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Old 06-19-2008, 09:19 PM   #1
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A/C Current Question


Have a 40 amp 240v service feeding an a/c unit that is tripping the breaker. The plate on the unit says it is supposed to draw 30amps total. Checked the 2 feeds to the unit to see what it was pulling. Each leg was pulling 20amps. My question is.... do you combine the amps of each leg to get the total current?

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Old 06-19-2008, 10:28 PM   #2
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No. each half of a 40 amp breaker is designed to limit current to 40 amps.

If you have a unit drawing 20 amps on a 40 amp breaker and tripping it, either you are misreading something or the breaker is bad.

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Old 06-19-2008, 10:30 PM   #3
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I'm no electrician, but to my way of thinking you would NOT combine the amperage in each line to get the combined amperage.

In a 220 volt circuit to a dryer, you have a 30 amp fuse on the black and a 30 amp fuse on the red. That doesn't mean the dryer draws 60 amps, it means it draws 30 amps because that's a 30 amp plug and receptacle you plug that dryer in with. Ditto for the 50 amp service to electric stoves.

I'm wondering if the problem is that the compressor motor is drawing more than 40 amps when it's first starting. Is there such a thing as a "slo blo" 40 amp breaker?
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Old 06-19-2008, 11:15 PM   #4
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essentially, yes. It is an HACR rated breaker. All motors draw much more than plate current rating on start up. A typical breaker does allow an excess current for a limited time but an HACR rated breaker gives it a bit more.

there should be a max circuit size on the unit as well as a minimum ampacity rating. With a motor, youcan go 250% of nameplate for most motors so that would allow a 50 amp breaker but do not do this without some further discussion. A 40 would typically hold unless there were problems which I suspect there is.
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Old 06-20-2008, 10:35 AM   #5
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If the AC is rated at 30 amp, that would be the correct size breaker for it. Was the 40 amp breaker installed at the same time as the AC, and did the HVAC guys install it?
If you have a 30 amp AC with a 40 amp breaker and it is tripping, it sounds to me like the breaker is bad, provided the AC is not the problem.
It also could be a loose connection, bad cable ect.........
Make sure the AC nameplate says "30 amp FLA" (Full Load Amps). Then go and get a 30 amp double pole breaker that will fit your panel. The wire or cable should be at minimum # 10 AWG. For a 40 amp breaker you should have at least # 8 wire or cable.
Install the new 30 amp breaker and see if this corrects the problem. If not there is another issue that will require someone with electrical troubleshoooting experience.
I will put my money on a bad breaker. Please note, I'm not that lucky.

Ps....Check any literature that came with the AC unit. It may require a 40 amp breaker. Check this first or contact the manufacturer.
Let us know how this works out.
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Old 06-20-2008, 04:28 PM   #6
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hold on a minute there JV. Newer AC units will list a minimum circuit ampacity and a maximum OCPD. The wire must be sized for the minimum ampacity circuit BUT the OCPD can often be several steps larger due to special rules for this type of equipment.

rusty, provide all the info you can on the unit including manuf., model, min circuit ampacity, max OCPD, and anything else you can toss up here. No such thing as too much info.

I still suspect a breaker but if anybody had done any work on this lately, all bets are off.
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Old 06-20-2008, 06:35 PM   #7
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Also, provide what size wire is being used. Undersized wire can trip a perfectly good breaker. Also, how long is the run between the panel and the AC unit? That's probably not the problem but it could be if it's a very long run.
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Old 06-20-2008, 07:15 PM   #8
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Also, provide what size wire is being used. Undersized wire can trip a perfectly good breaker. Also, how long is the run between the panel and the AC unit?

You lost me on that statement. how on earth would the wire size trip a breaker?


I've wired many ac units with #14 awg fed from a 30 amp breaker...
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Old 06-20-2008, 07:24 PM   #9
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I suspect he was referring to the fact that an undersized wire would cause a voltage drop and resulting current increase which could cause a tripped breaker.

Not sure but that's my story and I'm sticking to it!
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Old 06-20-2008, 07:54 PM   #10
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I suspect he was referring to the fact that an undersized wire would cause a voltage drop and resulting current increase which could cause a tripped breaker.

Not sure but that's my story and I'm sticking to it!
I was thinking that also, but I have never witnessed that in my life. If the voltage is that bad, the motor would never start.
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Old 06-20-2008, 08:04 PM   #11
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I was thinking that also, but I have never witnessed that in my life. If the voltage is that bad, the motor would never start.
generally you are right, especially with an A/C compressor.
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Old 06-20-2008, 09:57 PM   #12
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We are going in circles. Did this unit ever work? If so, was anything changed since it worked correctly?
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Old 06-21-2008, 03:19 PM   #13
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Originally Posted by Rusty53 View Post
Have a 40 amp 240v service feeding an a/c unit that is tripping the breaker. The plate on the unit says it is supposed to draw 30amps total. Checked the 2 feeds to the unit to see what it was pulling. Each leg was pulling 20amps. My question is.... do you combine the amps of each leg to get the total current?
If the rated FLA is 30 Amps, the unit may potentially use 4 to 8 times this current at startup. In plain English, the unit could use between 120 Amps & 240 Amps at startup. As a consequence, a "motor start" circuit breaker is required. If this type of circuit breaker is not used under these conditions, nuisance tripping will result.
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Old 06-21-2008, 03:32 PM   #14
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Originally Posted by chris75 View Post
I've wired many ac units with #14 awg fed from a 30 amp breaker...

Are you serious??????
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Old 06-21-2008, 03:34 PM   #15
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I suspect he was referring to the fact that an undersized wire would cause a voltage drop and resulting current increase which could cause a tripped breaker.
Most of the time there will be a buzz and the thing won't start. Although, it is possible that the breaker will trip. It is a possibility and wire size should be accounted for, it COULD be a factor here.

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