Equipment Ground to an MCC ?

dduffee260 · Sep 20, 2005

We have a Motor Control Center which we are running 2 sets of 350 Kcmil XHHW aluminum for service from a 400 amp disconnect. The service is 480 volt 3 phase 3 wire. We are going to install 1- 4/0 aluminum XHHW ground wire in one conduit for the service. The other conduit will just have 3 350 Kcmil wires for the conductors. Do we need 1-4/0 wire for each conduit or can we get by with just one wire in one conduit?

ryan_618 · Sep 20, 2005

Re: Equipment Ground to an MCC ?

What kind of raceway is it?

iwire · Sep 20, 2005

Re: Equipment Ground to an MCC ?

250.122(F) requires a full size ground in each raceway.

If this is a 400 amp feeder Table 250.122 shows that you will need at least a 1 AWG AL.

I say at least as 250.122(B) may require you to up size the EGC.

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 circular mil area of the ungrounded conductors.

You say you are running two sets of 350 ALs when 2 sets of 250 AL would do.

[ September 20, 2005, 06:20 PM: Message edited by: iwire ]

paul · Sep 20, 2005

Re: Equipment Ground to an MCC ?

YES

250.122(F)Conductors in Parallel. Where conductors are run in parallel in multiple raceways or cables as permitted in 310.4, the equipment grounding conductors, where used, shall be run in parallel in each raceway or cable.

dduffee260 · Sep 20, 2005

Re: Equipment Ground to an MCC ?

You say you are running two sets of 350 ALs when 2 sets of 250 AL would do.

Bob, this is a situation where a customer was going to let their people do this work, then they decided that it would be best to let us install the electrical equipment. We turned down the job twice because we were too busy at the time, they agreed to give us ample time to do this. They already had the wire purchased and on site. We have since bought everything else. By the way, we are running the 350 aluminum in 3" PVC conduit underground about 300 foot. I know the code says to run in parallel but since your equipment is 400 amps would the set of parallel wires have to be good for 400 amps of grounding?

iwire · Sep 21, 2005

Re: Equipment Ground to an MCC ?

Originally posted by dduffee260:
I know the code says to run in parallel but since your equipment is 400 amps would the set of parallel wires have to be good for 400 amps of grounding?

I am not sure of the question here.

In regards to 250.122(F)

If you run a 400 amp feeder in one PVC raceway you will need one - 1 AWG AL as an EGC.

If you run this same 400 amp feeder in two PVC raceways you still need a one - 1AWG AL in each raceway.

Now we have to think about 250.122(B)

250.122(B) requires you to up size the EGC due to the fact you are running 500 amps worth of conductor for a 400 amp feeder.

And that increased cEGC will need to be installed in each raceway.

charlie b · Sep 21, 2005

Re: Equipment Ground to an MCC ?

I make it that you need to run 2/0 AL EGC in each raceway. Since you planned to run 4/0, you meet the minimum requirements.

The math:

Minimum for 400 amps = two sets of 250 AL

You are running two sets of 350 AL

350 / 250 = 1.4 (i.e., a 40% increase in size)

Minimum EGC is #1 AL (83,690 circular mils)

1.4 times 83690 = 117,166 circular mils

The smallest cable that is at least 117,166 cm is #2/0

tom baker · Sep 21, 2005

Re: Equipment Ground to an MCC ?

There is a big difference between a grounded neutral size and a feeder equipment grounding conductor size, for conductors in parallel. For a feeder the EGC is full size in each raceway. For a service its only based on the size of the ungrounded conductors in each raceway...

Equipment Ground to an MCC ?

dduffee260

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ryan_618

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iwire

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paul

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dduffee260

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iwire

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charlie b

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tom baker

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