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Old 12-30-2018, 11:26 AM   #1
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Overloaded circuits -damage to A/C units


I live in a house, 3 years old, with 2 A/C units - one a 5 ton (rated 50 amps), the other a 4 ton (rated 40 amps). They are wired to 2 breakers - 220 volts - 50 amps and 40. I recently discovered that the builder used 10 gauge wires between the units and the breakers. I know that it should be 6 and 8 gauge wire, respectively. I am working on having this re-wired. My question is what everyone thinks is the likelihood of damage to the AC units, which have run for 3 seasons with this inadequate wiring?
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Old 12-30-2018, 11:44 AM   #2
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Re: Overloaded circuits -damage to A/C units


IR voltage drop is not good for electric motors especially during start up phase. Have you noticed compressor motor being sluggish getting up to speed?


A prolonged start up current will heat up the windings and if long enough will burn the insulation. So not great for a long trouble free in-service life.


Since it was new its odd the inspector didn't catch this.


I'd go to the manufacturer's web site and pull the installation PDF and see what it says for branch breaker and wire gauge. Maybe 10AWG is ok but the breaker is too big? Easy fix, two new breakers. There are special rules for wire gauge and breaker sizing for AC units but I'm not familiar with them.


I saw this on another forum but can't attest to its accuracy.
..."It will say two things... "minimum circuit ampacity" and "maximum fuse or breaker size". Compressors have a totally different set of rules. You size your wire to the "minimum circuit ampacity" (or bigger, if you want to be wasteful) and you size your breaker to the "maximum fuse or breaker size" (or smaller, if you want to tempt a call-back). With this in mind, you can see #12 with a 30 or 40 amp breaker, for instance, and it would be completely legal."...
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Old 12-30-2018, 11:57 AM   #3
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Re: Overloaded circuits -damage to A/C units


That quote is correct. You can easily see a 40A or 50A breaker on a #10 wire. The large breaker is there to ride out any initial surge without tripping but only need a #10 wire for the unit's current usage. And with a soft-start on the compressor, it might only be in the low 20A range.



Jim, can you post photos of the info plates on each unit so we can see the values?



My prior house had a Water Furnace GEO unit with a 50A breaker on a 22A draw.
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Old 12-30-2018, 12:29 PM   #4
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Re: Overloaded circuits -damage to A/C units


Thank you. I am trying to figure out how I can post the 2 pictures I just took (jpg) of the 2 labels.
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Old 12-30-2018, 12:37 PM   #5
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Re: Overloaded circuits -damage to A/C units


If this worked, it is the 5 ton label.
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Old 12-30-2018, 12:40 PM   #6
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Re: Overloaded circuits -damage to A/C units


This is the 4 ton label. Sorry if they come out sideways. I don't know why.
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Old 12-30-2018, 03:59 PM   #7
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Re: Overloaded circuits -damage to A/C units


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Originally Posted by jim3902 View Post
I live in a house, 3 years old, with 2 A/C units - one a 5 ton (rated 50 amps), the other a 4 ton (rated 40 amps). They are wired to 2 breakers - 220 volts - 50 amps and 40. I recently discovered that the builder used 10 gauge wires between the units and the breakers. I know that it should be 6 and 8 gauge wire, respectively. I am working on having this re-wired. My question is what everyone thinks is the likelihood of damage to the AC units, which have run for 3 seasons with this inadequate wiring?
There may be nothing wrong with the size wire you have! What do the name plates on the AC units say?

This is by far the most misunderstood section of the code.

Many electricians still do not know this code and refuse to believe that it is possible to wire smaller wire to larger breakers.

Try wiring a large motor that requires a 250 amp breaker and how small the wire can be.
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Old 12-30-2018, 04:03 PM   #8
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Re: Overloaded circuits -damage to A/C units


Jim3902, the units are wired to code, and wired the same way I would have wired them.

What issues are you having with the AC units that make you think the wiring was done incorrectly?
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Old 12-30-2018, 04:31 PM   #9
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Re: Overloaded circuits -damage to A/C units


As you can see, the smaller wires can carry that much current,
The down side is that they will run quite a bit hotter,
Not good for long term reliability.
Weather any damage has been done ?
Whilst it is theoretically possible !

The chances are quite low.
I would not worry too much.
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Old 12-30-2018, 04:37 PM   #10
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There is no issue. The #10 is rated for 30 amps and the unit calls call for at least 29 amp capacity. The unit s not going to run hot or anything else. Even posters in this thread do not understand the rules.
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Old 12-30-2018, 06:33 PM   #11
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Re: Overloaded circuits -damage to A/C units


Seems if there was a break in the wire on the motor and if it began drawing, let's say 45 amps, wire would overheat but breaker wouldn't trip on the 50 amp circuit. Small likelihood, but why not size the wire to the breaker size.
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Old 12-30-2018, 06:50 PM   #12
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Re: Overloaded circuits -damage to A/C units


If my opinion is they weren't done much good being run hot on undersized wire what would you do?
They still run don't they? change the wire out of course and run 'em till they die, you're aware that you may have to replace motors, so budget accordingly, the air handler isn't a big deal but the compressor motor is the big question isn't it? \_(ツ)_/
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Old 12-30-2018, 07:06 PM   #13
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Re: Overloaded circuits -damage to A/C units


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Originally Posted by jim3902 View Post
Seems if there was a break in the wire on the motor and if it began drawing, let's say 45 amps, wire would overheat but breaker wouldn't trip on the 50 amp circuit. Small likelihood, but why not size the wire to the breaker size.
Your logic is flawed. Any issue with the compressor and the thermo overloads will shut down the power. If the compressor is locked up, then the current draw will trip the breaker.

The breaker is only there for short circuit protection, and sized for the inrush current of the compressor starting up.
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Old 12-30-2018, 07:29 PM   #14
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Quote:
Originally Posted by Sparkomatic View Post
If my opinion is they weren't done much good being run hot on undersized wire what would you do?
They still run don't they? change the wire out of course and run 'em till they die, you're aware that you may have to replace motors, so budget accordingly, the air handler isn't a big deal but the compressor motor is the big question isn't it? \_(ツ)_/
The units are not going to run hot. The wire is sized for the run current. The breaker is sized for the inrush current. Everything is fine.
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Old 12-30-2018, 07:29 PM   #15
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Re: Overloaded circuits -damage to A/C units


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Originally Posted by dmxtothemax View Post
As you can see, the smaller wires can carry that much current,
The down side is that they will run quite a bit hotter,
Not good for long term reliability
.
Weather any damage has been done ?
Whilst it is theoretically possible !

The chances are quite low.
I would not worry too much.
Why would the wires run 'quite a bit hotter' ??
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