Sizing equipment grounding conductor for sub panel

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Enrique Lara

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FL, USA
Hello,

I built a sub panel that is 200 amps 3 phase. I used 4/0 to feed the load carrying conductors and went with a 6 AWG off of table 250.122 for my equipment grounding conductor and failed my inspection. I was told that 250.122 (B) says that my equipment grounding conductor “shall be increased in size proportionately according to the circular mil area of the ungrounded conductors. I did increase my ungrounded conductors from 3/0 to 4/0. I thought I would be ok because in Table 250.122 it says it goes up to anything below 300 amps over current device. What do you think?
 
You need a larger EGC. #6 is required for #3/0 copper conductors @ 200 amps, since you went up one size you'll need to increase the size of the EGC in this case one size to a #4.

Welcome to the Forum. :)
 
I agree with Rob. Look at the wording of 250.122(B). The "increased in size" sentence was recently revised to clarify that it means "increased from a size that would have been sufficient for the required ampacity." In your case, a 3/0 would have had sufficient ampacity for a 200 amp load, and you used a larger wire than that.
 
I agree as I stated in my pm before the thread was started but I do understand where the op is coming from. If I only need a #6 equipment grounding conductor when I have a 250 amp breaker why do I need a larger conductor when the breaker size is smaller. It is hard to wrap your head around as this section doesn't always make sense.
 
I believe it's has to do with making sure the OCPD operates quickly with a short circuit.

Also, the OCP for the conductors could be increased in the future to raise the capacity.

That it's a percentage instead of a step in the table makes me think it's the first idea.
 
Dennis Alwon said:
I agree as I stated in my pm before the thread was started but I do understand where the op is coming from. If I only need a #6 equipment grounding conductor when I have a 250 amp breaker why do I need a larger conductor when the breaker size is smaller. It is hard to wrap your head around as this section doesn't always make sense.
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Yep you and many others have called BS on these situations. There are many sizing situations where you get unreasonable EGC results under the existing wording. I think we are going to see a big change in this in the 2020 edition. The talk is to rewrite all this along the lines of the size of the conductors and not the size of the OCPD. Similar to 250.102(C)(1) but different values.
If I was the inspector I would probably give them a pass.
 
Our mod Don has been proposing for years that the EGC be sized according to the circuit conductor size not the OCPD that would help to end this nonsense.
 
infinity said:
Our mod Don has been proposing for years that the EGC be sized according to the circuit conductor size not the OCPD that would help to end this nonsense.
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Yes, it really does lead to some bizarre results in some situations. I think Don will ultimately get his way as I think this makes the most sense.
 
infinity said:
Our mod Don has been proposing for years that the EGC be sized according to the circuit conductor size not the OCPD that would help to end this nonsense.
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There would have to something in place for sizing HVAC equipment grounds, unless the wire size of grounds is made larger in general (if that makes sense). When we get up to bigger 30-40 ton units, sizing based on the conductors for HVAC equipment will give us undersized grounds.

500 copper can, and has been fused at 450amps if the RTU calls for it. If we size the ground based on 500's max amperage at 75 degrees, it's 380 amps and we would get a #3 ground according to 250.122. I realize im mixing apples and oranges a bit here, but i don't know what the mentioned moderator has proposed as far as the new formula for sizing EGCs. Only that it should be done based off conductor size, not OCPD. As its stands today i would have to pull a #2 equipment ground for that same RTU.

Not saying im against the change, i like the idea. Im just wondering how the more experienced guys would see a situation like this being handled by the NEC.

Sent from my SM-G950U using Tapatalk
 
Hopefully they don't change the value on the smaller size wires as all the nm & ser cables are sized based on 250.122.
 
petersonra said:
the whole problem is solved by using metallic raceways as the EGC. :)
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Unfortunatly convincing the AHJ’s this may be problematic as most I’ve dealt with want to see the wire EGC and by first hand experience I agree with them, as metallic raceway fittings become loose and/or corroded.
A ground fault condition happened in a barn where the couplings were loose subsequently showering sparks down onto some straw.......Good thing people were around when it happened.
 
but in the case of NM and SER cables arent the grounds based upon the size of the Live conductors? So if you switch from 8 NMb to 14 NMb isnt the ground smaller in the 14 than the 8? Thus, the size of the ground is based upon the size of the live wire?

I have heard of proposals that the ground and neutral should always be the same size as the live wire, but in UK am allowed to size the egc or ground smaller, though there is a specific table for such use.
 
mopowr steve said:
Unfortunatly convincing the AHJ’s this may be problematic as most I’ve dealt with want to see the wire EGC and by first hand experience I agree with them, as metallic raceway fittings become loose and/or corroded.
A ground fault condition happened in a barn where the couplings were loose subsequently showering sparks down onto some straw.......Good thing people were around when it happened.
Click to expand...

the code specifically allows for this. it is not up to the ahj.

in any case if the metallic raceway fittings are not done correctly so they become loose, what makes you think the wire was done right?

I have seen some cases where metallic raceways have come loose due to damage or poor installation techniques. But failing to repair them when it is noticed is the issue.
 
petersonra said:
the code specifically allows for this. it is not up to the ahj.

in any case if the metallic raceway fittings are not done correctly so they become loose, what makes you think the wire was done right?

I have seen some cases where metallic raceways have come loose due to damage or poor installation techniques. But failing to repair them when it is noticed is the issue.
Click to expand...


Unfortunatly you you do not have control over how a building moves and causes conduit fittings to become loose. Go back and check your fittings after 10 years and tell me they’re still all tight.
 
petersonra said:
the code specifically allows for this. it is not up to the ahj.

in any case if the metallic raceway fittings are not done correctly so they become loose, what makes you think the wire was done right?

I have seen some cases where metallic raceways have come loose due to damage or poor installation techniques. But failing to repair them when it is noticed is the issue.
Click to expand...

Loose fittings is an installation issue, not a product issue, and that what the code panel would answer if you submitted a change.
but I agree run an EGC.
 
infinity said:
Our mod Don has been proposing for years that the EGC be sized according to the circuit conductor size not the OCPD that would help to end this nonsense.
Click to expand...

The theory is the larger the circuit conductors the more energy they can contribute to a fault, so the EGC needs to be larger. There was a study done some years ago sponsored by the Steel Tube Institute, saw it explained in Soares Book On Grounding. Later there was an interesting software program called GEMI, that have various parameters, ex length of circuit and voltage, solved for size of EGC
 
Dennis Alwon said:
I agree as I stated in my pm before the thread was started but I do understand where the op is coming from. If I only need a #6 equipment grounding conductor when I have a 250 amp breaker why do I need a larger conductor when the breaker size is smaller. It is hard to wrap your head around as this section doesn't always make sense.
Click to expand...

You'd need a #4 Cu with the 250A breaker. But you are correct, this can lead to clever work-arounds, meeting the wording of the NEC.

Such as a 125A circuit, that upsized from #1 Cu to #3/0 Cu, and the EGC is required to be upsizes from #6 to #3. You could swap your 125A breaker for a 200A breaker, and then the 3/0 wire is compliant with the default #6 EGC. One would think that a larger OCPD in general means a larger EGC, yet here, a larger-than-necessary OCPD in the same range means the EGC doesn't need to be upsized as much.
 
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