# Wiring for a 3-phase 25 HP motor



## John D in CT (Nov 22, 2010)

Hi all. I need to run cable from my service panel to a 25 HP 3-phase motor (air compressor). Motor draws 63 amps at 230 volts; my 3-phase service runs at 212 volts, and can pull 150 amps. (I know I'll lose some HP, but from my experience the motor should be OK with the slightly low voltage - stop me if I'm wrong). I have the correct magnetic starter w/ heaters; I'd just like some second opinions on the size of the wires I should use. The total run will be about 60 feet max. I'm thinking #4 (four separate #4 wires - 1 each red, white, black and green, copper strand, in a conduit). I'd appreciate a confirmation or correction.

Also, will a 60-amp breaker be sufficient? It seems to me that the motor probably won't be actually drawing its full 63 amps when in operation.

Thanks, John D.


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## jbfan (Jul 1, 2004)

1-Skip the white wire and use black, red, blue and green.
2-According to the nec, your motor at 208 volts has a FLC of 74.8 amps.
3-#4 thwn-2 is rated at 95 amps, so you will be good there.
4- No way this works on a 60 amp breaker.

74.8 times 1.25 =93.5= 100 amp breaker.

How are you controlling the starter?


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## John D in CT (Nov 22, 2010)

jbfan - thanks for the reply.

Definitely will go with 100-amp breaker in my service panel.

I'd like to just use white wire (availability on EBay) and mark it with blue electrical tape at connections. Kosher?

I'll be controlling the magnetic starter with a 110-volt switch between one of the hot legs coming into it and the "2" terminal on the (contactor?).
I'm thinking of doing the same on my 7.5 HP 3-phase compressor, which I now turn on and off with the safety switch itself. Seems as if this will cut down on wear and tear of the safety switch. Again, is this kosher? This is how it was wired in the shop the compressor came out of.

The fuses in the (100-amp) safety switch box are B-14, and the heaters in the starter are T57's, which should be correct.

Lastly, I'm not sure of the size that the ground wire needs to be. Does it need to be #4, or can it be smaller? Does it need to be insulated, or can it be bare? And what does the ground wire get attached to?

The motor has an insulated green wire coming out of it that's either 6 or 8 gauge. Does this simply get attached to the magnetic starter box? The starter box and safety swith box are connected by a short metal conduit, so is the ground wire from the service panel simply connected to the safety switch box? 

Thanks.


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## Know It ALL (Nov 13, 2010)

John D, I don't know your overall electrical supply. Your 25 horse motor is an industrial size load. Is your MCC breaker adequately sized for this load including other necessary loads and will the compressor starting current cause any voltage drops that could interfere with voltage sensitive loads? Just looking ahead. 
What size main breaker are you using?


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## John D in CT (Nov 22, 2010)

K-I-A - 

I have 150 amps of 3-phase service (all that the existing in-slab conduit could accommodate) at around 212 volts so I should be fine. The lights might dim a little, but everything including the computer should be OK. Thanks for your concern, and feel free to correct me if I'm wrong.


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## Jim Port (Sep 21, 2007)

Re-identifying the white as a ungrounded or hot conductor would be a code violation. You must use the proper colors. 

Conductors in conduit can be marked with tape only if #4 or larger. White and gray are still reserved for grounded conductors. Green is reserved for grounding conductors.


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## John D in CT (Nov 22, 2010)

"Conductors in conduit can be marked with tape only if #4 or larger". 

Hi Jim -

The conductors will be #4. Now, since you say that "White and gray are still reserved for grounded conductors", does that mean that I couldn't re-identify a white #4 conductor to blue, but could pull a red and two blacks, re-identifying one of the blacks to blue?

Any help you could give me with my other questions (especially size, insulation, and connection points of the ground wire) would be greatly appreciated. 

Thanks, John D.


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## Speedy Petey (Feb 1, 2004)

Based on your questions, your implied ability and the scope of this web site my ONLY advice is to hire a qualified electrician. 

Wiring up a 25HP 3-phase motor is ABSOLUTELY NOT a job for a DIYer or someone unqualified!


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## frenchelectrican (Apr 12, 2006)

The issue you will run into is the conductor you need minum of 25mm² {#4 AWG } and also check your motour nameplate to make sure it is set up to run on 208 volts it will on the nameplate.

you will need 100 amp breaker on this motour due the inrush current during start up. { and yes NEC code allow up to 250% oversized for short circuit protection and limited protection on the motour. }

You do not need white conductor on this useage due full three phase system you will need Black . Red . Bleu and Green 

Your 150 amp service may squeak by with this size of motour but if start unloaded it not too bad but with loaded start then a good chance the main breaker may trip.

