- Location
- New Jersey
- Occupation
- Journeyman Electrician
I see EE's make this mistake quite often too, but 250.122(B) isn't so complex that an electrician can't figure out what the proper size EGC is and take appropriate action.
Equipment Grounding Conductor Sizing For Voltage Drop
- Thread starter ramIII
- Start date
- Status
I see EE's make this mistake quite often too, but 250.122(B) isn't so complex that an electrician can't figure out what the proper size EGC is and take appropriate action.
- Location
- Windsor, CO NEC: 2017
- Occupation
- Service Manager
No, you said...
You assumed that everyone is using cable and screwing it up; I was proposing that the installers may well be installing conduit and using "THHN/THWN".
I did not call a dozen supply houses to confirm or deny this, however.
ramIII said:
You assumed that everyone is using cable and screwing it up; I was proposing that the installers may well be installing conduit and using "THHN/THWN".
I did not call a dozen supply houses to confirm or deny this, however.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
To OP
What you are missing here is the distance. If you have a 20 amp circuit and you run #6 to the equipment with a #12 equipment grounding conductor over say 20', then I have no doubt that the breaker will trip under almost any fault condition. Now take that #12 and run it hundreds of feet then the impedance on the wire is now much greater than it was before and may have trouble tripping the breaker under some fault conditions. The code cannot take all conditions into account so they made a rule that is standard for all situations.
We know that the #6 with a 10 equipment grounding conductor will work but again throw in hundreds of feet and it may not. If you need to account for VD then one assumes the distance is very long and thus we must act accordingly.
It sounds like your solution is to use pipe and do it correctly.
Another possibility is if you are running single phase then just buy a 4 wire mc and don't use the equipment grounding conductor but use one of the other conductors as the equipment grounding conductor.
What you are missing here is the distance. If you have a 20 amp circuit and you run #6 to the equipment with a #12 equipment grounding conductor over say 20', then I have no doubt that the breaker will trip under almost any fault condition. Now take that #12 and run it hundreds of feet then the impedance on the wire is now much greater than it was before and may have trouble tripping the breaker under some fault conditions. The code cannot take all conditions into account so they made a rule that is standard for all situations.
We know that the #6 with a 10 equipment grounding conductor will work but again throw in hundreds of feet and it may not. If you need to account for VD then one assumes the distance is very long and thus we must act accordingly.
It sounds like your solution is to use pipe and do it correctly.
Another possibility is if you are running single phase then just buy a 4 wire mc and don't use the equipment grounding conductor but use one of the other conductors as the equipment grounding conductor.
jumper
Senior Member
- Location
- 3 Hr 2 Min from Winged Horses
A simple and elegant solution and one that Gus proposed earlier, but the "Horse Lords" of the NEC CMP will not allow it because if the conductors are smaller than #4 you cannot re-identify a phase conductor as an EGC or neutral.
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
It is not necessary to upsize the EGC to match the current carrying conductors; it just needs to be upsized by the same relative amount that the CCC's are upsized to counteract voltage drop. That may make the ECG match the CCC's or it may not.
jumper
Senior Member
- Location
- 3 Hr 2 Min from Winged Horses
The OP has 15, 20, and 30 amp circuits-so the EGC has to match the size of the ungrounded conductors in this case.
I have priced a job where we need to add some circuits in an existing building. To get to the areas we need to get the circuits across some hard ceilings. The directive was to use mc cable.Because of the distance from the existing panels we have to upsize some of the circuits to #6 or #8 wire. When I went to get pricing on the cable with the full size EGC's I found out there was a 3 to 6 week lead time and a minumum 2000' order for these cables. I needed about 1500' of cable total between the 2 sizes so I priced all #6. I stated in my proposal about the lead time on the cable and the customer told me that both of my competitors told him they could start immediately.
Volta
Senior Member
- Location
- Columbus, Ohio
Can you comply with 250.119(B)(1)?
This could possibly work but the maintenace man at this facility is also the janitor. I'm not sure we can meet the qualified person maintaining the installation,but that I guess will be up to the AHJ. I will make a call.
Thanks
JDBrown
Senior Member
- Location
- California
- Occupation
- Electrical Engineer
Out of curiosity, why isn't AC cable acceptable for this job? It may be worth suggesting a substitution of AC cable instead of MC cable, and emphasizing to the customer that if the other guys are planning to use standard off-the-shelf MC, they won't be giving him the quality, NEC-compliant job he's paying for.
Every now and then we'll have a client who gets a "money-saving suggestion" from a contractor or another engineer and wants us to use it in our design. Almost without fail, those suggestions are not code-compliant. Once we explain to the client that, while it might save some money up front, it's also likely to result in the Inspector refusing to sign off on the job until it's been fixed -- and how long would that take? -- almost without fail the client says to do whatever we have to in order to avoid trouble/delays with the Inspector, because every day it takes to correct a failed inspection is a day of revenue lost.
You might play a little game of "imagination" with your customer. "Picture this: You hire a licensed electrician to do your remodel. He says he can start tomorrow, and he'll be done in three weeks, so you tell him to get started. The electrician buys his materials, gets everything installed, and then calls for inspection, and what happens? The Inspector fails the job because the ground wire is too small and doesn't meet Code. Now your electrician has to rip everything out and start over. He has to order custom cables which take six weeks to arrive before he can even start re-installing. In the meantime, your facility sits empty, unable to be used.
