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Old 07-21-2014, 06:29 AM   #76
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Max HVAC Breaker Size


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Originally Posted by Jump-start View Post



I agree. Seen it first hand. The motor overload is supposed to protect the wiring. And Im sure that's what the engineers have in mind when designing them; but more than once I have seen and certainty heard the same from others that the breaker trips before an overload opens. Certainly not in all cases but its definitely not a rarity either.












Case in point: Had an AC unit once with a condenser coil packed full of debris. Liquid line (small pipe) leaving the unit toward the air handler was so hot I swear you could've fried eggs with it. By all means the compressor was being overworked. Breaker would keep tripping half hour. Unit had a minimum circuit ampacity around 12 amps #14 on a 20 amp breaker as per max breaker size. The unit started normal but as it ran current draw ran up to 25 amps and kept steadily rising until the breaker tripped around 30 amps. No overload ever tripped.

With smaller units, the circuit breaker and the thermal motor protection are going to be very close to the same trip curve, so its not surprising if the circuit breakers thermal protection opens before the motors... but, when you get to much larger motors, this simply isn't the case.


Here are the acceptable trip curve allowances.... This of course is done with a single breaker at 40o Celsius, this is why the 80% rule exists.

200%, 0-30A, 2 minutes
200%, 31-50A, 4 minutes
135%, 0-50A, within one hour
100%, never trip (test until temperature stabilizes)

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Old 07-21-2014, 12:03 PM   #77
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Max HVAC Breaker Size


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Originally Posted by stickboy1375 View Post
With smaller units, the circuit breaker and the thermal motor protection are going to be very close to the same trip curve, so its not surprising if the circuit breakers thermal protection opens before the motors... but, when you get to much larger motors, this simply isn't the case.


Here are the acceptable trip curve allowances.... This of course is done with a single breaker at 40o Celsius, this is why the 80% rule exists.

200%, 0-30A, 2 minutes
200%, 31-50A, 4 minutes
135%, 0-50A, within one hour
100%, never trip (test until temperature stabilizes)

Still IMO the overload should have cut out. The 20 amp breaker certainly did trip within the correct time frame (135% of 20 is 27 amps) Holding 27 amps for 30 minutes is acceptable for this breaker. However, since the minimum circuit amps dictates 14 guage wire a trip curve similar to a 15 amp breaker should be seen in the overload. 135% of 15 is 20.25. Actual amperage was 25 and rising which is well over 135%, closer to 200%. A standard 15 amp breaker takes about 3 minutes to trip on a 150% overload worst case 10 minutes. A 180% overload should be no more than 5 minutes. No standard 15 amp breaker should hold 27 amps for 30 minutes. If the overload was subjected to a UL time current curve test it would fail.

Now if the minimum circuit amps was 17 (#12 wire) for this unit and the 20amp breaker tripped first I could understand time current curves being close that one, the other or both might trip.
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Old 07-21-2014, 04:56 PM   #78
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Still IMO the overload should have cut out. The 20 amp breaker certainly did trip within the correct time frame (135% of 20 is 27 amps) Holding 27 amps for 30 minutes is acceptable for this breaker. However, since the minimum circuit amps dictates 14 guage wire a trip curve similar to a 15 amp breaker should be seen in the overload. 135% of 15 is 20.25. Actual amperage was 25 and rising which is well over 135%, closer to 200%. A standard 15 amp breaker takes about 3 minutes to trip on a 150% overload worst case 10 minutes. A 180% overload should be no more than 5 minutes. No standard 15 amp breaker should hold 27 amps for 30 minutes. If the overload was subjected to a UL time current curve test it would fail.

Now if the minimum circuit amps was 17 (#12 wire) for this unit and the 20amp breaker tripped first I could understand time current curves being close that one, the other or both might trip.

The above is only true if the circuit breaker is in a 40o Celsius environment.

The issue is that the breaker isn't supplying the overload protection, the motor is, so who knows what that trip curve really is, but in all honesty, I still wouldn't lose sleep over it because who knows what any insulation for any conductor is actually rated for, but I can guarantee that there is a huge safety fudge factor built in between the NEC and UL....

FYI, #14 awg is good for 20 amps when dealing with AC units.

