MINIMUM CIRCUIT AMPACITY

[fifty60]

When a refrigeration compressor states the max fuse size and a minimum circuit ampacity, does this mean that for the maximum fuse size you can use the stated minimum circuit ampacity?

For example, a max fuse size of 15A and a minimum circuit ampacity of 9.7A. would I be able to use 16AWG wire (ampacity 14A). It is a 120V circuit..

 

[iwire]

When a refrigeration compressor states the max fuse size and a minimum circuit ampacity, does this mean that for the maximum fuse size you can use the stated minimum circuit ampacity?

For example, a max fuse size of 15A and a minimum circuit ampacity of 9.7A. would I be able to use 16AWG wire (ampacity 14A). It is a 120V circuit..

16 AWG is not an option, 14 AWG with 15 amp fuse would be acceptable.

 

[fifty60]

Understood. A different example. If the Max Fuse Size is 30A, and the minimum circuit ampacity is 19A, would I be able to use a 30A fuse and #12AWG wire?

Does the MCA coincide with the max fuse size, or should they be taken separately...

 

[iwire]

Understood. A different example. If the Max Fuse Size is 30A, and the minimum circuit ampacity is 19A, would I be able to use a 30A fuse and #12AWG wire?

Does the MCA coincide with the max fuse size, or should they be taken separately...

You can wire and select the fuse size per the MCA.

But what you are allowed to do and what you should do are often different things.

My personal preference is to always use the max breaker or fuse size allowed and to select a conductor size based on the MCA or larger.

 

[Ponchik]

Understood. A different example. If the Max Fuse Size is 30A, and the minimum circuit ampacity is 19A, would I be able to use a 30A fuse and #12AWG wire?

Does the MCA coincide with the max fuse size, or should they be taken separately...

Based on 2008 and your given example, IMO you can use 30A OCPD and #14 THHN/THWN for your circuit conductors.

 

[kwired]

Understood. A different example. If the Max Fuse Size is 30A, and the minimum circuit ampacity is 19A, would I be able to use a 30A fuse and #12AWG wire?

Does the MCA coincide with the max fuse size, or should they be taken separately...

Yes - it is done all the time. Your first example would be acceptable per art 430 or 440, but 210.19(A)(4) limits us to 14 AWG minimum for most applications.

With motors (and hermetic compressors - which art 440 basically sends you to 430 for those motors anyway) you only need a conductor capable of handling full load for ampacity (times 125% in most cases for continuous duty reasons) but you typically have two forms of overcurrent protection - the branch circuit device is usually for short circuit and ground fault protection and the motor overload protection also doubles as conductor overload protection. Each type of protection has a different trip curve - we don't want the faster trip curve typical for circuit breakers or even a time delay fuse for overload protection - or we will have a hard time starting the motor without tripping the overload, the overload has slower response time and is intended to heat up at a rate more similar to how the motor would heat up when under excess load conditions.

 

[rt66electric]

wire size max breaker size minimum

wire size max breaker size minimum

Does the customer want the bare cheepo minimum??

Or does he want a good job that last a long time.... is $20 in parts worth worrying about??

From experience And repairing many a cheepo-done job.

The A/C guys swear by running the wire size at at the MAX and install the breaker at the minimum. If a problem occurs in the unit, the the breaker will trip first and, possibly saving the equipment.

the industrial rule of thumb is... run the wire at 125% of the load and max design usage of 80%

heat is the Killer in wiring terminations, larger wire sizes allow for more cooling at stress points.

 

[iwire]

The A/C guys swear by running the wire size at at the MAX and install the breaker at the minimum. If a problem occurs in the unit, the the breaker will trip first and, possibly saving the equipment.

I have never heard that and doubt that using an overcurrent device listed for the unit will harm the unit.

I think it is much more likely the minimum sized breaker will have nuisance tripping issues.

 

[electricalist]

On the houses i worked on atleast around here we would have to breaker the ac. At the max fuse rating to pass inspection and not that it matters but we ran 6-2 to every unit. Too often ac guys would set diff unit at diff location.

 

[Ponchik]

Does the customer want the bare cheepo minimum??

Or does he want a good job that last a long time.... is $20 in parts worth worrying about??

From experience And repairing many a cheepo-done job.

The A/C guys swear by running the wire size at at the MAX and install the breaker at the minimum. If a problem occurs in the unit, the the breaker will trip first and, possibly saving the equipment.

the industrial rule of thumb is... run the wire at 125% of the load and max design usage of 80%

heat is the Killer in wiring terminations, larger wire sizes allow for more cooling at stress points.

If VD is not an issue I think 14AWG will work fine just as well as #10 will.

On the houses i worked on at least around here we would have to breaker the ac. At the max fuse rating to pass inspection and not that it matters but we ran 6-2 to every unit. Too often ac guys would set diff unit at diff location.

I have never installed #6 to any residential A/C unit, not even to the one that has a MAX breaker of 50Amps. I believe that is a waste of money and resources. Even if it $5 more it is still waste of money.

IMO, it has nothing to do with cheap work or quality work. If there is no voltage drop issue, the minimum conductor size will work fine and the over current device will work just as quick if the circuit is based on the minimum circuit ampacity of the name plate.

 

[electricalist]

I agree it seemed like a waste,but vd might have been part of the reason. Most of the houses were fairly large.

 

[JoeStillman]

The reason you can run wiring with a lower ampacity than the breaker size is because the breaker is only for short-circuit and ground-fault protection. The wire is protected from overload by the motor starter overloads in the condensing units.

I get asked this all the time and the inspectors always let us follow the nameplate.