NEC question on AC circuit breaker sizing

[Coppersmith]

(For a residential AC system) I got a call from an AC contractor I work with and I want to confirm the answer I gave him. He said an inspector was insisting that the breaker size has to match the "maximum fuse or circuit breaker" on the nameplate. I said It could be smaller matching the wire size for the "minimum circuit ampacity" (so 30 amp breaker for #10 wires). I tried to find this info in the NEC. Seems like it should be in 440, but I can't find anything related to circuit breaker sizing. Please tell me if I am correct and tell me where I can find this in the NEC so we can show the inspector. Thanks.

 

[LarryFine]

MCA determines the smallest conductor, disconnect, etc. size, to support continuous running current.

MOC determines the largest breaker or fuse (if specified) size, to allow for momentary starting current.

The breaker does not act as overload protection, which is built into the unit, only short-circuit protection.

I don't have time to pull up NEC sections right now.

 

[Dennis Alwon]

If you read these sections you can see that the overcurrent protective device can be larger than the circuit conductor because the ac unit has overload protection built in as Larry stated. The function of the overcurrent protective device is for ground fault and short circuit only.

440.22 Application and Selection.
(A) Rating or Setting for Individual Motor-Compressor. The
motor-compressor branch-circuit short-circuit and ground-fault
protective device shall be capable of carrying the starting
current of the motor. A protective device having a rating or
setting not exceeding 175 percent of the motor-compressor
rated-load current or branch-circuit selection current, whichever
is greater, shall be permitted, provided that, where the
protection specified is not sufficient for the starting current of
the motor, the rating or setting shall be permitted to be
increased but shall not exceed 225 percent of the motor rated load
current or branch-circuit selection current, whichever is
greater.
Exception: The rating of the branch-circuit short-circuit and ground fault
protective device shall not be required to be less than 15 amperes.

Part IV. Branch-Circuit Conductors
440.31 General. The provisions of Part IV and Article 310
specify ampacities of conductors required to carry the motor
current without overheating under the conditions specified,
except as modified in 440.6(A), Exception No. 1.
The provisions of these articles shall not apply to integral
conductors of motors, to motor controllers and the like, or to
conductors that form an integral part of approved equipment.
440.32 Single Motor-Compressor. Branch-circuit conductors
supplying a single motor-compressor shall have an ampacity
not less than 125 percent of either the motor-compressor rated load
current or the branch-circuit selection current, whichever
is greater.

 

[ramsy]

Inspectors may look to listing requirements of equipment, before becoming mired in code-specific calculations or tables.

In this case, the inspector is looking at Nameplate data from 440.4(B).

"the maximum rating of the branch-circuit short-circuit and ground-fault protective device"

The Nameplate may show Minimum and Maximum fuse, but the inspector is trained like a fundamentalist Bible thumper, and only their adopted version of section 440.4 can be the code-Bible verse, which must be literally interpreted as "Maximum Rating", not Minimum.

More industry experienced inspectors may adopt a more historical-critical context of code-Bible interpretation, and recognize Nameplate sources for "Minimum" fuses.

However, in the mean time the inspector's inspiration can only come from God, another ordained priest, or authorities having jurisdiction, such engineered plans.

Contractors are not generally considered such authorities, they are more akin to infidels expected to piss & moan to get their way.

 

[Jraef]

There is no "minimum" OCPD per code, there is only a maximum. Minimum COULD be smaller than the load itself so far as the code is concerned. It would not allow the unit to work, but it would not start a fire if it is always in a state of being tripped...

 

[wwhitney]

There is no "minimum" OCPD per code, there is only a maximum. Minimum COULD be smaller than the load itself so far as the code is concerned.

What about 210.20 and 215.3?

Cheers, Wayne