New Jersey
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- Journeyman Electrician
250.122(B) requires a proportional increase regardless if a smaller EGC would work, more so the question is will it work if you didn't increase the size of the EGC. You can use A/C cable and won't need to worry about the EGC at all.
250.122(B) requires a proportional increase regardless if a smaller EGC would work, more so the question is will it work if you didn't increase the size of the EGC. You can use A/C cable and won't need to worry about the EGC at all.
AC cable is not acceptable on this project. But in trying to make sense of why the code is written this way how does it make sense you can use A/C cable but cannot use a 8/2 mc with a #10 ground. I would think that the #10 ground in the mc cable would have less impedence than the A/C cable.
That will lead to distress
In having my suppliers try to look for #8 and #6 mc with a full size ground we have not found one manufacturer that stocks this type of cable. It can be made on a special order basis. We have not had one manufacturer be able to give us a distributor that stocks this cable. Based on this I would assume there are many installations where oversized mc cable has been installed for voltage drop but with the reduced EGC. Must be buildings burning down all over the USA.
Another area that this code section is not being applied correctly, in our area at least, is site lighting. I just looked at five sets of plans at our office by five different EE's. Everyone of them has the site lighting circuits at either 2 #8's and 1#10 ground or 2#6's and 1 #10 ground all on either 20 or 30 amp breakers.
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- Placerville, CA, USA
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- Retired PV System Designer
250.122(B) requires a proportional increase regardless if a smaller EGC would work, more so the question is will it work if you didn't increase the size of the EGC. You can use A/C cable and won't need to worry about the EGC at all.
One motivation for the upsizing of the EGC to match is that during a line to equipment metal fault, up to the time that the breaker trips, a fraction of the line voltage will appear on the bonded metal of the equipment. The hot lead and the EGC will form a resistive voltage divider and so the potential on the exposed metal will be higher if the line conductor is proportionally larger sized than the EGC.
May or may not have anything to do with the decision of the CMP, but it is a good rationalization at least.
One motivation for the upsizing of the EGC to match is that during a line to equipment metal fault, up to the time that the breaker trips, a fraction of the line voltage will appear on the bonded metal of the equipment. The hot lead and the EGC will form a resistive voltage divider and so the potential on the exposed metal will be higher if the line conductor is proportionally larger sized than the EGC.
May or may not have anything to do with the decision of the CMP, but it is a good rationalization at least.
Rationalization= 2 #8's and 1 #10 ground connected to 30 amp breaker =code violation
2 #8's and 1 #10 ground connected to 40 amp breaker = okay
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
this is a lot like the near prohibition on using cords for wiring. there is no good reason for it but no one wants to change it and make sense of it.
- Location
- Connecticut
- Occupation
- Engineer
It's easy enough to rationalize. Imagine you had a 24A, 120V load on a 30A breaker with a 100' conductor length to the load, and you increase the conductors and egc to #8 to keep the voltage drop to 3%.
And you also have a 32A, 120V on a 40A breaker with a 75' conductor length to the load, which gives you a 3% voltage drop without having to increase the conductor size.
The #8 and #8 egc at 100' will have a higher circuit impedance than the #8 and #10 egc at 75'.
The code doesn't limit it to voltage drop. You could have both of your ampacities above but make them both 20'. The 30 amp would be in violation while the 40 amp would not. In your first scenario you could also meet the code by changing the 30 amp breaker to 40 amp. This wouldn't change the impedence. It would seem to me that the 30 amp breaker would open quicker than the 40 but the 30 does not meet code.
- Occupation
- Licensed Electrician
It used to. As I understand it rub was guys would oversize the hots but still use a regular EGC and claim there was no voltage drop.
The change in the wording to fix this has resulted in 250.122(B) being one of the top two poorly worded sections in the NEC
It is something that must not be strictly enforced. There has to be a lot of installations where cable upsizing is required due to voltage drop on up to 30 amp circuits but the cable manufacturers or suppliers are not stocking the cable with full size grounds. Also as I stated earlier I have a number of drawings where the EE's are upsizing the current carrying conductors but are not upsizing the EGC's to match on these 15,20 and 30amp circuits.
- Location
- Windsor, CO NEC: 2017
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- Service Manager
In my experience, PVC and THHN fits the bill just fine for site lighting. Problem solved.
I use PVC and "THWN" for my site lighting. It is not a problem with my installations. I was just stateing that because many EE's are not upsizing the EGC to match the current carrying conductors on these 15,20 and 30 amp circuits when factoring in voltage drop on their drawings and the inavailability of the correct cables when factoring in voltage drop for circuits inside buildings that there probably are a lot of violations.
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