Compressor wiring Question

[Fishplate]

I was going to piggyback on the other compressor wiring thread, but this should be simple...

Air compressor: 230V, 15A, 3.7HP according to the nameplate. It's going to be a 60 foot run to the final location (30A disconnect and #10 whip to compressor).

A shorter run might squeak by with a 20A breaker and #12, but I'd be smart to use #10 and a 30A breaker...

Does that sound right?

 

[sberry]

12 would slide by with room to spare, this unit won't know the difference. The upside of it is that the circuit will accept a larger comp if ever needed.

 

[Aceman]

If it's 3.7HP, then legally you need to run #10.

 

[wyliesdiesels]

12 would slide by with room to spare, this unit won't know the difference. The upside of it is that the circuit will accept a larger comp if ever needed.

How do u figure?

 

[sberry]

It pulls 15A and its not 3.7 hp

But I am all for learning a little about code,, how does it say it again? If the current is not listed or listed on a plate.

It seems I saw a blue one about this a while back when cruising thru a job, I know it was contracted and inspected and I believe it had a 14 cord to 12/30. I think the wire cost come out of the sparkies pocket on this one.

Its a little like the welder picture, a guy buys a 211, ends up convinced he needs a number 6 circuit, anyone wanna bet the tune of about 90% of that would change if it was coming out of their pocket.

 

[sberry]

Are there any owners manuals that come with these comps? Just for the sake of discussion and I could be mistaken, there are so many NEMA and codes I cant remember them all but I believe that comp is in a class it can be wired with 12 cable, the 5's are listed for a 10 and the 7.5 call for an 8.

A 225 AC buzzer, 12 in single circuit in pipe but its 47.5 A listing was so it could slide in on a 10 cable.

 

[sberry]

I realize the only OM I have for one is 45 yrs old but it lists 3 hp and 20 A and 12 208/thru 230.

The real load is known from the plate,, the advertising HP is kind of moot or I would have to put a cord on my shop vac for a 6 hp motor? ha

 

[Aceman]

It pulls 15A and its not 3.7 hp

But I am all for learning a little about code,, how does it say it again? If the current is not listed or listed on a plate.

If the HP isn't listed on the plate, take the motor nameplate amps and compare it to the NEC tables. Pick the HP size closest to the nameplate amps. In this case, the closest motor amps in the table is 17 amps for a 3HP.

17 amps x 125%=21.25 amps. Since #12 romex is only good for 20 amps, he would need #10-2 Romex or #12 THHN in pipe.

If you think you'll ever upsize to a 5HP compressor, then I'd recommend running #8-2 Romex or #10 THHN in pipe instead.

 

[sberry]

Do we have to begin derating at 20 cause its a cable? How big of motor can we put on a 12 in pipe? The difference between sizing the wire for 15A and 17 is quite a bit.

My hoist list 14 for 15A,, actually 14A maybe.

Many get my question as disagreement that as a fundamental principle going up a size from the code minimum or mgf listing in most cases isn't a bad idea,,,, and some times even 2 is worthy in the case of certain welding circuits or people with plenty can afford to over wire just in case in places additions would be difficult, and just in case Johny brings home a 300 synchro,,, well, I am not a fan and think if its the case he can expand the service or circuit for it.

 

[sberry]

To tell the truth some of it would be about what I happened to have on hand and the expected duty cycle. This is not a serious working compressor in the sense that it would be a choice if manpower, painting cars on a regular basis was the intent.

If the guy needs to upgrade 2 sizes now is the time, return this and get the right comp.

 

[pattenp]

In a class many moons ago we were instructed to interpolate the table if your amp or hp didn't match the table. I think picking the closest next up where the ranges are small is okay. But where the ranges are large it will lead you to make the conductors larger than necessary. Here's some extracted text from a web page I found that states the same.

To find the F.L.C. of a motor rated more than one motor H/P listed and less than next Larger Motor H/P listed, you must interpolate the two sizes and calculate what that motor F.L.C. would be.

Example:
The F.L.C. of a 230 volt - single phase - 4 H /P motor would be calculated as follows; The F.L.C. of a 5 H/P motor is 28 amps. The F.L.C. of a 3 H/P motor is 17 amps. Therefore the difference between the two sizes in amps would be 11 amps. The difference in the horse power of a 5 H/P and the 3 H/P motors would be 2 H/P. Therefore you divide the F.L.C. difference 11 amps by the difference in H/P ratings of the two listed H/P motors “2 H/P”, which would equal 5.5 amps difference per H/P. (11 amps divided by 2 H/P = 5.5 amps) You may now discover the F.L.C. of your 4 H/P motor by adding the 5.5 amps {you received in that division of F.L.C. in amps}, to the F.L.C. rating found in the chart for a 3 H/P. (3 H/P=17 amps + 5.5 amps) {found to be the difference in amps per H/P found in your calculation}, and you will find the F.L.C. of a 4 H/P motor is 22.5 amps.

You could also find the same answer by subtracting the difference in F.L.C. amps per H/P 5.5 from the F.L.C. amp rating for a 5 H/P motor 28 amp - 5.5 amps = 22.5 amps for your 4 H/P motor.

