GEC size for 400 amp service

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suemarkp said:
Are your main disconnects now 200A each? If so, i dont think 2/0 copper is sufficient because each set it not carrying the full dwelling load. Therefore, the conductors to each transfer switch need to be rated for the rating of that breaker (or calculated load if within roundup limits). You cant use the residential derating factor if the conductor set doesnt feed the entire dwelling. I think you need 3/0 there and that probably puts you to a 1/0 GEC without any questions.
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Yes, each transfer sw has a 200 amp breaker.
mopowr steve said:
If not mistaken, you should be able to run 1- #1/0 from within 5’ where the waterline enters the home out to the transfer switch locations and make irreversible splices of #4 from each transfer switch to the 1/0 GEC.
At least that’s the way I would do it and I believe Mike Holt also has illustrations of this method also, just involving plain old disconnects. Should not be any different for service rated transfer switches
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I like the sounds of this, Steve. I will look for the same from MH docs. Where in the code is this supported?
 
I disagree with the need for 1/0 for the grounding electrode conductor. This is one of those strange anomalies in the code. If you have a 400 amp meter main then 1/0 is needed for the grounding electrode conductor however, if you have 2- 200 amp disconnects with 3/0 copper to each then a #2 is all that is required to the ground electrode.

I had one of our past state engineers tell me that wasn't true until he did the math.
 
Dennis Alwon said:
I disagree with the need for 1/0 for the grounding electrode conductor. This is one of those strange anomalies in the code. If you have a 400 amp meter main then 1/0 is needed for the grounding electrode conductor however, if you have 2- 200 amp disconnects with 3/0 copper to each then a #2 is all that is required to the ground electrode.

I had one of our past state engineers tell me that wasn't true until he did the math.
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Dennis, I think what you are saying for your case of 2 200 amp discos, is an installation where 230.40 ex 3 is used all the way back to the service point so there are no "common" conductors? That is an interesting scenario I don't think I have considered. It seems like the only time that scenario would happen are if the service has pad mount metering at the transformer spades, or the service point is at the meter (very uncommon here but I know it's common some places).

Now that MLOs are banned (worst code change ever) we might be seeing that type of install more commonly on commercial services that have metering at the transformer.
 
jjhoward said:
To clarify: We did not install the conductors for the service. There service was pre-existing. We are installing a 24Kw standby generator at the site. We have installed 2 200 amp transfer switches near the meter pan & running the necessary conductors for the gen feed. Fortunately, there is a run of 1 1/2 PVC (empty) from the site near the meter pan, the PVC runs across the property to the basement where the 2 150 amp breaker panels are (~100'). We have re-confgured the two 150 amp panels in the basement as sub panels, the 2 200 amp transfer switches (mounted by the meter pan) are getting their feeds from the double lugs in the 400 amp meter pan. We simply re-routed the original service feeds that were from the meter pan to the basement panels to be now fed from the transfer switches. We are looking for a solid answer on how to install a properly sized GEC.
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I think you found your solid answer by the gentlemen of the group. You need to size your GEC according to T. 250.66 which is based on the largest ungrounded conductor, which is your 400 MCM. You will need a 1/0 cu. GEC. Did you leave the the existing panels alone with 150A main breakers in each and install 200A "rated" transfer switches? I think that's what the questions above are related to.
 
Dennis Alwon said:
I disagree with the need for 1/0 for the grounding electrode conductor. This is one of those strange anomalies in the code. If you have a 400 amp meter main then 1/0 is needed for the grounding electrode conductor however, if you have 2- 200 amp disconnects with 3/0 copper to each then a #2 is all that is required to the ground electrode.

I had one of our past state engineers tell me that wasn't true until he did the math.
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Interesting. The original electrician installed a 400 MCM in the riser. Does the GEC need to be sized for that conductor (being the largest) even if the service is 300 amp? I always thought if conductors are paralleled we needed to add the circular mils together and treat them as one conductor size.
 
electrofelon said:
Dennis, I think what you are saying for your case of 2 200 amp discos, is an installation where 230.40 ex 3 is used all the way back to the service point so there are no "common" conductors? That is an interesting scenario I don't think I have considered. It seems like the only time that scenario would happen are if the service has pad mount metering at the transformer spades, or the service point is at the meter (very uncommon here but I know it's common some places).

Now that MLOs are banned (worst code change ever) we might be seeing that type of install more commonly on commercial services that have metering at the transformer.
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MLO panels are banned? Where is this stated?
 
