250.122

[shepelec]

If table 250.122 is used to size the EGC and this is based on the OCP, why does 250.122 (B) tell you to increase the EGC if oversized ungrounded conductors are installed?

This caught my eye while inspecting a sub-panel in an out building fed with #2 Cu and a #10 ground protected by a 60A breaker.

Any thoughts?

 

[infinity]

Yes, one thought, a violation. Although sometimes applying this section can be tricky in this case it's not. The ungrounded conductors have been increased in size therefore a proportional increase in the EGC is required.

 

[cpal]

If table 250.122 is used to size the EGC and this is based on the OCP, why does 250.122 (B) tell you to increase the EGC if oversized ungrounded conductors are installed?

This caught my eye while inspecting a sub-panel in an out building fed with #2 Cu and a #10 ground protected by a 60A breaker.

Any thoughts?

What is the length of run from feeder supply to the out building??

were the feeders increase in size to compensate for voltage drop?
were they increased for some other reason to ensure proper circuit operation??

 

[infinity]

were the feeders increase in size to compensate for voltage drop?
were they increased for some other reason to ensure proper circuit operation??

Does it matter? Either way they were increased in size.

 

[Volta]

If table 250.122 is used to size the EGC and this is based on the OCP, why does 250.122 (B) tell you to increase the EGC if oversized ungrounded conductors are installed?...

Because the larger conductors allow a greater amount of fault current to flow.

Assuming the lugs allowed 75 deg C terminations, a #6 would have been sufficient for the 60 amp circuit. If the wires were increased for voltage drop (or any other reason), the conductors were increased 2.53 times. The EGC must be increased proportionally, so CM of #10 = 10380 x 2.53 = 26251 circular mils minimum needed, so a #4 EGC should have been used.

If limited to 60 deg C, the same method would show a #8 as acceptable for grounding.

 

[shepelec]

This is where my thoughts were going on this but I wanted to be sure.

Yes the feeders were increased for VD. The total run is around 150'.

 

[LarryFine]

Another good reason is in case someone sees the upsized wires and decides to increase the OCP to take advantage of them.

 

[steve066]

Does it matter? Either way they were increased in size.

If its under the 2005 or 2008 NEC, I agree it doesn't matter.

But it might matter if they were installed under an older version of the NEC. I think the 2002 only required upsizing the ground if the phase conductors were upsized for voltage drop.

And I wonder when this rule first appeared in the NEC??

 

[Volta]

If its under the 2005 or 2008 NEC, I agree it doesn't matter.

But it might matter if they were installed under an older version of the NEC. I think the 2002 only required upsizing the ground if the phase conductors were upsized for voltage drop.

And I wonder when this rule first appeared in the NEC??

1975.

250-95 ... Where conductors are adjusted in size to compensate for voltage drop, grounding conductors, where required, shall be adjusted proportionately in size.

Two exceptions follow.

 

[mcclary's electrical]

1975.

250-95 ... Where conductors are adjusted in size to compensate for voltage drop, grounding conductors, where required, shall be adjusted proportionately in size.

Two exceptions follow.

02' does not mention voltage drop, simply, if the ungrounded are increased,the EGC is proportional

 

[Volta]

02' does not mention voltage drop, simply, if the ungrounded are increased,the EGC is proportional

I know, and I agree, but Steve066 was wondering when the rule first appeared in the NEC.

 

[charlie b]

Another good reason is in case someone sees the upsized wires and decides to increase the OCP to take advantage of them.

There is some value in that, I must admit. But I don't believe a present day installation should be required to protect against a future day bad decision. Anyone who makes a change is responsible to assure it is done correctly.

 

[charlie b]

Because the larger conductors allow a greater amount of fault current to flow.

Not to be critical of you in any way, but IMHO that is not a valid scientific basis for this requirement. Indeed, it is my belief that there is no valid scientific basis! I would be happy to see it removed. Consider this:

Case #1:
Run a 20 amp circuit 150 feet, using #12 wire and a #12 EGC. Postulate a fault at the load end. There will be some amount of fault current.

Case #2:
Run another 20 amp circuit 150 feet, using #10 wire and a #12 EGC. Postulate a fault at the load end. The amount of fault current will be more than in Case #1.

Case #3:
Run another 20 amp circuit 150 feet, using #10 wire and a #10 EGC. Postulate a fault at the load end. The amount of fault current will be more than in Case #1, and more than in Case #2 as well.

Discussion:
If we know that the fault current is higher in Case #2 than in Case #1, and therefore the chances of tripping the breaker are greater, why do we need to increase the fault current even more, by making the EGC bigger, as in Case #3? What makes anyone think that the fault current in Case #2 was not enough, given that the installation of Case #1 was legal, and that its fault current was enough by itself?

I don?t get it. :-?

 

[mcclary's electrical]

Not to be critical of you in any way, but IMHO that is not a valid scientific basis for this requirement. Indeed, it is my belief that there is no valid scientific basis! I would be happy to see it removed. Consider this:

Case #1:
Run a 20 amp circuit 150 feet, using #12 wire and a #12 EGC. Postulate a fault at the load end. There will be some amount of fault current.

