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Max Breaker Size and Double Pole Breaker


O
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Imagine this HVAC Manufacturer Info:

Max Breaker Size: 50 Amps

Min Circuit Ampacity: 32.6 Amps

Run Load Amps: 22.5

In the panel, a double-pole 30 Amp breaker, tied.

Is this breaker in the panel sufficient?

It seems to me that I can only assume that there is an even distribution of power between the two conductors. In theory, the circuit could carry up to 29 amps on each line, which would put total amps to 58 (above the max), without tripping the breaker. Should an inspector call this out as a possible issue, pending manufacturer's install specs?

I keep asking inspectors who offer me different answers, so I am trusting TIJ to break the stalemate ... what do you say? And thanks.

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First of all, you can't double the rating of a 2 pole breaker. A 30 amp rating is for the circuit, not each leg.

Minimum circuit ampacity means that all the circuit components (wiring and breaker) must be rated to meet or exceed the minimum current draw specified. If you have a 30 amp breaker, you don't meet the minimum specified ampacity. Assuming the wiring is # 8 or larger, you need only install a larger breaker.

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Originally posted by O

Imagine this HVAC Manufacturer Info:

Max Breaker Size: 50 Amps

Min Circuit Ampacity: 32.6 Amps

Run Load Amps: 22.5

In the panel, a double-pole 30 Amp breaker, tied.

Is this breaker in the panel sufficient?

Not per the data plate. The breaker should be at least 35 amps but not more than 50 amps. In reality, the 30-amp breaker might work fine, especially if the unit is relatively new.

It seems to me that I can only assume that there is an even distribution of power between the two conductors. In theory, the circuit could carry up to 29 amps on each line, which would put total amps to 58 (above the max), without tripping the breaker.

As Kevin explained, it doesn't work like that. In a 30-amp, 240-volt circuit, the power flows back & forth between the two legs. The power is always evenly distributed. Imagine a water pump and a long garden hose. Attach one end of the hose to the pump inlet and the other to the pump outlet. Now imagine that the pump can change directions 60 times a second. That's a good image to help understand how a 240-volt circuit in your house works.

Next, breakers don't necessarily trip right at their designated amp rating. Each breaker's time/current trip curve determines how quickly it reacts to current above its designated rating. For instance, a 30 amp breaker should never trip if the current running through it is 30 amps or less. A little more current, say 35 amps, might cause it to trip after an hour or so while a lot more current, say 100 amps might cause it to trip in 10-15 seconds. A gross overload, such as a short circuit, would activate the breaker's secondary trip mechanism and would cause it to trip nearly instantly. Different manufacturers have breakers with different trip curves. HACR breakers -- most breakers these days -- have more forgiving trip curves, similar to slo-blow fuses.

The 30-amp breaker that you saw wasn't tripping because, most of the time, the current that the compressor was drawing was well under 30 amps. On start-up, the current no doubt surged well over 30-amps but for such a short time that it didn't activate the breaker's trip mechanism.

Over time, this situation might change. A dirty coil or a clogged filter or an old compressor might cause the system to draw well over 30 amps for a sustained period and then the breaker would trip. People would percieve this as nuisance tripping.

Last, remember that the breaker on an AC (or heat pump) circuit isn't intended to provide overcurrent protection. It's only there to guard against shorts and ground faults. The compressor motor has its own built-in overcurrent protection. That's why it's so common to see large breakers feeding small wires on HVAC systems.

Should an inspector call this out as a possible issue, pending manufacturer's install specs?

It's up to the inspector. The conservative call would be to recommend changing the breaker to one that complies with the data plate -- that would mean a 35-, 40-, 45- or 50-amp breaker.

I keep asking inspectors who offer me different answers, so I am trusting TIJ to break the stalemate ... what do you say? And thanks.

Personally, I'd recommend installing a larger breaker to comply with the manufacturer's spec. However, I wouldn't argue with another inspector who didn't make that call. If the system is working and there aren't any nuisance trips, I really can't see any harm in leaving it alone. Especially considering that, if it were needed in the future, a new $40-amp breaker might cost $10.

- Jim Katen, Oregon

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