the problem is 250.122(B) says the size of the egc must be proportionally increased along with the other conductors. so for example (this is example only and the numbers are off) if you increase the required #10 circuit conductors for a 30 amp circuit by 50%, you have to calculate the circular mils of the ground and increase that as well.
8/3UF inside?
- Thread starter timewarp
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the problem is 250.122(B) says the size of the egc must be proportionally increased along with the other conductors. so for example (this is example only and the numbers are off) if you increase the required #10 circuit conductors for a 30 amp circuit by 50%, you have to calculate the circular mils of the ground and increase that as well.
I know this thread is a couple weeks old but I just had to add my $0.02. To the original poster, I'd ask the inspector, more than likely he'd OK it unless you got him mad or were wearing a sports jersey from his rival team.
Now to the code issue, I agree with MRB's interpretation of the NEC, however that's not what I believe that they had in mind when they wrote it. When push comes to shove, it's the AHJ that has the last word.
What I think they meant to say was that when you increase the size of the ungrounded conductors, you then have to increase the size of the ground to whatever ground size would be required for that rating of wire.
For example, looking at table 250.122
If you used cable rated for 60A but fed it with a 30A breaker you would need a 10awg ground. It just so happens that both 30A and 60A rated wire both require a 10awg ground.
Now if you use a cable rated for 100A but feed it with a 60A breaker, you would need a 8awg ground because 100A wire needs an 8awg ground.
MRB's interpretation is correct from my opinion, but the code is written wrong.
Lets try another one, I have a garage 300ft away from my house and want to feed it with 100A service as a subpanel fed from my house. I would need 1awg aluminum conductors for 100A at 75C. But for voltage drop and available wire sizes in aluminum, I use 4/O. I've increased 4 sizes, so per 250.122 100A breaker would have been a 6awg al cable, but upsizing that 4 sizes is a 2awg aluminum ground (based on circular mil). Looking at 250.122, a 2awg al ground is good for a 300A breaker!!
So legally according to the NEC and MRB, I can
1: run 100A feeder breaker to the garage using 4/O, 4/O, 4/O, 2awg
2: run 180A feeder breaker to the garage using 4/O, 4/O, 4/O, 4awg
(4/O is rated at 180A at 75C)
I agree with MRB's interpretation, I just think the NEC wording is incorrect and that the above example shows this. The NEC doesn't _require_ you to take voltage drop into consideration, so in #2, it would pass everything except excessive voltage drop at 180A...
As others have stated, when you oversize wire for voltage drop, you affect short circuit impedance. However I believe table 250.122 takes that into account.
This thread has brought to my attention that I need to talk to my AHJ to OK my garage feeder plans as I had intended to use a 100A breaker and a 4awg ground...
OK, so maybe it was $0.04
Brian
I edited this post after doing the math on circular mil ratio.
Now to the code issue, I agree with MRB's interpretation of the NEC, however that's not what I believe that they had in mind when they wrote it. When push comes to shove, it's the AHJ that has the last word.
What I think they meant to say was that when you increase the size of the ungrounded conductors, you then have to increase the size of the ground to whatever ground size would be required for that rating of wire.
For example, looking at table 250.122
If you used cable rated for 60A but fed it with a 30A breaker you would need a 10awg ground. It just so happens that both 30A and 60A rated wire both require a 10awg ground.
Now if you use a cable rated for 100A but feed it with a 60A breaker, you would need a 8awg ground because 100A wire needs an 8awg ground.
MRB's interpretation is correct from my opinion, but the code is written wrong.
Lets try another one, I have a garage 300ft away from my house and want to feed it with 100A service as a subpanel fed from my house. I would need 1awg aluminum conductors for 100A at 75C. But for voltage drop and available wire sizes in aluminum, I use 4/O. I've increased 4 sizes, so per 250.122 100A breaker would have been a 6awg al cable, but upsizing that 4 sizes is a 2awg aluminum ground (based on circular mil). Looking at 250.122, a 2awg al ground is good for a 300A breaker!!
So legally according to the NEC and MRB, I can
1: run 100A feeder breaker to the garage using 4/O, 4/O, 4/O, 2awg
2: run 180A feeder breaker to the garage using 4/O, 4/O, 4/O, 4awg
(4/O is rated at 180A at 75C)
I agree with MRB's interpretation, I just think the NEC wording is incorrect and that the above example shows this. The NEC doesn't _require_ you to take voltage drop into consideration, so in #2, it would pass everything except excessive voltage drop at 180A...
As others have stated, when you oversize wire for voltage drop, you affect short circuit impedance. However I believe table 250.122 takes that into account.
This thread has brought to my attention that I need to talk to my AHJ to OK my garage feeder plans as I had intended to use a 100A breaker and a 4awg ground...
