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Old 07-04-2008, 08:03 PM   #1
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Proper wiring of A/C fuse box


I had some electrical problems with my outdoor AC unit. The repair guy replaced the twin 50 amp circuit breakers as well as the fuse box. When he put everthing back and tried to connect the system, there was a short and the breaker would trip. No matter what he did, the short was there.

Today, I started playing with the two wires on the circuit breaker where I reversed them (not that it should make a difference, and it didn't) and I also changed some of the positions of the L and R wires in the fuse box but nothing changed. With the A/C unit off, I'm still getting a short. The last thing I hope for is the the condenser unit is bad but everything was working prior. Is it possible that the wires are bad or the fuse box is no good? Can it be that even though there was no short in the condenser unit one has developed now? How do I check for these things? Thanks.
Gary

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Old 07-04-2008, 08:31 PM   #2
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Proper wiring of A/C fuse box


if the breaker trips when the condenser is off it is very unlikely you have a bad condenser(unless the contator is stuck in) disconnect the wires at the disconnect box and at the contactor in the condenser and take an ohm reading (resistance) if you messure any resistance betwen the two wires and the are not touching replace the whip. wires if sized properly and without a power surge or mice should melt or short . the wires should be sized to handle the breaker size or smaller but not smaller then the cond fla

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Old 07-04-2008, 08:45 PM   #3
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Proper wiring of A/C fuse box


Is the contactor in the condenser something I get at by opening up the A/C unit? Also, what is a "whip"? Thanks.
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Old 07-04-2008, 10:29 PM   #4
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Proper wiring of A/C fuse box


now since there was no short before anything was replaced, is it possible to wire the fuse box incorrectly? i looked at the fuse box for the downstairs unit and everything seems identical. the two black wires from the whip go to the L and R, and the green wires goes to the ground connector and then there are 3 more wires coming from a box which i assume comes from the main circuit breakers. the green wire here goes to ground as well and then the two black wires go to the two Live connectors towards the top of the fuse box. changing around the wires made no difference.

i will try 8 Ball's suggestion of disconnecting the wires in the whip on the condenser side and checking for a trip.
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Old 07-04-2008, 11:08 PM   #5
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Proper wiring of A/C fuse box


I reread your post, and accidentaly deleted my reply. Is the 50 amp breaker in your main panel, and fuse or disconnect out by the condensor?

Why did they get replaced in the first place?

Yes it is possible to wire a disconnect incorrectly.

Look at the fuse holder. If there are two lugs at the top and two at the bottom, power to the top, load to the bottom, with the fuses in between. If there are two lugs on eighther side of the fuse holder, outboard are power, inboard are load.

If you pulled both wires off the 50 amp breaker, and it wouldnt reset, you probably have a bad breaker.

With the disconnect and fuses pulled, the 50 amp breaker trips, you really should call a licensed electrician.

By the way, if there is a problem with the condensor, and there are fuses in the disconnect, the fuses should blow before the breaker trips. What size are the fuses.

Last edited by 8 Ball; 07-04-2008 at 11:40 PM.
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Old 07-05-2008, 12:20 AM   #6
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Proper wiring of A/C fuse box


Yes the twin 50 amp breaker is in the main panel and the fuse by the condenser. I replaced the breaker because it would not set and I thought it was bad. After taking the old breaker out and leaving it on my desk for a day, I can now set and reset it so maybe it was not bad afterall but had overheated or something.

The fuse box was replaced because the plastic piece inside was broken and the white pull out disconnect was not sitting properly. There are two lugs on top marked LIVE. The wires that go to these lugs come from a junction box right below the fuse box. The two lugs at the bottom, one marked L and the other R have wires that come from what I guess you're calling the whip. Then there are two green ground wires that go to the middle.

With the fuses in and the disconnect in, if I try to turn the circuit breaker on I see a spark and it shuts off. With the fuses in, the disconnect out and the circuit breaker on, trying to put the disconnect in causes a spark and the circuit breaker goes off. I have used both 50 amp and 60 amp fuses.
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Old 07-05-2008, 07:43 AM   #7
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Proper wiring of A/C fuse box


Sounds like your wired correctly. It also sounds like you have a short in the condensor.

On the side of the unit there should be a tag with your model and serial number. In the electrical data, it will say max fuse size, put the proper fuse in the disconnect.

Turn off the main 50A breaker, turn off the furnace, pull the disconnect, then take the access cover off the condensor. Inside you will see where the power wires terminate at the contactor. A contactor is a large normaly open relay. There should also be a capacitor and additional wiring for the fan. Verify that there is no power to the unit, and try pushing the middle of the contactor in with an insulated screwdriver. If it doesent move, or appears to already be pulled in, theres your problem. If it does move freely, look at the bottom of the compressor. If there is a metal band with two wires terminating at the power side of the contactor, carefully disconnect them, tape them off, and try reseting your power. If anything else looks burnt or scorched, thats probably where the short is.

Never leave a unit open or unattended if there is any chance a child or animal could have access to it. BE CAREFUL...even qualified service techs and electricians are electrocuted every year doing exactly what you are doing. At any point you feel uncomfortable with what your doing, or you feel it is beyond you capabilities, stop, and call a pro.

Breakers are designed to respond to heat. If there is a short or too much amperage traveling thru it that creates heat, the breaker reacts, and trips same as fuses. When the breaker cools off, it can be reset, fuses get tossed.

Look forward to hearing from you.

Last edited by 8 Ball; 07-06-2008 at 08:46 AM.
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Old 07-06-2008, 08:33 AM   #8
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Proper wiring of A/C fuse box


shut the stat off no fan no cooling......ring out the wiring in OHMS from the condenser.....shut the CB off and leave the fuses in out at the condenser,with your meter go to the copper line and at the compressor contactor hit each wire at the top(see if either of the 208V lines are picking up a ground) .......anything and then take the meter and put a lead on each wire at the top of the contactor(to see if the 208V is shorting to each other coming out)the rasoning to read it from the top of the compressor contactor is to read the fuse holder section out back out to the house CB.suggestion do not use the CB to turn the condenser on use the stat or the outside disconnect.your 24Vs for the condenser comes off your TR in the furnace/air handler thats why the compressor contactor is in with the fused disconnect down and the condenser CB off only if you have a seperate CB for the furnace.at the CB for the condenser remove the wires and turn it on anf read 208Vs..if it doesn't trip(NO LOAD) something running out is grounding.if it tris with the wires off the CB is NG

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