Rick 0920 said:
Interesting. The original electrician installed a 400 MCM in the riser. Does the GEC need to be sized for that conductor (being the largest) even if the service is 300 amp? I always thought if conductors are paralleled we needed to add the circular mils together and treat them as one conductor size.
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A couple points:. Note the OCPD has no bearing on the gec size. It is based on conductor size. Also note that the conductors from the meter to the panels are NOT in parallel they are two "sets"
 
Rick 0920 said:
MLO panels are banned? Where is this stated?
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I believe this is what was being referenced. We can no longer have 2-6 disconnect in one enclosure for a service. The reason is that there is no way to disconnect the busbar from being energized in a mlo panel.


(B) Two to Six Service Disconnecting Means.
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Two to six service disconnects shall be permitted for each service permitted by 230.2 or for each set of service-entrance conductors permitted by 230.40, Exception No. 1, 3, 4, or 5. The two to six service disconnecting means shall be permitted to consist of a combination of any of the following:
  • (1)
    Separate enclosures with a main service disconnecting means in each enclosure
  • (2)
    Panelboards with a main service disconnecting means in each panelboard enclosure
  • (3)
    Switchboard(s) where there is only one service disconnect in each separate vertical section where there are barriers separating each vertical section
  • (4)
    Service disconnects in switchgear or metering centers where each disconnect is located in a separate compartment
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electrofelon said:
A couple points:. Note the OCPD has no bearing on the gec size. It is based on conductor size. Also note that the conductors from the meter to the panels are NOT in parallel they are two "sets"
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Great point on the the two sets! I wasn't basing it on the OCPD, just pointing out that we are looking at a 300 A service. With a 400 MCM cu. conductor going up the riser to the service point, isn't that considered a service conductor? That's where my confusion is here. I was basing the GEC to sized for that conductor since T. 250.66 is based on largest ungrounded conductor size.
 
If the 400 mcm is larger than the 2- 3/0 coppers than we would have to use that. In this case that is correct so 1/0 is correct
 
I think we have 2 scenarios going on here. One is the op question with 2-150 amp panels and now we have 2- 200 amp panels. I am confused....
 
Rick 0920 said:
Interesting. The original electrician installed a 400 MCM in the riser. Does the GEC need to be sized for that conductor (being the largest) even if the service is 300 amp? I always thought if conductors are paralleled we needed to add the circular mils together and treat them as one conductor size.
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two runs to two panels is not paralleled. If you splice them together then tap to each panel - the common run is then paralleled though. One might do that for voltage drop reasons in some cases.
 
Rick 0920 said:
Great point on the the two sets! I wasn't basing it on the OCPD, just pointing out that we are looking at a 300 A service. With a 400 MCM cu. conductor going up the riser to the service point, isn't that considered a service conductor? That's where my confusion is here. I was basing the GEC to sized for that conductor since T. 250.66 is based on largest ungrounded conductor size.
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Yes in the case of an overhead service the riser is certainly a service conductor. I was making the point that for underground sometimes the conductors on the line side of the meter are power company conductors so in that case you would not have any common service conductors just the two sets going to each service disconnect. I think that's the scenario Dennis was talking about.
 
electrofelon said:
Yes in the case of an overhead service the riser is certainly a service conductor. I was making the point that for underground sometimes the conductors on the line side of the meter are power company conductors so in that case you would not have any common service conductors just the two sets going to each service disconnect. I think that's the scenario Dennis was talking about.
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Ahhhh! exactly what I was thinking. If we don't install the service laterals, they don't get figured in sizing our GEC, right? Now my question is this. They are not "paralleled" per say, but we have 2 sets of 3/0 cu. on the load side of our meter serving (2) 200A disconnects, do we size our GEC based on a single 3/0 which would be a #2 cu. or, do we double the cm of a 3/0 cu. conductor? It would still work out the same in this case, but could make a difference in others.
 
Rick 0920 said:
Ahhhh! exactly what I was thinking. If we don't install the service laterals, they don't get figured in sizing our GEC, right? Now my question is this. They are not "paralleled" per say, but we have 2 sets of 3/0 cu. on the load side of our meter serving (2) 200A disconnects, do we size our GEC based on a single 3/0 which would be a #2 cu. or, do we double the cm of a 3/0 cu. conductor? It would still work out the same in this case, but could make a difference in others.
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Actually one 3/0 would be a #4 but you need to double it so it would need #2
 
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