EGC Sizing

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Muneepit

Muneepit
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Houston
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Electrician
Hi brainiacs,

Need some clarification.

I have a 2000 amp 277/480v service to an office building. The 2000 amp transfer switch failed and we are installing a new one. I am going to use the existing transfer switch cabinet as a J-box. I am running 5 parallel 3" conduits for the utility power and 5 parallel 3" conduits for the load side of the transfer switch. Each conduit is going to have 4 - 500 kCmil conductors. What would be the EGC size?

Table 250.122 says 250 kCmil based on the size of the O.C. device.

250.122 (F) (1) (b) says to use table 250.122 for multiple raceways for feeders or branch circuits. Do I not use this table for the main service?

If I do use this table, the last sentence of 250.122(F)(1)(b) states: Metal raceways or auxiliary gutters in accordance with 250.118..........shall be permitted as the equipment grounding conductor.

250.118 (4) Says EMT can be used as an EGC.

Does this mean that I do not need to install a separate EGC in the conduit?
 
I'm a little confused where is the service disconnect? Also 5 sets of 500 kcmil is only good for 1900 amps. 380*5=1900
 
Muneepit said:
Hi brainiacs,

Need some clarification.

I have a 2000 amp 277/480v service to an office building. The 2000 amp transfer switch failed and we are installing a new one. I am going to use the existing transfer switch cabinet as a J-box. I am running 5 parallel 3" conduits for the utility power and 5 parallel 3" conduits for the load side of the transfer switch. Each conduit is going to have 4 - 500 kCmil conductors. What would be the EGC size?

Table 250.122 says 250 kCmil based on the size of the O.C. device.

250.122 (F) (1) (b) says to use table 250.122 for multiple raceways for feeders or branch circuits. Do I not use this table for the main service?
Click to expand...

You use table 240.102 where the conductors are ahead of the main service OCPD. It is unclear from your post where this applies to your installation.


If I do use this table, the last sentence of 250.122(F)(1)(b) states: Metal raceways or auxiliary gutters in accordance with 250.118..........shall be permitted as the equipment grounding conductor.

250.118 (4) Says EMT can be used as an EGC.

Does this mean that I do not need to install a separate EGC in the conduit?
Click to expand...

As long as the EMT and associated fittings bond everything together properly, then ... Yes. The 'as long as' part might be more involved than it sounds.

You might want to review in particular 250.97, and 250.92 which it refers back to.
 
infinity said:
I'm a little confused where is the service disconnect? Also 5 sets of 500 kcmil is only good for 1900 amps. 380*5=1900
Click to expand...

I agree with your assessment of the 1900 amps. I am working with an existing service.

To clarify, I am coming off of the load side of the Main Service Breaker to the Utility side of the transfer switch.
Load side of the transfer switch to the building.
Generator side of the transfer switch to the generators.

The existing size of the EGC is 3/0. Which looks like the original installer might have used 250.66.
 
Yes EMT can be used as the EGC for your 2000 amp feeder. If you chose to use a wire type EGC then it is sized with Table 250.122 based on the size of the OCPD which as you've stated is 250 kcmil copper for 2000 amps in each parallel raceway.
 
infinity said:
Yes EMT can be used as the EGC for your 2000 amp feeder. If you chose to use a wire type EGC then it is sized with Table 250.122 based on the size of the OCPD which as you've stated is 250 kcmil copper for 2000 amps in each parallel raceway.
Click to expand...

Ok. Thank you for the clarification. You have been a great help.
 
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