Isolated Ground bar in 400A panel...

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I am installing an isolated ground bar in a 400A 120/208V 3P 4W panel to serve isolated ground receptacles. What size isolated ground conductor do I need routed back to the MDP ground bus bar to accomodate this? where would I even find this infomation?

Thanks
 
raider1 said:
It should be sized the same as any equipment grounding conductor in accordance with 250.122.

Chris
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Sized per the MCB of the panel (400A) or on the branch circuit sizes that are feeding the receptacles (20A)?

Thanks
 
Eng said:
Sized per the MCB of the panel (400A) or on the branch circuit sizes that are feeding the receptacles (20A)?

Thanks
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The main equipment ground should be sized to the 400 A OCPD. The branch circuit would in turn be sized per 20 amp cir.
 
Dennis Alwon said:
The main equipment ground should be sized to the 400 A OCPD. The branch circuit would in turn be sized per 20 amp cir.
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Off topic. I'm talking about the isolated ground from the MDP to the 400A for connection to the isolated ground busbar. Not the equipment ground.
 
Is there any mention in the NEC of sizing a single IG to accommodate a IG ground bar serving multiple devices or circuits? So far we have one vote for making it the same size as the EGC in the panel feeder. IMO if it supplies only 20 amp circuits then it can be sized for 20 amps but I can't find a decent reference either way. :)
 
infinity said:
Is there any mention in the NEC of sizing a single IG to accommodate a IG ground bar serving multiple devices or circuits? So far we have one vote for making it the same size as the EGC in the panel feeder. IMO if it supplies only 20 amp circuits then it can be sized for 20 amps but I can't find a decent reference either way. :)
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250.146(D) has implications? It allows the IG run with circuit conductors to pass through panelboards without termination therein. Installing a IG bar will have those IG's terminating in the panelboard...and the IG run with the feeder is not run with circuit conductors. It also specifically calls the IG wire an equipment grounding conductor.
 
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infinity said:
Is there any mention in the NEC of sizing a single IG to accommodate a IG ground bar serving multiple devices or circuits? So far we have one vote for making it the same size as the EGC in the panel feeder. IMO if it supplies only 20 amp circuits then it can be sized for 20 amps but I can't find a decent reference either way. :)
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This is what my supervisor and the signing engineer is telling me. I am serving 6 separete IG 20A-120V receptacle circuits from this IG busbar.
 
Eng said:
This is what my supervisor and the signing engineer is telling me. I am serving 6 separete IG 20A-120V receptacle circuits from this IG busbar.
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The way I see it the IG only needs to be sized to the 20 amp OCPD. The EGC in the feeder needs to open a 400 amp OCPD during a fault, the IG only needs to open a 20 amp OCPD. As I stated before the code isn't really clear on how to size this IG.
 
infinity said:
The way I see it the IG only needs to be sized to the 20 amp OCPD. The EGC in the feeder needs to open a 400 amp OCPD during a fault, the IG only needs to open a 20 amp OCPD. As I stated before the code isn't really clear on how to size this IG.
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I think I agree with Trevor. The feeder egc is for the ground fault for the 400 amp but the isolated ground is really just a continuation of the #12 circuits back to the main panel.
 
Just curious, what load do these IG recptacles serve ? Do these IG recptacles provide any

added benifit or protection ? Elimination of noise ? Eng, hang out here long enough &

you will get a feeling for the general disdain of IG recptacles.
 
I think you treat it just like the EGC in the panel. If you had a 400A feeder to a panel, but only had a few 20A circuits in it, that does not allow you to run a #12 EGC in the feeder. EGCs are sized by the overcurrent device supplying the ungrounded conductors in the circuit, doesn't matter whether the ground is isolated or not.
 
infinity said:
The way I see it the IG only needs to be sized to the 20 amp OCPD. The EGC in the feeder needs to open a 400 amp OCPD during a fault, the IG only needs to open a 20 amp OCPD. As I stated before the code isn't really clear on how to size this IG.
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I would agree with you if each circuit IG ran through the panel without termination.

What if these six (6) 20A circuits were the only circuits in the panel? Still think the "feeder" IG only needs sized for 20A?
 
Dennis Alwon said:
I think I agree with Trevor. The feeder egc is for the ground fault for the 400 amp but the isolated ground is really just a continuation of the #12 circuits back to the main panel.
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As I replied to Trevor (infinity), that might be the case if you ran a #12 IG for each circuit back to the main panel.
 
suemarkp said:
I think you treat it just like the EGC in the panel. If you had a 400A feeder to a panel, but only had a few 20A circuits in it, that does not allow you to run a #12 EGC in the feeder. EGCs are sized by the overcurrent device supplying the ungrounded conductors in the circuit, doesn't matter whether the ground is isolated or not.
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In that case the EGC is protected by a 400 amp OCPD so it would need to be sized for 400 amps. In the case of a 20 amp OCPD why would you need an EGC that sized for 400 amps?


Smart $ said:
I would agree with you if each circuit IG ran through the panel without termination.

What if these six (6) 20A circuits were the only circuits in the panel? Still think the "feeder" IG only needs sized for 20A?
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We're allowed to install one EGC in a raceway for multiple circuits based on the size of the largest OCDP protecting the mulitple circuits. I see this installation the same way regarding the singular IG back to MDP. I could install those 6 branch circuits and install only one IG sized for 20 amps so I'm ot sure why installing more branch circuit IG's would change anything.
 
infinity said:
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We're allowed to install one EGC in a raceway for multiple circuits based on the size of the largest OCDP protecting the mulitple circuits. I see this installation the same way regarding the singular IG back to MDP. I could install those 6 branch circuits and install only one IG sized for 20 amps so I'm ot sure why installing more branch circuit IG's would change anything.
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Yes, when run with the circuit conductors. The IG wire going back to the main is not bering run with the circuit conductors.

Also, running one IG wire for six circuits isn't a violation... but many would think it defeats the purpose of installing IG receptacles.
 
I don't know why one would need all 6 back to the panel but if that were the case then sharing a larger conductor would not be any better if one were trying to get a true isolated receptacle. I don't believe a #12 not a #3 would be appropriate for an true isolation
 
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