200 A Subpanel downstream from 100A Main Service

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BMacky

Senior Member
Location
Foster City, CA
Would someone please reference any or all related Code articles that would justify the installation of a 200A subpanel (distribution only) downline from a 100A meter/main switch service as OK?

We're at the limit space-wise, so I as many of my peers do and suggest doing, install a 20/40 sized panel, typically a 200A sub with distribution only, to accomodate all the breakers.

The inspector I queried on this had me a bit confused and made me wonder if I was missing something. As far as I can tell, if the demand on the panel is less than the rating of the main switch, then the downline panel could be any size so long as the feeders are protected with a overcurrent device that is properly sized for the wire.

Chime in, please.

Thanks!

Bob
 
There is no problem with it providing that your load calculation will permit the 100 amp feeder.
 
You are correct.
You need a panel large enough to serve your load and can install as large as you wish beyond that.
Likewise, your feeder must be large enough to serve the load and must be protected per Art 240.
You could install a 60 amp feeder supplied from a 60 amp OCP device and supply a 600 amp panelboard if you wish.
 
from NEC 2011 I am sure it is the same for 2008 NEC

408.36 Overcurrent Protection.
In addition to the requirement of 408.30, a panelboard shall be protected by an overcurrent protective device having a rating not greater than that of the panelboard. This overcurrent protective device shall be located within or at any point on the supply side of the panelboard.

also
215.2 Minimum Rating and Size.
(A) Feeders Not More Than 600 Volts.
(1) General. Feeder conductors shall have an ampacity not less than required to supply the load as calculated in Parts III, IV, and V of Article 220.
 
I see this issue scenario often. Why somone thinks that you need to supply a 200 amp panel with 200 amps. All you need to do is properly size the feeder for the load and not to exceede the panel rating!..

We may see in the future panel buss ratings quite higher than the main in order to comply with the every increasing co-generation of power. I refer the to the 120% buss backfeed max.
 
And here is the response I received from the local building authorities, as far as my querying them on this practice:

"With your permit application please submit a single line drawing showing the proposed electrical system, with wire size, OCPD rating, derating calculations, load calculations, equipment locations and manufacturer's specification sheets for new equipment. I have attached an example of a specification sheet. Once these are reviewed and approved you will be able to provide your client with an approved job copy of the plans."

So basically I get to do all of the back office work as well, and hope they approve it. I hope I have not set a precedence for my peers in the field!!
 
I think that request should be the minimum when applying for an electrical permit.
 
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