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12 gauge multi-stranded wire and 40 amp breaker


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I have not found a maximum ampacity for multi-stranded wire but know that it can carry more than single strand. This HVAC unit had a 40 amp breaker and a 12 gauge multi-strand copper wire. Wire from unit to breaker panel was only a few feet. I think this may be ok but would like to find a chart for ampacity of multi-strand. Anyone?

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I have not found a maximum ampacity for multi-stranded wire but know that it can carry more than single strand. This HVAC unit had a 40 amp breaker and a 12 gauge multi-strand copper wire. Wire from unit to breaker panel was only a few feet. I think this may be ok but would like to find a chart for ampacity of multi-strand. Anyone?

The NEC doesn't give stranded wire and greater ampacity than solid wire. The type of insulation on the wire makes a difference though, in terms of its temperature rating and will affect the ampacity. Just use table 310.16 as you would for solid wire.

Remember that, in an air conditioning circuit, the breaker isn't there to provide overcurrent protection. The AC data plate will list the minimum necessary ampacity of the wire and the maximum breaker or fuse size. It might be fine to have a circuit with a #12 wire and a 40-amp breaker.

There are two key questions:

What does the data plate require?

What kind of stranded wire was it, and what is its temperature rating?

- Jim Katen, Oregon

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Remember that, in an air conditioning circuit, the breaker isn't there to provide overcurrent protection.

- Jim Katen, Oregon

Jim,

Can you explain that? What would happen if the data plate listed a max fuse rating of 25 AMPS and they had a 40 AMP breaker (see this all of the time on older houses that have had systems replaced). I would think this could cause a problem but maybe I am missing something. I always give them 5 AMPS for compressor load (start up) draw and that is commonly done around here.

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Jim,

Can you explain that? What would happen if the data plate listed a max fuse rating of 25 AMPS and they had a 40 AMP breaker (see this all of the time on older houses that have had systems replaced). I would think this could cause a problem but maybe I am missing something. I always give them 5 AMPS for compressor load (start up) draw and that is commonly done around here.

Forget the 5-amp thing. It's meaningless. Just go by the data plate. With an air conditioning compressor -- as with lots of large motor loads -- the compressor motor itself has overload protection built in. That'll protect the wire from overloads. The required breaker is only there to protect against short circuits & ground faults.

Why then, you may ask, does the size of the breaker matter? If it's only there to protect against faults, wouldn't any larger breaker size do? And if you were to ask those questions, they'd be good ones. The answer is that smaller breakers have steeper trip curves, even for faults, so they provide slightly (ever so slightly) better protection.

- Jim Katen, Oregon

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Just follow the requirements noted on the data plate. If it says that the MAX Fuse/Breaker size is: 40-amps ... Then that is what you need to see in the panelboard.

I just went through an 8-hour CE session on electrical this past Friday and the electrical consultant was adamant about following the rules. He more than once commented that you need to follow the data plate per the manufacturer's requirements.

He noted that most electrical guys do things in a certain way as "that's the way we've always done it" ... does not make it the "right" way.

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Jim,

Can you explain that? What would happen if the data plate listed a max fuse rating of 25 AMPS and they had a 40 AMP breaker (see this all of the time on older houses that have had systems replaced). I would think this could cause a problem but maybe I am missing something. I always give them 5 AMPS for compressor load (start up) draw and that is commonly done around here.

Forget the 5-amp thing. It's meaningless. Just go by the data plate. With an air conditioning compressor -- as with lots of large motor loads -- the compressor motor itself has overload protection built in. That'll protect the wire from overloads. The required breaker is only there to protect against short circuits & ground faults.

Why then, you may ask, does the size of the breaker matter? If it's only there to protect against faults, wouldn't any larger breaker size do? And if you were to ask those questions, they'd be good ones. The answer is that smaller breakers have steeper trip curves, even for faults, so they provide slightly (ever so slightly) better protection.

- Jim Katen, Oregon

Thank you Jim!

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Jim,

Can you explain that? What would happen if the data plate listed a max fuse rating of 25 AMPS and they had a 40 AMP breaker (see this all of the time on older houses that have had systems replaced). I would think this could cause a problem but maybe I am missing something. I always give them 5 AMPS for compressor load (start up) draw and that is commonly done around here.

Forget the 5-amp thing. It's meaningless. Just go by the data plate. With an air conditioning compressor -- as with lots of large motor loads -- the compressor motor itself has overload protection built in. That'll protect the wire from overloads. The required breaker is only there to protect against short circuits & ground faults.

Why then, you may ask, does the size of the breaker matter? If it's only there to protect against faults, wouldn't any larger breaker size do? And if you were to ask those questions, they'd be good ones. The answer is that smaller breakers have steeper trip curves, even for faults, so they provide slightly (ever so slightly) better protection.

- Jim Katen, Oregon

Motor wiring is supposed to be sized to 125% of rated load,10gauge wire is actually a little less than required but most inspectors would accept it.

As far as the breaker size if it is just used as a disconect in the box outside by the unit it doesnt really matter if its oversized,the breaker feeding the circuit in the panel is what matters.

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