I'm also curious about your scenarios, were the other circuit breakers around the AC breaker overloaded, causing extra heat to the AC circuit breaker? Where were most of the panels located, hot or cool areas? Were the MAXIMUM OCPD used with all the AC installs? See how many variables can change the outcome of every install?

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Old 07-22-2014, 08:33 AM   #79
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The above is only true if the circuit breaker is in a 40o Celsius environment.

The issue is that the breaker isn't supplying the overload protection, the motor is, so who knows what that trip curve really is, but in all honesty, I still wouldn't lose sleep over it because who knows what any insulation for any conductor is actually rated for, but I can guarantee that there is a huge safety fudge factor built in between the NEC and UL....

FYI, #14 awg is good for 20 amps when dealing with AC units.

I'm also curious about your scenarios, were the other circuit breakers around the AC breaker overloaded, causing extra heat to the AC circuit breaker? Where were most of the panels located, hot or cool areas? Were the MAXIMUM OCPD used with all the AC installs? See how many variables can change the outcome of every install?

True, but keep in mind all newer breakers are 'ambient compensated' so its not that much of a deviation. Temperature sure does have an effect, but deviations in excess are restricted.


I do agree with you in most conditions the amperage of a wire is much higher. Of interesting note is that the IEC uses an equation based method for sizing wires. Wires stapled up on an open wall can go to double the current as apposed to them imbedded within an insulated wall. NEC wire tables assume the worst scenario and apply it to all scenarios. Rumor Ive heard regarding the huge fudge factor being more about voltage drop rather than temperature limitations. In countries that use 230/400 volts wires as a whole in general have a greater current rating than NEC applications. And certainly you could get away with it. A #14 NM cable can hold 22 amps for ever when run across none insulated rafters in a basement ceiling. But, on a 150 ft run, 120 volts will produce a much greater voltage drop over 230 volts, so ignoring application while automatically bumping to #12 (actually #10 if a 22 amp OCPD existed) will take care of voltage drop. Not something ever mentioned, but somewhere in the whole concocting of wire gauges voltage drop has to have been of influence.


None of the other breakers were overloaded that I know. Most of the panels where in basements or average temperature rooms. Granted a none working AC will cause higher temps, but nothing above 100 degrees. I don't remember the min max on all the units, but Id say in the middle with most wires sized to the breaker rather than minimal circuit ampacity as is the custom around here. But definitely MCA one all these units called for smaller wire. I have seen units that would have allowed a breaker of 2 sizes up over the MCA. 30amp MCA calling for a 50amp breaker.
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Old 07-22-2014, 05:11 PM   #80
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None of the other breakers were overloaded that I know. Most of the panels where in basements or average temperature rooms. Granted a none working AC will cause higher temps, but nothing above 100 degrees. I don't remember the min max on all the units, but Id say in the middle with most wires sized to the breaker rather than minimal circuit ampacity as is the custom around here. But definitely MCA one all these units called for smaller wire. I have seen units that would have allowed a breaker of 2 sizes up over the MCA. 30amp MCA calling for a 50amp breaker.
I never run a cable sized to the MAXIMUM breaker, that is just a waste of someone else's money, I generally pick the smallest wire allowed by the nameplate, lets face it, even that number has already been bumped up 125%, and I always install the maximum circuit breaker to avoid future service calls when the units get older and has a harder time starting.

IMO, a lot of residential electricians just don't understand the NEC and don't even know you can ignore the asterisk in table 310.16, people freak out when they see a much larger circuit breaker on a smaller sized wire...

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Old 07-22-2014, 05:19 PM   #81
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I never run a cable sized to the MAXIMUM breaker, that is just a waste of someone else's money, I generally pick the smallest wire allowed by the nameplate, lets face it, even that number has already been bumped up 125%, and I always install the maximum circuit breaker to avoid future service calls when the units get older and has a harder time starting.

IMO, a lot of residential electricians just don't understand the NEC and don't even know you can ignore the asterisk in table 310.16, people freak out when they see a much larger circuit breaker on a smaller sized wire...
I have to agree with this. Code allows for this type of setup but a lot of sparkies or inspectors just don't know better. Ive heard of HIs and inspectors failing this

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