Sunday, June 20, 2010

Breaker Sizing According to Art. 210.3

Recently in the field I have run across a few panelboards that utilized 25 Amp breakers (over Current Protective Devices - OCPD) for lighting circuits. It struck me as odd because you normally don't find anything other than 15A or 20A OCPD's for lighting. I suspected that whomever installed it was attempting to fit extra luminaries on the circuit for some reason. However - this is not an allowable installation per the NEC Article 210.3. We'll cover why this is so in a minute.

After removing the deadfront, I was at least pleased to see #10 wire being utilized for the branch circuit conductors. According to 310.16, a #10's rating is from 30 to 40 Amps, depending on insulation type and other de-rating factors. Granted, a #12 THWN or THHN would be rated 25 to 30 Amp according to the 75* & 90* columns (T310.16), but due to the so called "small conductor rule," a #12 is still restricted to a maximum of 20 Amps in ampacity. Even if if you could get around that ruled, you'd most likely hit a derating factor of 80% or more if you placed more than 3 current carrying conductors in the same raceway.

Understanding the rule in 210.3 takes a bit of referencing definitions and cross referencing a few other articles. The rule in 210.3 states "The rating for other than Individual Branch Circuits shall be 15, 20, 30, 40, and 50 amperes." <Looking at Article 100 under "Branch Circuit, Individual," we see that defines it as a BC having a SINGLE piece of utilization equipment, i.e. a single motor etc... Under "Branch Circuit, General Purpose," we find that to include most lighting and receptacle loads. There-in lies the problem with the application I talked about earlier. A 25A OCPD shall be used only for a specifically designated and designed for a  SINGLE piece of equipment, NOT for a general lighting load. The same principle applies for 35A and 45A OCPD sizes as well (note Article 240.6(A))

There are of course, exceptions to this rule, but those fall outside the scope of this blog. this article in 210.3 is an oft overlooked, albeit important rule. It also lends to uniformity and conformity in field installations, whereby a technician can immediately identify a 25A or 35A OCPD as having it as a supply for a specific piece of equipment. In any case, always remember to size your circuit by the size of the OCPD FIRST, then size BC wiring to match that, keeping in mind any aspects of voltage drop or de-rating factor considerations. Finally, insure that your OCPD size conforms to the rule in 210.3.

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