I have a roll of 10/3 romex that I was given so I want to use that to run for my compressor (80 gallon Kobal from Lowes). I have a fairly straight run with only one curve that totals about 60 feet. Reading on here and what I have read about code in regards to romex in emt, it seems if I use 3/4 emt I should be under the fill limit as this is a total enclosed run of emt from breaker box to outlet. My question is do I need to be concerned with derating for this short of a run? Any suggestions or help is appreciated.
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Compressor wiring with 10/3 and emt question
- Thread starter ml504
- Start date
Falcon67
Well-known member
If it's a 5HP like most, he'll draw around 16A @ 240V. I don't know about the conduit fill, but seems like the wire will be the right size. The Lowes spec sheet shows 18.5A which seems a bit high for 4.7HP.
MrMark
Well-known member
Derating for romex in conduit akin to bundling issues with 9 or more conductors? Haven't seen it but good question. Shows you are thinking.
i think you just need to consider the fill (and that is based on the cross section of the whole cable treated as one conductor. You cant calculate the fill based on the individual conductors inside the romex. I dont know if 3/4" would cut it for 10/3 (and i dont envy anyone who gets to pull that) I would sell the romex and buy the right wire.
Bill_Brasky
Active member
I just ran 12/3 thru 20 ft of 1/2" emt with 2 bends with no problems. Are you sure you need 10/3? The motor on my 7.5 HP (prob a generous rating) is 15 A.
eta: sorry, I see the 10/3 was given to you.
eta: sorry, I see the 10/3 was given to you.
Last edited:
Charles (in GA)
Well-known member
I just ran 12/3 thru 20 ft of 1/2" emt with 2 bends with no problems. Are you sure you need 10/3? The motor on my 7.5 HP (prob a generous rating) is 15 A.
That would be a very optimistic maximum "starting" horsepower. Based on the amp load (assuming 120v) would only be about 1 hp. Even if 240v, only about 2 hp running.
Charles
Falcon67
Well-known member
I just ran 12/3 thru 20 ft of 1/2" emt with 2 bends with no problems. Are you sure you need 10/3? The motor on my 7.5 HP (prob a generous rating) is 15 A.
You have something wrong there - real 7.5 HP is 23 amps at 240V, not including any power factors or whatnot. 745w per HP - 5587 watts. That's 10 gauge easy, maybe 8. If it really draws 15A, it aint 7.5HP IMHO.
Bill_Brasky
Active member
You have something wrong there - real 7.5 HP is 23 amps at 240V, not including any power factors or whatnot. 745w per HP - 5587 watts. That's 10 gauge easy, maybe 8. If it really draws 15A, it aint 7.5HP IMHO.
I don't have anything wrong. I said it was a generous rating. I didn't do the math, but I knew the 7.5 HP sticker on the compressor didn't coincide with the 15A rating on the motor.
There are three issues here:
1) The rating of the compressor. Consumer oriented compressors are notorious for claiming a horsepower higher than theoretically possible. I have small Craftsman 110V that runs on a 15 Amp circuit just fine and claims to be 7.5 "peak" horsepower. I also have an industrial Speedaire 2-stage compressor that needs a 50 Amp 6 gage circuit. The speedair is also listed at 7.5 hp.
2) Rating of wire size: If your compressor needs 16 Amps then you should have 12 gage wire. By the book (and there are some exceptions for motor inrush, run length, conduit, etc) here are the guidelines:
It does not matter if the circuit is 220V or 110V for the amperage rating (if questions about why this is true, post back and I'll do my best to explain).
In the NEC, there is significant exception in wire size for running electric motors. The reason is when an electric motor starts, it uses a huge amount of current compared to it's "normal" operating current requirement. The NEC recognizes this and allows you to use a wire size based more on normal than start current. If you had to size for start current, that would be much larger wire. You are also allowed to use slow blow fuses for electric motors.
For you application, since you say you need 16 Amps, you will need 12 gage wire as a minimum. 10 Gage is great if you have it but not strictly required.
3) Romex (NM or NM-B) in conduit: There is debate wether or not NM wire can be used in conduit and still be code compliant. I've looked hard for this in the NEC and haven't found a clause prohibiting it for inside use (can someone prove me wrong on this, please?). However, most inspectors where I live will fail you for running NM in conduit. It's clear by code that NM is not allowed in conduit for underground or outside applications.
1) The rating of the compressor. Consumer oriented compressors are notorious for claiming a horsepower higher than theoretically possible. I have small Craftsman 110V that runs on a 15 Amp circuit just fine and claims to be 7.5 "peak" horsepower. I also have an industrial Speedaire 2-stage compressor that needs a 50 Amp 6 gage circuit. The speedair is also listed at 7.5 hp.
2) Rating of wire size: If your compressor needs 16 Amps then you should have 12 gage wire. By the book (and there are some exceptions for motor inrush, run length, conduit, etc) here are the guidelines:
14 Gage: 15 Amp
12 Gage: 20 Amp
10 Gage: 30 Amp
8 Gage: 40 Amp (for larger sizes, the exceptions start to matter)
6 Gage: 50 Amp (for larger sizes, the exceptions start to matter)
12 Gage: 20 Amp
10 Gage: 30 Amp
8 Gage: 40 Amp (for larger sizes, the exceptions start to matter)
6 Gage: 50 Amp (for larger sizes, the exceptions start to matter)
It does not matter if the circuit is 220V or 110V for the amperage rating (if questions about why this is true, post back and I'll do my best to explain).
In the NEC, there is significant exception in wire size for running electric motors. The reason is when an electric motor starts, it uses a huge amount of current compared to it's "normal" operating current requirement. The NEC recognizes this and allows you to use a wire size based more on normal than start current. If you had to size for start current, that would be much larger wire. You are also allowed to use slow blow fuses for electric motors.
For you application, since you say you need 16 Amps, you will need 12 gage wire as a minimum. 10 Gage is great if you have it but not strictly required.
3) Romex (NM or NM-B) in conduit: There is debate wether or not NM wire can be used in conduit and still be code compliant. I've looked hard for this in the NEC and haven't found a clause prohibiting it for inside use (can someone prove me wrong on this, please?). However, most inspectors where I live will fail you for running NM in conduit. It's clear by code that NM is not allowed in conduit for underground or outside applications.
Aceman
Well-known member
2) Rating of wire size: If your compressor needs 16 Amps then you should have 12 gage wire. By the book (and there are some exceptions for motor inrush, run length, conduit, etc) here are the guidelines:
14 Gage: 15 Amp
12 Gage: 20 Amp
10 Gage: 30 Amp
8 Gage: 40 Amp (for larger sizes, the exceptions start to matter)
6 Gage: 50 Amp (for larger sizes, the exceptions start to matter)
Those aren't the correct cable ampacities for motor circuits. You need to look at Table 310.16 and 240.4(D) in the 08 NEC.
3) Romex (NM or NM-B) in conduit: There is debate wether or not NM wire can be used in conduit and still be code compliant. I've looked hard for this in the NEC and haven't found a clause prohibiting it for inside use (can someone prove me wrong on this, please?). However, most inspectors where I live will fail you for running NM in conduit. It's clear by code that NM is not allowed in conduit for underground or outside applications.
This will clear it up:
http://forums.mikeholt.com/showthread.php?t=133895&highlight=romex+in+conduit