Breaker Sizing and HVAC MCA

[ericwg]

Even though I think Jraef already nailed this one and I have joined this post way late, I wanted to add something I hadn't seen pointed out thus far. There have been a couple of people that referenced 220.18 as justification/reference that the NEC says a circuit includes the conductor and the OCPD. That is not true. 220.18 states that "the total load shall not exceed the rating of the branch circuit, and it shall not exceed the maximum loads specified in 220.18(A) through (C) under the conditions specified therein." That section focuses on the rating of the "branch circuit". So, let's now go back to Article 100 and read the definition of a branch circuit - "The circuit conductors between the final over-current device protecting the circuit and the outlet(s)." It specifically says where the conductors are...BETWEEN the OCPD and outlet. It does not INCLUDE the OCPD. It is ONLY the conductors. Since it is probably more applicable, we should also review the definition of a multiwire branch circuit. "A branch circuit that consists of two or more ungrounded conductors that have a voltage between them, and a grounded conductor that has equal voltage between it and each ungrounded conductor of the circuit and this is connected to the neutral or grounded conductor of the system." Again, the only component inclusive of the definition is conductors. No OCPD was even mentioned in the multiwire branch circuit definition. Even though Jraef and others addressed this, to the question can you have an MCA of 36.9A and an MOCP of 35A? Absolutely. As someone else pointed out...as long as your conductors are sized for the 36.9A you could legally install a 15A breaker on that circuit if you wanted. Would you be inviting nuisance tripping? Yes. Is it logical and well thought out? No. However, the point is that the prescribed minimum (ampacity of the conductors) was met and the maximum (OCPD rating) was not exceeded. The criteria for code legality has been met.