Case #2:
Run another 20 amp circuit 150 feet, using #10 wire and a #12 EGC. Postulate a fault at the load end. The amount of fault current will be more than in Case #1.

Case #3:
Run another 20 amp circuit 150 feet, using #10 wire and a #10 EGC. Postulate a fault at the load end. The amount of fault current will be more than in Case #1, and more than in Case #2 as well.

Discussion:
If we know that the fault current is higher in Case #2 than in Case #1, and therefore the chances of tripping the breaker are greater, why do we need to increase the fault current even more, by making the EGC bigger, as in Case #3? What makes anyone think that the fault current in Case #2 was not enough, given that the installation of Case #1 was legal, and that its fault current was enough by itself?

I don?t get it. :-?

Charlie, your example is very interesting, however the distance of 150 feet, should be changed to 1500 ft. I myself have witnessed long runs with too small of conductors, where a line to ground fault will not trip a breaker. With that number change, Does that change your opinion?

 

[charlie b]

Charlie, your example is very interesting, however the distance of 150 feet, should be changed to 1500 ft.

In that case, I calculate a voltage drop of 80%!

I myself have witnessed long runs with too small of conductors, where a line to ground fault will not trip a breaker.

Making the phase conductors larger will improve that situation. Making the EGC larger will improve it even more. But where is the proof that the one was not enough by itself, and that the second step is necessary?

 

[Dennis Alwon]

But where is the proof that the one was not enough by itself, and that the second step is necessary?

You know where it is Charlie. Some dang electrical engineer (sorry Charlie):grin: came up with this and that's why it's there.

 

[iwire]

But where is the proof that the one was not enough by itself, and that the second step is necessary?

Considering the NEC is a minimum safety code I would say the perspective has to be where is the proof it is enough?

Much like the 25 ohm rule, I have to prove that I have at least 25 ohms, the inspector does not have to prove I do not.

 

[shepelec]

In that case, I calculate a voltage drop of 80%! Making the phase conductors larger will improve that situation. Making the EGC larger will improve it even more. But where is the proof that the one was not enough by itself, and that the second step is necessary?

I could be wrong but if the voltage drop would be around 80% on a phase conductor would you not have the same drop on the EGC? So if you installed oversized conductors for voltage drop but left the EGC the minimum size for the circuit the VD would be around 40% going back to the grounding buss, and would that draw enough fault current to trip the OCP?

Or do I have no clue as to what I speak of.:grin:

 

[steve66]

Not to be critical of you in any way, but IMHO that is not a valid scientific basis for this requirement. Indeed, it is my belief that there is no valid scientific basis! I would be happy to see it removed. Consider this:

Case #1:
Run a 20 amp circuit 150 feet, using #12 wire and a #12 EGC. Postulate a fault at the load end. There will be some amount of fault current.

Case #2:
Run another 20 amp circuit 150 feet, using #10 wire and a #12 EGC. Postulate a fault at the load end. The amount of fault current will be more than in Case #1.

Case #3:
Run another 20 amp circuit 150 feet, using #10 wire and a #10 EGC. Postulate a fault at the load end. The amount of fault current will be more than in Case #1, and more than in Case #2 as well.

Discussion:
If we know that the fault current is higher in Case #2 than in Case #1, and therefore the chances of tripping the breaker are greater, why do we need to increase the fault current even more, by making the EGC bigger, as in Case #3? What makes anyone think that the fault current in Case #2 was not enough, given that the installation of Case #1 was legal, and that its fault current was enough by itself?

I don’t get it. :-?

Charlie:

I always thought the idea was to make sure the ground wire could handle the fault current without burning (maybe melting is a better word) away.

Picture a 200 amp feeder with a 3/0 phase conductor, and a #6 ground. Now assume the engineer changes the 3/0 wire size to 500 KCM. And consider that a short circuit happens between the phase and ground, and say the short happens close to the source. The 500's will provide more fault current than the 3/0 wire would. It may be enough to burn (melt) the #6 wire in half before the OCP has a chance to trip. Especially if ground fault protection is not provided, or if it has a high setpoint and a long delay.

So I don't think it is that the ground wire provides more fault current, its just able to handle the larger fault currents that the bigger phase conductors can provide.

Steve

 

[mcclary's electrical]

In that case, I calculate a voltage drop of 80%! Making the phase conductors larger will improve that situation. Making the EGC larger will improve it even more. But where is the proof that the one was not enough by itself, and that the second step is necessary?

Well gosh Charlie, I didn't mean run 14 AWG for 1500 feet!!

My point is your 150 ft example will always work and alway trip the OCPD.

For discussion, an electrician ran conductors(doesn't matter what size) ,,,for 1500 feet. Due to VD, he has upsized his conductors 3 standard sizes. But basing his EGC on your earlier theory, he left the EGC the original size. Do you still call that a safe install? See the difference?