You could adjust up from the smaller motor rated, or down from the larger motor rated, after you know the difference in F.L.C. amps per H/P listed. You must calculate each F.L.C. listed above and below the motor F.L.C. amp rating you are working with, that is not listed, because the difference in F.L.C. amps per H/P is not always the same as you change H/P ratings.

If the HP isn't listed on the plate, take the motor nameplate amps and compare it to the NEC tables. Pick the HP size closest to the nameplate amps. In this case, the closest motor amps in the table is 17 amps for a 3HP.

17 amps x 125%=21.25 amps. Since #12 romex is only good for 20 amps, he would need #10-2 Romex or #12 THHN in pipe.

If you think you'll ever upsize to a 5HP compressor, then I'd recommend running #8-2 Romex or #10 THHN in pipe instead.

 

[Aceman]

Do we have to begin derating at 20 cause its a cable? How big of motor can we put on a 12 in pipe? The difference between sizing the wire for 15A and 17 is quite a bit.

There's no derating, Romex just has a lower ampacity than THHN.

#12 Romex=20 amps
#10 Romex=30 amps

#12 THHN=25 amps
#10 THHN=35 amps

To answer your second question, I'd say a 3HP is the most you're going to get on 12's in pipe.

In a class many moons ago we were instructed to interpolate the table if your amp or hp didn't match the table. I think picking the closest next up where the ranges are small is okay. But where the ranges are large it will lead you to make the conductors larger than necessary. Here's some extracted text from a web page I found that states the same.

That is how I was taught.

If someone comes up with some random piece of machinery that uses a 4HP motor(I've never seen one), I would still wire it like a 5HP. When that motor smokes one day, I would not be surprised if they source a 5HP to replace it because a 4HP would be rare and probably hard to find=expensive.

We may go over all the details and the how's and the why's on here but when I'm wiring a customers piece of equipment, most of the time they don't care how I do it or what size wire I use. They only care about the end result, the equipment working.

 

[Fishplate]

For the record, I got 60' of 10-2 NM-B, and a 30A breaker. Better safe than sorry...12-2 would have been pushing things too hard, and the extra cost wasn't that oppressive.

 

[sberry]

Ok, got it,,, one more question. In another thread wylee quoted a 15A comp motor and listed 12, now this thread we use 17A and call for a 10,, which is it? hahaha

 

[Aceman]

Ok, got it,,, one more question. In another thread wylee quoted a 15A comp motor and listed 12, now this thread we use 17A and call for a 10,, which is it? hahaha

I couldn't tell you how Wylie comes up with his figures.

 

[wyliesdiesels]

Ok, got it,,, one more question. In another thread wylee quoted a 15A comp motor and listed 12, now this thread we use 17A and call for a 10,, which is it? hahaha

Wire for motors is sized at 125% FLC. IIRCC the thread youre referring to, the OP didnt know the HP rating so I said #12 could work because 15a x 125% = 18.75a and #12 NM is good upto 20a.

In answer to your question: "which is it?" it all depends on the equipment as you already know.

 

[Aceman]

Wire for motors is sized at 125% FLC. IIRCC the thread youre referring to, the OP didnt know the HP rating so I said #12 could work because 15a x 125% = 18.75a and #12 NM is good upto 20a.

In answer to your question: "which is it?" it all depends on the equipment as you already know.

Have you read 430.6(A)1?

 

[pattenp]

In regards to NEC 430.6. This is where it would be nice if the NEC would use plain English and give an informational note has to how to apply the table values. The 15 amps interpolate to be 2-3/5HP by the table, but there is no 2-3/5HP listing. The thing is in figuring the HP from the amps, the code section says to interpolate if necessary and to use the corresponding HP value. Corresponding means to be like or matching, but there is no corresponding HP to 2-3/5HP, so the reasoning is to take the next higher value of 3HP even though code does not explicitly say to do that. The code section mentions the use of interpolation and I have been taught to also interpolate the HP, so based on interpolation #12 is correct and based on taking the next higher listed HP of 3 then #10 is correct. So what is the correct way of applying the table values to determine the motor amps?

 

[sberry]

To tell the truth I would have done it just like the OP did especially if I didn't have wire on hand and had to buy it at the time. But,,, I wouldn't have had any sleepless nights connecting it with a 12 a bud had on hand in his home garage either,,, based on 2 things,,, one being its no mystery how much it draws, (not only does it meet 125% criteria it is only 3/4 loaded, the second is having seen several 3 hp comps run on 12 for 30+ years, seen installs for them way back by licensed guys dam near when they invented codes,,, ha

 

[Fishplate]

So what is the correct way of applying the table values to determine the motor amps?

I don't know from correct, but I'm gonna go out on a limb and say that if your interpolated HP value is not on the list, you can't use the smaller value that it's closest to because that would mean your conductors would be undersized.

Plain language would say "if you are between values, use the next largest one".

 

[pattenp]

You're right, I corrected my post. I meant to say next higher value.

I don't know from correct, but I'm gonna go out on a limb and say that if your interpolated HP value is not on the list, you can't use the smaller value that it's closest to because that would mean your conductors would be undersized.

Plain language would say "if you are between values, use the next largest one".