electrical wiring question

Hi i got an electrical question, not plc yet, .. I got a panel in one building with an AC unit , the wire size looks to be number 10 wire and the double pole breaker is a 60 amp. Should this not be a 30 amp breaker ? or is it 2 x 30 amps that means its 60 amp. i dont know the motor size up on the roof but it is a standard AC unit with motor. Is the 60 amp breaker right or what? I am confused because the wire i am sure is 10 awg wire. ??

According to this site http://xtronics.com/reference/wire_gauge-ampacity.htm the #10 wire is only good for 33 amps if in conduit. I figure that is what you have. The wire is to small for the 60 amp breaker on the circuit. According to the same chart from above, you would need #6 to handle 60 amps and this is all figured on CU wire. This probably needs to be fixed before you have a problem.

thats what I thought

yes thats what i thought , but its a breaker in a building thats been there for 10 or 20 years. So i wondered if its a multiple of the 2 wires for 220 volt which would be like 60 amps, the 30 amps through one wire and 30 through the other. But it still should say 30 amps on the breaker right ? not 60 as it does say. Its not in use , but was years ago when the AC units were working. I am surprised it is there like that.

grey said:

yes thats what i thought , but its a breaker in a building thats been there for 10 or 20 years. So i wondered if its a multiple of the 2 wires for 220 volt which would be like 60 amps, the 30 amps through one wire and 30 through the other.
REPLY on a 220 circuit with a balanced load ie motor 30 amps on one leg is equal to 30 amps on other.

But it still should say 30 amps on the breaker right ? not 60 as it does say. Its not in use , but was years ago when the AC units were working. I am surprised it is there like that.

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REPLY
IF it were mine I would
1. Determine motor starting current and running current at full load. Read motor nameplate - OR - use a clamp on ammeter.
2. Now that you know motor amps and starting amps (use locked rotor for this) You now can size your wire.
3. Heat pump compressors (which is what you have) are often hard starting due to compressor head pressure they have a heavy current draw. Therefore to a limit the bigger the conductor the better.
4. Generically
# 12 is good for 20
10 for 30
8 for 40
6 for 50.
THIS depends on insulation type and number conductors in conduit and lenght of wire run among other NEC requirements.
5. IF it were mine and it were a one or two ton unit I would size off the locked rotor current.
6. I would have overload protection set at 110 to 125% of full load amps.
7. The circuit breaker should be able to not trip on starting current IF you size it for 125% of running current. IF it trips on start head pressure is too high wait a half hour to try restart.

Some will argue this is well beyond NEC reqs they are right - but my motors start, I don't have callbacks and fires -- so far --(need symbol for crossed fingers). Have fixed my share of blown out controllers though.

Now for some honesty. Based on your calculations of a 220 V motor load you are in over your head. Do the above steps. Then get a GOOD motor control electrician to check things out - note I said GOOD not licensed or bonded just someone who knows what he is doing. THEN go ahead and rewire as needed.

Dan Bentler

remember...it's the wire size that determines overall ampacity of the circuit, not breaker size....all breakers,overloads & fuses should be rated from FLA of the device. If the wire is #10 (the NEC says 10 can be used in special conditions up to 35 amps, but 30 amps max most of the time) then the circuit can SAFELY carry 30 amps before the properties of the wire begin to break down. If it is on a 60A breaker, then the breaker will not trip until a short circuit occurs, and by that time, a fire could already have caused severe property damage.

To answer the question..NO it is not 2 x 30 = 60, each 10ga wire is rated at 30 amps, a 60 amp breaker will allow 60 amps of current per leg (wire). I would remove that 60 amp breaker and get it far from that box or replace it with a 30.

As mentioned its an accident waiting to happen.

leitmotif said:

REPLY
6. I would have overload protection set at 110 to 125% of full load amps.
Dan Bentler

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actually, the NEC says that motors are to be rated at 125% FLA for overload & overcurrent protection.

Based on the NEC, under the right situation, it is perfectly legal to place a 10AWG conductor in a 60AMP breaker, under the right conditions I could place a 10AWG conductor in a 250AMP breaker, but those are for motor circuits, you said you have an AC unit. There is an article in the NEC which deals with AC units Article 440, you do not size conductors/overloads/disconnects for AC units as you do for motors.

Generally (meaning I can’t see your installation, there might be exceptions), the rating of the conductors and equipment is based on the rated-load current on the nameplate (there are exceptions). The short-circuit and ground-fault protection shall not exceed 175% of the nameplate, unless this is not high enough to carry the starting current of the motor, in which case you can increase to 225%. The conductors shall have an ampacity of not less than 125%.

So is it possible to place a 10AWG conductor in a 60AMP breaker used on an AC unit? Yes.

Is it ok to do so in your installation? I don’t know, I can’t see your installation.

I have to admit I was wrong

I am not seeing an exception that allows the wiring to be rated less than the circuit protection and dont think I would EVER use wire rated less than the load and circuit protection even if there is some kind of exception.
BUT
A 10ga copper Type PFA, FEP, and FEPB is rated 60 amps, not more than 3 conductors in raceway, in free air with ambient temp less that 104C its rated 90 amps and nickel or nickel coated copper is 107 amps.

I made the fatal mistake of thinking in general terms where THxx or equivalent is used, which at 10ga is rated 30 amps.

Is there another smaller breaker (30Amp) upstream of the 60AMP breaker? The 60 amp may just be used as a disconnect means, not for overcurrent protection. The upstream breaker 30A would be for overcurrent protection.

I Am New On This Site. Do Have Any Idea How To Connect Master Sefty Relay Msr210p With The E-stop
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Check for the actuall load of the circuit before deciding if the wire or breaker is sized wrong. In my expierance it's seems that usually it's the wire that's sized improperly and not the breaker. Either way, somthing has to be changed.

elevmike said:

Check for the actuall load of the circuit before deciding if the wire or breaker is sized wrong. In my expierance it's seems that usually it's the wire that's sized improperly and not the breaker. Either way, somthing has to be changed.

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WHY? IMO the poster is WAY over his head.

Kasongomoma, technically I have the same opinion about you, by asking that question the REAL answer is get an electrician (control tech).

The first step to figuring out how is RTFM http://literature.rockwellautomation.com/idc/groups/literature/documents/in/440r-in001_-en-p.pdf

Just in case that is not enough, which I assume because the question was asked, here is an example.

kasongomoma said:

I Am New On This Site. Do Have Any Idea How To Connect Master Sefty Relay Msr210p With The E-stop
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Being new on the site, welcome! However, hi-jacking threads is not polite. Read the how to use this site info at the top of the main page. Start your own thread about a topic that is important to you.