No power to AC rooftop unit - Is there an extra breaker somewhere?

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Old 08-12-15, 08:18 AM
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No power to AC rooftop unit - Is there an extra breaker somewhere?

Hi all;

Just found this forum, and am mighty impressed. Maybe you can help me on this one: My central air, rooftop gas/electric package unit stopped blowing cool the other day. It's as old as the house--mid 70s, but I think the compressor has been replaced. Anyway, the capacitor was oily and dirty, so I had a friend help me replace it. Once we did, the outside fan kicked on, but not the compressor.

While we were fiddling with it, trying to troubleshoot it, we shorted the capacitor against the unit. Now, I have no power to the unit at all. The breaker is NOT tripped at the breaker box, and the fuses in the AC disconnect are still good. So why no power?

I have turned off the AC breaker, and checked voltage to the 50A wires coming from it (actually using probe on the exposed wire from breaker, not terminal)...there is no voltage. When breaker is on, I get voltage. So the breaker is good, and there is power leaving the breaker box.

At the disconnect on the roof with the unit, I have no power at all. I have even disconnected the wires completely at the ac disconnect, so I could test them. No power.

Is there ANOTHER breaker or fuse between my breaker box and the ac disconnect? Would it be in the attic (the only realistic place for it in my house design)?

Is it possible we fried the main 50-amp wire from the breaker box to the unit, WITHOUT shorting it (a short would cause the breaker to trip, wouldn't it)?

Any ideas? I'm baffled....
 
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Old 08-12-15, 09:09 AM
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Not likely you fried the wire. It would be highly unusual that there would be a another breaker in the circuit. Did you turn off the breaker and then back on. Some times they looked like they didn't trip, reset the breaker just in case. Shorting out the cap should have tripped it.
 
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Old 08-12-15, 05:12 PM
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Well, if you have the CORRECT voltage at the breaker then the breaker is reset,it's unlikely there is a second fused disc. but possible,you may have to try and trace the pipe or cable just to be sure,check around the air handler and see what is going on there.
 
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Old 08-12-15, 06:55 PM
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I have turned off the AC breaker, and checked voltage to the 50A wires coming from it (actually using probe on the exposed wire from breaker, not terminal)...there is no voltage. When breaker is on, I get voltage. So the breaker is good, and there is power leaving the breaker box.
Are you using an inductive - non contact probe? All that probe tells you is that you have dangerous voltage in the area. You could have 120vac (missing the second leg) and your probe will show ok.

You MUST use a voltmeter to check for 240vac at each location.
 
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Old 08-12-15, 07:06 PM
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How are you testing? Are you using a meter and reading from line to line? Are you getting 240 volts?

I also highly doubt you burned up a wire, if you still do not have voltage there is likely a junction box someplace with a bad splice.
 
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Old 08-14-15, 11:10 AM
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Hi all;

Thanks for the input--here are answers to your questions:

1. I did reset (turn off, turn back on) all the breakers (main, ac, etc), just to make sure. They haven't tripped again.

2. Voltage at breaker is correct--am using a multimeter with contact probes. I'm actually touching the little bit of wire exposed coming from the breaker. When breaker is off, no juice. When it's on, I get 240V.

3. I'll look for another junction box or connector, but it would almost certainly be in the attic (and it's 115F in Phoenix right now, so I'll wait until morning.

If I run some number 6 wire in a "jumper" from the breaker to the ac disconnect, and the unit's fan comes on, that would mean my problem is between the ac disconnect and the breaker box, right?
 
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Old 08-14-15, 11:20 AM
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If I run some number 6 wire in a "jumper" from the breaker to the ac disconnect, and the unit's fan comes on, that would mean my problem is between the ac disconnect and the breaker box, right?
Correct. You should also take your meter at the disconnect and measure both legs to ground. You should get 120 volts to ground. Likely you only lost one leg so you might be able to just "jumper" one leg to see if you can get the unit going.

I also agree that a junction box would likely be in the attic. At 115 air temp it is likely 160-180 degrees up there so be careful!
 
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Old 08-14-15, 04:18 PM
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Aha! Stupid alert!

After further examination, I discovered that I have been checking the wrong breaker, i.e., the electric range. The previous owner had mislabeled it on the box. Lesson learned--don't trust labels.

Now I'm sure there's a short somewhere, since the "right" breaker won't re-set.

Argh.
 
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Old 08-14-15, 05:16 PM
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I thought you said you had 240v at the service disconnect ??
Did you even check at the service disconnect ?

Disconnect the wires at the breaker first and then see what breaker does.
If breaker sets off..... remove service pull out by condensor unit and try again.
 
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Old 08-14-15, 05:26 PM
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Disconnect the wires from the breaker and see if it will reset. If it does, then the breaker is good, if not, you have a bad breaker.

If the breaker is good, turn off the disconnect and measure continuity between the wires, and from each wire and ground. You should have none. If you do, then you have a short between the breaker and the disconnect. If you do not, then you have a short on the load side of the disconnect. This might mean your unit is fried.
 
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Old 08-17-15, 06:35 PM
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Problem solved--

Hi all; Just an update, to let you know what happened.

I was able to restore power with a new breaker, and installed a new capacitor. The compressor would not come on, however. Called an AC guy, and he ohm checked compressor, it was ok. While we were waiting to figure it out, the compressor came on--apparently there's a 5-minute start delay.

Anyway, problem solved, it's blowing cool, and life is good. thanks for all your help!
 
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Old 08-18-15, 06:53 AM
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That's great to hear! Thanks for letting us know the outcome.
 
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