Wire Size for Large Compressor

[EasyE]

20 HP compressor with twin 10HP motors that start sequentially. It is about 75' from the breaker panel. I plan to wire in a disconnect outside where the compressor is.

I have 120/208 Wye service. Will be running these motors at ~208V.

What size wire will I need?

See pics:

 

[exranger06]

I'm not sure you'd be able to use that GE motor, since it doesn't appear to be rated for 208V. But assuming you could use it, I think you'd need wire with an ampacity of 70A, which means #4 copper wire (table 310.16).

How I arrived at that answer:
According to 430.24, you need to take 125% of the FLC of the biggest motor, and 100% of the FLC of all of the smaller motors and add them all up to get the total ampacity required. To find the FLC, you look it up in table 430.250. A 10 HP motor at 208V 3 phase has an FLC of 30.8 amps.

Since both motors are 10 HP, they should both have an FLC of 30.8A. 125% of 30.8 is 38.5A. 38.5 + 30.8 = 69.3A. Round it up to 70A.

Someone correct me if I'm wrong.

 

[TRWham]

Looks correct to me, but there is a note on the GE nameplate that says it is usable on 208V.

 

[EasyE]

Hmm I wonder if the internal jumper wiring for the GE motor is different for 208 than for 230, Ill have to pull the cover and check.

 

[Aceman]

Units like that leave the factory designed for a specific voltage. The motor only being one piece of the puzzle.

Have you verifed the starters, coil voltages, overload settings, internal wiring, terminal blocks, etc are all appropriate for operation at 208v?

 

[wyliesdiesels]

I'm not sure you'd be able to use that GE motor, since it doesn't appear to be rated for 208V. But assuming you could use it, I think you'd need wire with an ampacity of 70A, which means #4 copper wire (table 310.16).

How I arrived at that answer:
According to 430.24, you need to take 125% of the FLC of the biggest motor, and 100% of the FLC of all of the smaller motors and add them all up to get the total ampacity required. To find the FLC, you look it up in table 430.250. A 10 HP motor at 208V 3 phase has an FLC of 30.8 amps.

Since both motors are 10 HP, they should both have an FLC of 30.8A. 125% of 30.8 is 38.5A. 38.5 + 30.8 = 69.3A. Round it up to 70A.

Someone correct me if I'm wrong.

Huh?

It clearly says 208-230/460 on the nameplate...

 

[EasyE]

Units like that leave the factory designed for a specific voltage. The motor only being one piece of the puzzle.

Have you verifed the starters, coil voltages, overload settings, internal wiring, terminal blocks, etc are all appropriate for operation at 208v?

Sure haven't I just got it set. I'm used to wiring multi tap motors but thats about the extent of my experience. Can you give me some 101 on what to look for on these?

I did briefly run both motors at separate times on 208V to make sure it functioned, but I need to really shake it down to make sure I don't damage anything that isn't setup right.

So consensus is 4/3 huh. Ill probably see if I can get that in romex I'm sure its $5/ft...

 

[matt_i]

Your easiest clue on what-voltage the unit was formerly wired at is how the motors are configured.

If 4-5-6 are jumpered together then was on low voltage (208/240).

If you have L1-1, L2-2, L3-3 then was on high voltage.

You can normally read the coil voltage on the contactors themselves. It might have a control transformer instead of sending 240/480vac thru the pressure switch. In that case the transformer likely has a jumper configuration like the motors.

 

[EasyE]

Your easiest clue on what-voltage the unit was formerly wired at is how the motors are configured.

If 4-5-6 are jumpered together then was on low voltage (208/240).

If you have L1-1, L2-2, L3-3 then was on high voltage.

You can normally read the coil voltage on the contactors themselves. It might have a control transformer instead of sending 240/480vac thru the pressure switch. In that case the transformer likely has a jumper configuration like the motors.

Thanks, I wired them to low voltage to test them but didn't look at the other odds and ends.

 

[wyliesdiesels]

Why bother with Romex?

Get some EMT, black #4 THHN and 3 rolls of tape- red blue and green.

Can you post pics of the starter box with cover on and off as wel as a close up of the contactors and the overload relay block?

In addition to a possible coil change, You may need different overload heaters or if solid state, an adjustment to the setting.

 

[EasyE]

Why bother with Romex?

Get some EMT, black #4 THHN and 3 rolls of tape- red blue and green.

Can you post pics of the starter box with cover on and off as wel as a close up of the contactors and the overload relay block?

In addition to a possible coil change, You may need different overload heaters or if solid state, an adjustment to the setting.

Yeah I will be able to get some shots friday or Saturday and post them.

What size EMT you think I will need for that run?

 

[exranger06]

Huh?

It clearly says 208-230/460 on the nameplate...

