Sizing OCPD and Feeder Wires for 125A Rated Subpanel

6 replies [Last post]
nickmcb's picture
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Joined: 2015-08-26

Good evening,

I am a Nevada County Building Inspector in CA. I had this come up twice today in consecutive inspections.

A detached garage is fed from the main service panel with a 2-pole, 40A breaker and two No. 8 copper plus neutral and ground. The panelboard in the garage has two 20A breakers for outlets and one 15A breaker for lights. The branch circuits are sized correctly. I am being told that the OCPD and feeder wires from the house to the garage must be sized to the rating of the panelboard in the garage, which is 125A. At 100 ft distance from house to garage this is approx a No. 1 copper conductor. This guy only installed a 3/4 in. conduit. Do I size to the load or the panelboard rating? And if its to the panelboard rating must it be equal to or greater to the panelboard rating or must it be less then?

Thanks in advance for your assistance.

elmundogrande's picture
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Joined: 2013-05-20

The wires are sized to the Main OCPD 40A with No. 8 wires. The only thing that might pose a problem would be voltage drop if they try to use the 30+ amps at 100 ft. The panelboard is a 125A, but it can only load it the what the main OCPD is 40A and UL recommends not to load a breaker more than 80%.

tuttle's picture
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Joined: 2015-10-10

Sorry to insert this question here but I have a similar panel board question and being a first time user cannot figure out for the life of me how use this forum. Can someone please advise me as to where I start in oder to ask my question? Thanks

tuttle's picture
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Joined: 2015-10-10

Here is my question: Is it permissible by NEC 2014 to install a 150A Main Breaker panel board fed with only No. 6 AWG? The No. 6 is fused at 40A at the Main Distribution Panel. Is this not at the very least a "neat and workman-like manner" violation?

MIkeHonz's picture
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Joined: 2015-05-21

Just because a load center or panelboard has a bus rating of 125A does not mean that it must be fed with a feeder of the same rating. Whoever is telling you that is wrong.

Frank Blas's picture
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Joined: 2013-11-12

NEC 408.36 -- The panelboard shall be protected by an overcurrent device having a rating not greater than that of a panelboard. In other words, if the panelboard is rated at 125A you cannot protect it with a 150A OCPD.

Spence2014's picture
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Joined: 2014-05-02

We are installing an emergency power distribution switchboard fed from (2) 1,250kVA generators in parallel. The switchboard is rated at 5,000A and is entirely devoted to the emergency distribution system in the building. There is a totally separate switchboard for the normal power distribution system located elsewhere in the facility. The building fire pump is fed from an 800A circuit breaker in the switchboard lineup to feed the fire pump combination controller and transfer switch. The engineer is saying we need a separate cubicle in the switchgear for the fire pump emergency power. We contend that the separate cubicle requirement of NEC Sec. 695.3(A)(1) only pertains to the Electric Utility normal power connections. Which is right?

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