OK, so maybe it was $0.04
Brian
I edited this post after doing the math on circular mil ratio.
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i also should note, than when getting a code exception from your AHJ you need to have it in writing from the building official. An inspector saying 'ok' is not sufficient and could bite you later (for example he could be replaced right after you finished installing the cable and his replacement could make you change it)
gatchel
Well-known member
I love these discussions...One similar to it actually convinced me to change out my 100A subpanel breaker to a 90A one...
For code questions I search Garagejournal.com first
For code questions I search Garagejournal.com first
I love these discussions...One similar to it actually convinced me to change out my 100A subpanel breaker to a 90A one...
For code questions I search Garagejournal.com first
If the wiring was not undersized in the first place, one would not have to change out the breaker......
gatchel
Well-known member
If the wiring was not undersized in the first place, one would not have to change out the breaker......
...If one would be able to get good information from his "so called inspector" and other "so called electricians" one would have never installed a 100A breaker to begin with. Thanks for your help. Seriously...
Anyone want to buy a 100A Siemens two pole breaker? Used for about 2 months...
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Frank The Plumber
Well-known member
The problem with interpretting the code wrong will be that the insurance could be voided.
I disagree.
Many houses do not meet code. The code has evolved over the years making many houses noncompliant.
To not cover a claim because of "not to code" opens a legal hornets nest. Did the "not to code" cause the loss? Who did the "not to code"? Was the "not to code" "according to code" at some time in the past? Was the "not to code" a minor issue or was it gross incompetence? Was the work inspected but not discovered? etc....
tdkkart
Well-known member
Agree'd, my house was built in 1976, the electrical was all done very well from what I can see, however much of it is not up to current code, especially regarding stuff like mechanical protection of romex etc(romex running unprotected below instead of within floor joists)
So, if the house burns down and it's found to be an electrical issue, who did it?? Was it original to the house, or did I just do it last week. The only things that I know of that an inspector might have been in the house for in the last 30 years are a generator switch and a furnace replacement.
I have not had an insurance company rep in my house for a look-see in over 20years. I think they've figured out that few of the issues are worth the fight.
remember this: insurance comapnies are not in business to write checks.
have a fire and your 50 year old cloth covered romex is invloved and you will probably be ok, have a fire and they find burned up new romex and no corresponding building permit and youve got a problem.
I have come to the conclusion you really do not want to make a claim. My deductible on my house is now $5,000.
I really doubt that an insurance company would decline a claim based on "not to code". It is just too hard to substantiate. "not to code" does not mean "imminent danger".
Now, if it could be proven you were the idiot to do an "imminent danger" "not to code" I can see where you will likely have a real hard time finding future insurance.
I really doubt that an insurance company would decline a claim based on "not to code". It is just too hard to substantiate. "not to code" does not mean "imminent danger".
Now, if it could be proven you were the idiot to do an "imminent danger" "not to code" I can see where you will likely have a real hard time finding future insurance.
ive seen it firsthand (commercial property)
Alright, Norcal and MRB, you guys seem to know how to interpret some of this oversizeing stuff, so please clarify a bit further with this example.
lets assume the original poster was using wire in conduit. He's got 8awg THHN copper wire. per table 310.16 THHN is good for:
55A @ 90C
50A @ 75C
40A @ 60C
He's connecting to his circuit breaker which only is listed for 60C terminals. Thus the wire would be properly rated at 40A and he installs a 40A breaker. Later on he replaces the breaker with a "new"40A breaker and this one is rated for 75C terminations. Now he has a 40A breaker on a system that could use a 50A breaker due to it meeting the 75C rating. If he replaces the 40A 60C breaker with a 40A 75C breaker, must he increase the equipment ground because the system is now rated for a higher temp?
With Romex, it's a bit clearer because it's all rated for 60C per code.
I guess it all comes down to what you use for the wire's "rating"
Brian
lets assume the original poster was using wire in conduit. He's got 8awg THHN copper wire. per table 310.16 THHN is good for:
55A @ 90C
50A @ 75C
40A @ 60C
He's connecting to his circuit breaker which only is listed for 60C terminals. Thus the wire would be properly rated at 40A and he installs a 40A breaker. Later on he replaces the breaker with a "new"40A breaker and this one is rated for 75C terminations. Now he has a 40A breaker on a system that could use a 50A breaker due to it meeting the 75C rating. If he replaces the 40A 60C breaker with a 40A 75C breaker, must he increase the equipment ground because the system is now rated for a higher temp?
With Romex, it's a bit clearer because it's all rated for 60C per code.
I guess it all comes down to what you use for the wire's "rating"
Brian