I wouldn't say it CLEARLY says 208v. It says 230/460 on the left side. I had to zoom in a lot, and even then I could barely make out where it says compatible with 208v.

 

[alfredeneuman]

Why bother with Romex? Get some EMT, black #4 THHN and 3 rolls of tape- red blue and green.

70-100A circuits only need a #8 Cu ground wire except it's got to have green insulation, or stripped for it's entire visible length (not in the conduit)

 

[EasyE]

70-100A circuits only need a #8 Cu ground wire except it's got to have green insulation, or stripped for it's entire visible length (not in the conduit)

Ok ground size was my next question. What size conduit fits 3 - #4s and 1 - #8? 1”?

 

[Jim greengo]

Huh?

It clearly says 208-230/460 on the nameplate...

I think that's the marathon plate for other motor you're looking at.
The ge motor says 230v,although I've seen plenty of 230 motors run on 208 before.
Might run a little warmer.

 

[wyliesdiesels]

I wouldn't say it CLEARLY says 208v. It says 230/460 on the left side. I had to zoom in a lot, and even then I could barely make out where it says compatible with 208v.

Yup my mistake.

I was only looking at the first motor...

 

[yatg]

Ok ground size was my next question. What size conduit fits 3 - #4s and 1 - #8? 1”?

You don't need a ground wire if you use EMT conduit.

 

[Sokoloff]

1" EMT for 3 #4 (or 3 #3) and 1 #8 (which is optional as above)

https://www.southwire.com/calculator-conduit

 

[mike93lx]

If you want to save some money, use #2 AL XHHW

1" emt will still work if you don't run a ground, but if it is a complex run, stepping up a size would probably be worth it

 

[mike93lx]

Also, is your POCO good with two 10hp motors? If this isn't industrial service, it might be a problem

 

[EasyE]

Also, is your POCO good with two 10hp motors? If this isn't industrial service, it might be a problem

They better be. I gave them a list of all the equipment I have when I built this place. I have 400 amp service.

They pulled 500KCM wire for my service ~ 375'.
See Pic.

 

[mike93lx]

They better be. I gave them a list of all the equipment I have when I built this place. I have 400 amp service.

They pulled 500KCM wire for my service ~ 375'.
See Pic.

It isn't about wire size, it's the transformers.

In residential service, 10hp is generally a no go, from what I have read here

 

[EasyE]

It isn't about wire size, it's the transformers.

In residential service, 10hp is generally a no go, from what I have read here

I can take a pic of the transformers, but they had to set a new pole and 3 large transformers off the highway for my service. From the load sheet I turned in I would hope they got this right.

 

[EasyE]

For my disconnect I will go with a GE outdoor non fused to match other I have around the shop, however I see I can get a 20HP rated disconnect, but it only has a 30 amp rating. I am expecting to run a 70 amp breaker at the panel and pull more than 30 amps should I upsize this? Or is it rated differently since we are dealing with motors?

 

[EasyE]

Ok I got the wire ran and all that finished up, however I took some pics of the wiring inside the panel at the compressor since it looks like a lot of the components were setup at 460V. My service now is 208V.

It looks like the control transformer I can rewire for ~240 volts. I am assuming this will work with 208V?

The rating on the contactors shows they are only good for 7.5hp at 208V so I am assuming I will need to upgrade these? Any good places to source this?

The overload relays appear to max out at ~16 amps. I will need something closer to 30 amps going to 208V. Where can I source these?

Also the relay in the center... Not sure whats going on with that.

Also the leads going to the motor are 12 awg. This was probably fine for 460V. Will I need to rewire these to 10 awg for the increased amperage on 208v?

Here are the pics:

 

[OccupantRJ]

The relay in the center could possibly be there to control it as a flip flop system where the motors alternate in running.

 

[alfredeneuman]

however I see I can get a 20HP rated disconnect, but it only has a 30 amp rating.

Huh?
I noticed also you didn't list your location on your profile (everyone else on this thread has)
Different PoCos have different rules re: max hp they'll allow on their system.
For instance So Cal Edison's rule is a 10HP max on resi services

 

[EasyE]

Huh?
I noticed also you didn't list your location on your profile (everyone else on this thread has)
Different PoCos have different rules re: max hp they'll allow on their system.
For instance So Cal Edison's rule is a 10HP max on resi services

I am in South Louisiana and no residential service here.

I was able to find a disconnect with the correct ratings.

Update on the controls I posted pics of earlier. I called klockner-moeller and the parts guys were helpful with crossing over all the part #'s to versions that will work with the higher amperage at 208V. However its nearly $500 so I am currently looking on ebay for some better pricing.

 

[wyliesdiesels]

I wouldnt run the control transformer 240v windings on 208v. the 120v output might be too low for the contactor coil and thus cause chattering.

