Ground conductor sizing

[ohmhead]

Question table 250.122 grounding conductor required to match ampere of phase conductors. Looking at the table in 250.122 i see 100A -200A- 300A if the wire is over but not passed the next higher ampere rage which one do you use? lets say its a 1/0 alu conductor 310.15(b)(16) it calls out at 120amps at 75deg column. Its on a 100Amp breaker can the grounding conductor be a No. 6 alu awg or does it have to be a no. 4 alu awg grounding conductor. Can it be up to but not over 200amps to stay at a No. 6 awg?

 

[augie47]

It seems to me the key is in 250.122(B): Increased in size:
Where ungrounded conductors are increased in size from the minimum size that has sufficient

ampacity for the intended installation, wire type equipment grounding conductors, where installed, shall be increased in size proportionately according to the circular mil area of the ungrounded conductors


If your conductor increased in size from the conductor sufficient for the ampacity, such as in your situation, the EGC must be increased proportionally.

 

[ohmhead]

It seems to me the key is in 250.122(B): Increased in size:
Where ungrounded conductors are increased in size from the minimum size that has sufficient

ampacity for the intended installation, wire type equipment grounding conductors, where installed, shall be increased in size proportionately according to the circular mil area of the ungrounded conductors


If your conductor increased in size from the conductor sufficient for the ampacity, such as in your situation, the EGC must be increased proportionally.

So Augie even its 20amps over or 2 amps above the 100amp mark on table 250.122 I must increase the ground to a number 4#from a no.6# at 100amps .

 

[infinity]

The 100 amp Al conductor @75° c is a #1 so you have increased it in size therefore you must proportionally increase the size of the EGC.

 

[ohmhead]

The 100 amp Al conductor @75° c is a #1 so you have increased it in size therefore you must proportionally increase the size of the EGC.

Yes the conductors are 3 phase No.1 alu ABCN the ground called for by the engineer is a no. 6# alu the no.1 alu is rated at 120amps in 310.16 going to the 250 .122 it shows no.6# so in every case if the conductors are passed the ampere rating of the chart 250.122 we must increase the size if its just one ampere over .If I had 810 amp conductors I would go to the 1000 amp ground size could not go to 800amp . Yes /NO

 

[augie47]

Yes the conductors are 3 phase No.1 alu ABCN the ground called for by the engineer is a no. 6# alu the no.1 alu is rated at 120amps in 310.16 going to the 250 .122 it shows no.6# so in every case if the conductors are passed the ampere rating of the chart 250.122 we must increase the size if its just one ampere over .If I had 810 amp conductors I would go to the 1000 amp ground size could not go to 800amp . Yes /NO

No.
You don't necessarily have to increase to the next listed size. You increase proportionality.
A #1 would suffcie your yoru 100 amp OCP device and a #1/0 was used

A 1/0 is 105600 c,. a #1 is 83690 so you had an increase of 126%
Your EGC would normally be a #6 AL with a CM of 26240. Increasing it by the same % would result in a conductor with a minimum CM of 33062, which in this case would be a #4 but its' not always the next listed size.
For example, if in this case you increased your conductor to a 250kcmil, your ration would be 250,000/83690 or 300% so your #6 AL CM would now be required to be 26240 X 3 or 78,720 so your EGC would be a#1. Same 100 amp breaker.

 

[Carultch]

It seems to me the key is in[/URL] 250.122(B): Increased in size:
Where ungrounded conductors are increased in size from the minimum size that has sufficient

ampacity for the intended installation, wire type equipment grounding conductors, where installed, shall be increased in size proportionately according to the circular mil area of the ungrounded conductors


If your conductor increased in size from the conductor sufficient for the ampacity, such as in your situation, the EGC must be increased proportionally.

What becomes "the minimum size that has sufficient ampacity for the intended installation", when increasing the size of the conductors prompts a change in the number of parallel sets?

Example: a 400A circuit, normally installed with 600 kcmil Cu wire and a #3 Cu ground. Alternatively, it could be installed as parallel #3/0 Cu in separate conduits. To curtail voltage drop, it is upsized to two sets of 350 kcmil, in separate conduits.

Which calculation is correct, to determine how to upsize the #3 Cu ground?
(350 + 350)/600 *52.6 kcmil = 61.37 kcmil (#2 Cu)
Or:
(350 + 350)/(168 +168) * 52.6 kcmil = 109.58 kcmil (#2/0 Cu)

 

[ohmhead]

What becomes "the minimum size that has sufficient ampacity for the intended installation", when increasing the size of the conductors prompts a change in the number of parallel sets?

Example: a 400A circuit, normally installed with 600 kcmil Cu wire and a #3 Cu ground. Alternatively, it could be installed as parallel #3/0 Cu in separate conduits. To curtail voltage drop, it is upsized to two sets of 350 kcmil, in separate conduits.

Which calculation is correct, to determine how to upsize the #3 Cu ground?
(350 + 350)/600 *52.6 kcmil = 61.37 kcmil (#2 Cu)
Or:
(350 + 350)/(168 +168) * 52.6 kcmil = 109.58 kcmil (#2/0 Cu)

Thanks for everyone's input .

 

[ohmhead]

No.
You don't necessarily have to increase to the next listed size. You increase proportionality.
A #1 would suffcie your yoru 100 amp OCP device and a #1/0 was used

A 1/0 is 105600 c,. a #1 is 83690 so you had an increase of 126%
Your EGC would normally be a #6 AL with a CM of 26240. Increasing it by the same % would result in a conductor with a minimum CM of 33062, which in this case would be a #4 but its' not always the next listed size.
For example, if in this case you increased your conductor to a 250kcmil, your ration would be 250,000/83690 or 300% so your #6 AL CM would now be required to be 26240 X 3 or 78,720 so your EGC would be a#1. Same 100 amp breaker.

Thanks I see the calculation now this is great you been helpful Augie

 

[charlie b]

Others have answered your question. But let me speak to this:

Question table 250.122 grounding conductor required to match ampere of phase conductors.

That is not correct. The minimum EGC size is based on the rating of the overcurrent device. After we figure out what that EGC size would be, we then look to see if the ungrounded wire size was larger than the minimum ampacity required by the circuit.