100A Subpanel fed from 40A breaker on 100A main : raise any issues?

[brycenesbitt]

I'm looking for a quick primer on any rules or gotchas with derating sub-panels.

In this particular case it is an existing 100A main with two 40A breakers leading
to subpanels. One of the subpanels it itself a 100A panel fed by three #4 wires
plus ground, in about 30 feet of 1.5" flexible metal conduit.

Does the derated subpanel cause any concern?

 

[charlie b]

You have not derated anything here. You are simply not taking advantage of an items's full rated capacity. The only relevant rule is that the panel has to be protected against overcurrent, with the OCPD set no higher than its rating (408.36). A 40 amp breaker can certainly protect a panel rated at 100 amps. The only concern I would have is the possibility that the calculated load (you did calculate the load, right?) on one of the subpanels might exceed 40 amps. That would get you into trouble with 215.2(A)(1).

 

[lakee911]

Breakers aren't coordinated, but that's okay. Feeder breaker trips before subpanel main.

 

[GoldDigger]

Breakers aren't coordinated, but that's okay. Feeder breaker trips before subpanel main.

And anyone who cannot later figure out why the subpanel cannot support 100A worth of load deserves the grief he gets.

 

[MasterTheNEC]

Breakers aren't coordinated, but that's okay. Feeder breaker trips before subpanel main.

Not required to be coordinated...but I agree with GoldDigger....he who forgets to calculate reaps the perils....

 

[jaggedben]

Breakers aren't coordinated, but that's okay. Feeder breaker trips before subpanel main.

It wasn't even stated that the sub has a main breaker. Could be MLO.