Get a transformer rated for 208v or better yet, since you need a new contactor anyways, just get one with a 208v rated coil and bypass the control transformer all together.

have you asked your PoCo about max HP limits on their system?

The in-rush current on larger motors can be very high...

 

[Bert_]

For my disconnect I will go with a GE outdoor non fused to match other I have around the shop, however I see I can get a 20HP rated disconnect, but it only has a 30 amp rating. I am expecting to run a 70 amp breaker at the panel and pull more than 30 amps should I upsize this? Or is it rated differently since we are dealing with motors?

I think a 30 is good for 20hp on 600v 3 phase. You have 208 so it will probably take a 100A disconnect.

 

[mike93lx]

have you asked your PoCo about max HP limits on their system?

The in-rush current on larger motors can be very high...

He's been asked this several times and deflected or ignored it. I'm guessing he doesn't want the real answer.

 

[Bert_]

I wouldnt run the control transformer 240v windings on 208v. the 120v output might be too low for the contactor coil and thus cause chattering.

Get a transformer rated for 208v or better yet, since you need a new contactor anyways, just get one with a 208v rated coil and bypass the control transformer all together.

Kind of agree with just getting 208 coils. That cabinet is pretty messy as it bigger starters aren't going to help.

If you go to eBay stick with name brands. Used NEMA or new IEC are both fine for your purpose.

 

[Bert_]

He's been asked this several times and deflected or ignored it. I'm guessing he doesn't want the real answer.

I'm not sure why anyone thinks this is residential. He has 3 phase. I doubt this is in his back garage.

 

[Norcal]

Kind of agree with just getting 208 coils. That cabinet is pretty messy as it bigger starters aren't going to help.

If you go to eBay stick with name brands. Used NEMA or new IEC are both fine for your purpose.

With the control transformers means the controls are 120V, but the OP would either need to get control transformers that have a 208V primary, or supply a 120V control circuit, I would not try to change it. Since the OEM starters are IEC type, I would not try to fit considerably physically larger NEMA style starters in their place even though I personally prefer NEMA over IEC, but do have to agree with sticking with name brand starters, A-B is preferred by me just because have had pretty good luck with them, and will be easy to get accessories for them.

 

[Bert_]

You could just bring a neutral to the cabinet from the breaker panel. Get power from one of the line wires on the starter. If you use a small control fuse the neutral could be #14.

Keeps everything 120v and no need for a separate control power breaker.

 

[wyliesdiesels]

I'm not sure why anyone thinks this is residential. He has 3 phase. I doubt this is in his back garage.

I wasnt thinking it was resi. But I have seen limits on HP for small 3-phase service.

Especially for end of line runs or where the PoCo installed an undersized trany for the service and the cutouts arent rated very high.

 

[EasyE]

I wouldnt run the control transformer 240v windings on 208v. the 120v output might be too low for the contactor coil and thus cause chattering.

Get a transformer rated for 208v or better yet, since you need a new contactor anyways, just get one with a 208v rated coil and bypass the control transformer all together.

have you asked your PoCo about max HP limits on their system?

The in-rush current on larger motors can be very high...

Good thought on the 208 contactor I will take a look and see what I can find.

As for the poco requirements on motor size. I built this shop and sent the power company a build sheet of all the equipment I would be running. They came out and set a new pole and hung 3 transformers for the service based on my build sheet. I would think they would have seen the numerous motors and machine tools and sized them accordingly. I am in an industrial area so I don't think I would call that the end of the line. My shop has 400 amp service, I would be surprised if motor start up exceeds 200 amps.

As for controls I am looking at moeller - eaton stuff so should be decent quality I would think.

 

[Milton Shaw]

Nobody else has mentioned this, but make sure your motor rotations are correct before you put belts on and connect the whole thing. I would hate to see what mess two 10 hp motors would make if they were turning the wrong directions or what amps they would pull. Just a word or warning as even the experienced electricians will sometimes forget this and have something burn up or run without oil pressure from reverse rotation.

 

[sberry]

I doubt 2 10's are a problem, they set it knowing he has 400 and a reason for 3 ph. I have 400 single, ****** is a 15. I have small motors, biggest is 5, no flutter from it. I got with engineering before they set this, got a big fat wire from the ******, got it in writing to spec it.
Had a resident hook up the other day, poco used the same wire that was on a 60 to feed new 200, really looked strange,, ha. 4 to 4/0.

 

[sberry]

As for checking, I was at a job a while back, the installer didn't bother checking voltage, they were still ting to sort out who's fault it was, spose to be 230, was 480. . They didn't want to pay to have supervision,,, how hard can it be,,, what could go wrong? Had another one not too long ago, didn't want to pay the 2 g to manage it,,, 25 later,, ha.