Upsize EGC?

[mark32]

I will be putting #2 cu thhn (Ampacity @ 75c is 115a) on a 100a breaker and use a #8 cu egc. A thought though crossed my mind, when upsizing conductors to improve vd one must also upsize the egc, vd is not a concern in this case, I'm only using #2 instead of #3 because that's what I have already. So, should I go up on the egc in this scenario?

 

[raider1]

Yes.

The code requires that if ungrounded conductors are upsized for any reason not just voltage drop the EGC must be proportionally increased as well.

Chris

 

[charlie b]

Sadly, I must agree. I never liked this rule (because I doubt it has a justifiable techncial basis), but a rule it is.

 

[K8MHZ]

I will be putting #2 cu thhn (Ampacity @ 75c is 115a) on a 100a breaker and use a #8 cu egc. A thought though crossed my mind, when upsizing conductors to improve vd one must also upsize the egc, vd is not a concern in this case, I'm only using #2 instead of #3 because that's what I have already. So, should I go up on the egc in this scenario?

The reason you up size is irrelevant. See 250.122 (B). It says that if you increase the size of the ungrounded conductors, you must also increase the size of the EGCs proportionately. There is no mention of any exception not involving voltage drop.

 

[Ponchik]

it still has to be upsized even though we are not dealing with VD.

what is the reason?

 

[charlie b]

it still has to be upsized even though we are not dealing with VD. what is the reason?

Probably because installers made a habit of bypassing the rules. The code used to require upsizing the EGC only when the phase conductors were upsized because of VD. So installers would upsize the phase conductors, and say it was because of some other reason. So the code writers saw the need to close that loophole. But they could not come up with a revised rule that was simple to write, simple to understand, covered the appropriate types of installations, and made it acceptable not to upsize the EGC in other cases. The only way to handle the situation was to require upsizing the EGC in all cases. At least that is how I see it, and that is also why I made the "I doubt" statement in my earlier post.

 

[Wagbag]

Upsize egc

Upsize egc

I get that you want to know about the egc but what environment is this being installed? Will it be inside a building, in a dairy, underground? What?

 

[Ponchik]

in the commentary of the 08 handbook it states:

Where the ungrounded circuit conductors are increased in size to compensate for voltage drop or for any other reason related to
proper circuit operation, 250.122(B) requires that the equipment grounding conductors be increased proportionately.

what if the OP installation is not related to proper circuit operation. (for example) if he were to install #3 the circuit will work fine but he has chose to install #2 because that is what he had on the truck.

 

[Ponchik]

I get that you want to know about the egc but what environment is this being installed? Will it be inside a building, in a dairy, underground? What?

why does it matter?

 

[iwire]

in the commentary of the 08 handbook it states:

Where the ungrounded circuit conductors are increased in size to compensate for voltage drop or for any other reason related to
proper circuit operation, 250.122(B) requires that the equipment grounding conductors be increased proportionately.

what if the OP installation is not related to proper circuit operation. (for example) if he were to install #3 the circuit will work fine but he has chose to install #2 because that is what he had on the truck.

The handbook is just an opinion, not the code. The words in the actual code do not support the handbooks comments.

250.122(B) Increased in Size. Where ungrounded conductors are
increased in size, equipment grounding conductors, where
installed, shall be increased in size proportionately according
to the circular mil area of the ungrounded conductors.

 

[david luchini]

Looking at 110.14(C)(1)(a) for circuits rated 100 amperes or less, it says that the temperature limitation of terminations provisions shall be based on one of the following (items (1) thru (4).)

Item (2) says conductors with higher temperature ratings (than 60degC), provided the ampacity of the conductor is based on the 60degC ampacity.

The 60degC ampacity of #2 is 95, which is acceptable on a 100A c/b per 240.4(B) (assuming that 100% of the non-continuous load plus 125% of the continuous load does not exceed 95 amps).

I see no reason to upsize the EGC as the #2 feeder conductor would be a "normally" sized feeder per 110.14(C)(1)(a)(2).

 

[iwire]

IMO 3 AWG is the normal size for 100 amp breaker but that is always the tricky part.

 

[Ponchik]

The handbook is just an opinion, not the code. I know and agree. The words in the actual code do not support the handbooks comments. but the comments are valid comments

that is why i brought it up.

 

[George Stolz]

Any time conductor size increases, so does fault current. That's why EGCs always have to be adjusted too.

 

[charlie b]

Any time conductor size increases, so does fault current. That's why EGCs always have to be adjusted too.

That is the alleged technical basis with which I find fault (no pun intended, but it was unavoidable :roll. When the conductor size increases, the fault current does indeed also increase, making it likely that the breaker will trip faster. So why do we need to increase the fault current even more, by making the EGC bigger too? That makes no sense to me at all. :happysad:

 

[don_resqcapt19]

That is the alleged technical basis with which I find fault (no pun intended, but it was unavoidable :roll. When the conductor size increases, the fault current does indeed also increase, making it likely that the breaker will trip faster. So why do we need to increase the fault current even more, by making the EGC bigger too? That makes no sense to me at all. :happysad:

To make the breaker trip even faster.

 

[charlie b]

Let me give an example of what I tried to say in my earlier comment. Consider three scenarios (and yes, I know #2 is not legal, but this is an academic discussion):

1. Circuit with #12 hot and #12 EGC has a fault. Result: High fault current, and breaker trips.
2. Circuit with #10 hot and #12 EGC has a fault. Result: Fault current is higher, and breaker trips faster.
3. Circuit with #10 hot and #10 EGC has a fault. Result: Fault current is higher still, and breaker trips even faster.

I have a problem understanding why the trip in scenario #2 wasn’t fast enough (given that it was faster than #1, and we know #1 is fast enough, because that is a legal install). I just don't get why we have to make it even faster, by going to scenario #3.

 

[iwire]

but the comments are valid comments that is why i brought it up.

I think this is a perfect example of when the comments of the handbook are not vailid as they seem to indicate ther are times when we dont have to upsize the EGC with the circuit conductors.

When the conductor size increases, the fault current does indeed also increase, making it likely that the breaker will trip faster. So why do we need to increase the fault current even more, by making the EGC bigger too? That makes no sense to me at all. :happysad:

IMO to protect the EGC and make sure it is capable of tripping the breaker.

I like to think in extremes when contemplating things so bare with me. Lets say I have to run a 20 amp 120 volt circuit 1000', the load on the circuit is 10 amps. My VD calcs show I need to use 1 AWG copper for the two circuit conductors.

Now what will happen if I leave the 12 AWG EGC and I have a fault to the EGC out at the far end of the circuit?

I suspect the impedance in the EGC will prevent the breaker from opening at all.

 

[iwire]

I have a problem understanding why the trip in scenario #2 wasn’t fast enough (given that it was faster than #1, and we know #1 is fast enough, because that is a legal install). I just don't get why we have to make it even faster, by going to scenario #3.

IMO what you are forgetting is the impedance that comes with circuit length. Please see my post above this one.

 

[brother]

I agree with the impedence issue. More resistance, means more time to trip a breaker. ;-)