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240.4(B) Explanation?

posted Sep 21, 2012 01:50:49 by Jeff Smalley
Does this rule mean that an adjusted and corrected conductor ampacity can be less than an OCPD rating? I noticed this in Mike Holt's presentation when he was sizing his #8 wire.
"(B) Overcurrent Devices Rated 800 Amperes or Less.
The next higher standard overcurrent device rating (above
the ampacity of the conductors being protected) shall be
permitted to be used, provided all of the following conditions
are met:
(1) The conductors being protected are not part of a branch
circuit supplying more than one receptacle for cordand-
plug-connected portable loads.
(2) The ampacity of the conductors does not correspond
with the standard ampere rating of a fuse or a circuit
breaker without overload trip adjustments above its rating
(but that shall be permitted to have other trip or
rating adjustments).
(3) The next higher standard rating selected does not exceed
800 amperes."

This is contrary to this quote from HP106:
"Is the derated ampacity for the PV array conductors
greater than the rating of any overcurrent device (156
percent Isc)? [690.9]"

The 2012 NABCEP guide references this rule in one of their sample problems (pg156) but it just seems incorrect to underside a conductor.
"It is permissible to protect this conductor with a 20-amp circuit breaker according to NEC 240.4(B). A larger conductor should be considered unless the run is extremely short."
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1 reply
SarahElizabethRaymer said Jan 11, 2013 17:30:54
I beleive this question was answered in our Newsletter rather than here- Forgive that.
Mike Holt made a mistake here- the conductor definitely needs to be sized to carry a greater current than the OCPD- otherwise the OCPD wouldn't actually be able to protect the circuit.
If you have a conductor with an ampacity of 10A, and an OCPD of 15A- the conductor would fail before the OCPD could trip to protect the circuit.
If you have a conductor with an ampacity of 18A protected by a 15A OCPD, the circuit could potentially carry up to 18A, but the OCPF would trip at 15A so as to protect the system from that high current.

Much apologies for that error! Everyone makes mistakes sometimes! We will edit the Youtube video to include this correction.
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