"Or you can hire me, I'll order the correct cables now, and you can continue your normal operations for another five weeks. Then I'll start the demolition work, so that I'll be ready to start installing as soon as the cable arrives. But what would be even better is if you'll let me use AC cable instead of MC cable. With AC cable we won't have to worry about the size of the ground wire, because the cable armor IS the ground wire. And it's a standard shelf item, so I'll be able to start work right away."
I know if I was the customer, I'd be thinking AC cable looks pretty darned good right about now.
Every now and then we'll have a client who gets a "money-saving suggestion" from a contractor or another engineer and wants us to use it in our design. Almost without fail, those suggestions are not code-compliant. Once we explain to the client that, while it might save some money up front, it's also likely to result in the Inspector refusing to sign off on the job until it's been fixed -- and how long would that take? -- almost without fail the client says to do whatever we have to in order to avoid trouble/delays with the Inspector, because every day it takes to correct a failed inspection is a day of revenue lost.
You might play a little game of "imagination" with your customer. "Picture this: You hire a licensed electrician to do your remodel. He says he can start tomorrow, and he'll be done in three weeks, so you tell him to get started. The electrician buys his materials, gets everything installed, and then calls for inspection, and what happens? The Inspector fails the job because the ground wire is too small and doesn't meet Code. Now your electrician has to rip everything out and start over. He has to order custom cables which take six weeks to arrive before he can even start re-installing. In the meantime, your facility sits empty, unable to be used.
"Or you can hire me, I'll order the correct cables now, and you can continue your normal operations for another five weeks. Then I'll start the demolition work, so that I'll be ready to start installing as soon as the cable arrives. But what would be even better is if you'll let me use AC cable instead of MC cable. With AC cable we won't have to worry about the size of the ground wire, because the cable armor IS the ground wire. And it's a standard shelf item, so I'll be able to start work right away."
I know if I was the customer, I'd be thinking AC cable looks pretty darned good right about now.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
I realize the color code but give me a break. I am sure you could get by with that in most cases. I had a very strict inspector turn me down for taping a red wire in a 10/3 nm with green tape. This was back when 10/3 wo ground was allowed for a dryer. It was a 120 gas dryer but I ran the 10/3 in case they ever wanted to go to an electric dryer which was more common around here. I could have stripped the insulation but he agreed that would not be good and let it slide.
jumper
Senior Member
- Location
- 3 Hr 2 Min from Winged Horses
Dennis, I was agreeing with you, not arguing.
I despise the silly rule that disallows re-identifying phase colored wires smaller than #4 as EGCs or neutrals.
increased proportionately I hope - see if this works out.
Was a 12 awg
Now a 6 awg
Size the ground
Divide ? 6awg into 12 awg
12awg = 6530 (table 8 )
6awg = 26,240
26,240 divided by 6530 = 4.01
6530 x 4.01 = 26,185 c - mils
6 Awg
- Location
- Windsor, CO NEC: 2017
- Occupation
- Service Manager
No math is required for 15A, 20A, 30A - there is a 1:1 ratio between ungrounded and EGC conductors (240.4(D) and T250.122 are a 1:1 ratio.)
For other sizes,
Upsize Factor = Big / Little
Final EGC = Table 250.122 x Upsize Factor
For other sizes,
Upsize Factor = Big / Little
Final EGC = Table 250.122 x Upsize Factor
Thank You <smile >
Where ungrounded conductor sizes are not in kcmil (i.e. 4/0 or smaller AWG sizes)...
Upsized EGC = Table 250.122 + (normal ungrounded AWG - upsized ungrounded AWG)
For the "ought" sizes use...
0 for 1/0
-1 for 2/0
-2 for 3/0
-3 for 4/0
- Location
- Windsor, CO NEC: 2017
- Occupation
- Service Manager
250.122(B) makes it clear the adjustment is in circular mils. Seems easier just to reference the tables and stay with circular mils.
Your first statement is absolutely correct. As for the easier part, that is the route the section statement used because of the possibility of kcmil sizes in the calculation.
The problem is the table cmil values for AWG sizes are rounded to significant figures. Anyone knowing the rules for significant figures knows you only apply significant figures to the result of a calculation. If you use two values rounded to significant figures to start a calculation, it is very hard to determine the accuracy of the calculation result without rules for rounding. This section does not specify a protocol for rounding of values... we have to assume such. FWIW, AWG size differences are proportional when converted to true cmil values (i.e. quite often they are not using the table values).
- Location
- Windsor, CO NEC: 2017
- Occupation
- Service Manager
Smart, if the book has rounding errors, then questions based on the same book will have the same rounding errors. Doesn't it make sense to stick with the book?
Let me give you an example and then you tell me...!
60A circuit, nominal ungrounded #6 upsized to #4, nominal EGC #10.
Using table values, #6 to #4 is 1.5907 ratio, #10 to #8 is 1.5906 ratio. Does this mean we have to use a #6 EGC?
Last edited:
- Status