What namebrand starter you have in there so I can able double check the starter overload elements to make sure and with this size of motor you denfity have to use NEMA size 3 for sure.


*NOTE*
However I really suggest have a electrician to come out and have them check out the whole set up due the motor size is pretty big and can wreck hovac if not catch on time.



Merci.
Marc


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## John D in CT (Nov 22, 2010)

SpeedyPetey - 

Thank you for your concern. I assure you that I have several friends who are licensed electricians and will make a final check of my work before I put the equipment into service. In the meantime, I would appreciate answers to my specific questions infinitely more than uninvited criticisms of my judgement. It's a 3-phase electric motor. I'm not asking how to make a hydrogen bomb.

FrenchElectrician - 

Thanks for your response. The magnetic starter is a Dayton 5X157A (NEMA size 3). Yes, I will have an electrician check my work.

The motor (Dayton 3N029) nameplate says only that the motor can run at 230v or 460v, unlike the 10 HP Baldor (M3313T) motor on my smaller compressor that says 208-230v/460v. Are 3-phase motors constructed differently if they are to run at 208v, or will a motor that lists 230v on the nameplate run on 208v (or in my case, 212v) without significant problem? I can see it running slightly warmer, but I can't see it blowing up.

To all:

Unanswered questions on the table:

Can a #4 black conductor be re-identified as a blue conductor.

Is it code to control the magnetic starter with a 110-volt switch between a hot leg and the contactor mechanism. 

Can the ground wire be smaller than #4, and does it have to be insulated.

Where does the ground wire get attached (both the insulated one coming from the motor, and the one that will be coming from the neutral bar on the service panel).

Will a 3-phase motor that says only "230v/460v" on the nameplate run acceptably well on 212v? 

Thanks.


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## emolatur (Nov 8, 2010)

I get the idea this guy is going to do this right... Knows what questions to ask and isn't totally bewildered.



I am, however, trying very, VERY hard to imagine the environment this is being done in. It seems, if it were commercial/industrial, there would already be a "proper" electrician on staff to wire up this motor... yet I know very few people (two, actually) who have 3phase power in their homes!

and... that is a *BIG* motor!


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## frenchelectrican (Apr 12, 2006)

John D in CT said:


> SpeedyPetey -
> 
> Thank you for your concern. I assure you that I have several friends who are licensed electricians and will make a final check of my work before I put the quipment into service. In the meantime, I would appreciate answers to my specific questions infinitely more than uninvited criticisms of my judgement. It's a 3-phase electric motor. I'm not asking how to make a hydrogen bomb.


That fine as long you deal with that electrician and the codes requrired on this one. SpeedyPete is worry about if something go wrong there will be one heckva a mess to expain so therefore myself and Speedy we are Electrician and we did see some of crazy mess when someone try to hook up big arse motour when the service size is not big engough.



> FrenchElectrician -
> 
> Thanks for your response. The magnetic starter is a Dayton 5X157A (NEMA size 3). Yes, I will have an electrician check my work.


I will have to take a quick peek on that one to make sure I know which type I am dealing with heater size { AFAIK Granger carry SqD products so I should able find the answer pretty fast } 



> To all:
> 
> Unanswered questions on the table:
> 
> Can a #4 black conductor be re-identified as a blue conductor.


Oui, not a issue.



> Is it code to control the magnetic starter with a 110-volt switch between a hot leg and the contactor mechanism.


You may have to change the contractor coil to either 208/240 volt coil or get a control transfomer and use 24 volt coil.

That is only legit way I dealt with stright voltage system on the motour circuits.


> Can the ground wire be smaller than #4, and does it have to be insulated.
> 
> Where does the ground wire get attached (both the insulated one coming from the motor, and the one that will be coming from the neutral bar on the service panel).
> 
> Thanks.


16mm² { #6 AWG} is the smallest conductor I use up to 100 amp breaker but once it go over then you have to bump up to 25mm² { #4 AWG } for grounding conductors 

Merci.
Marc


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## frenchelectrican (Apr 12, 2006)

emolatur said:


> I get the idea this guy is going to do this right... Knows what questions to ask and isn't totally bewildered.
> 
> 
> 
> ...


I have few big three phase motours in my home and shop due I have combation property and have 400 amp 408Y277 service{ of course have transfomer to downstep for 208Y120 for general circuits } at Wisconsin location and in Paris France I have 200 amp 415Y240 volt service here as well. So I am well versed with it.

Merci.
Marc


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## John D in CT (Nov 22, 2010)

emolatur - 

Thanks for the vote of confidence, I appreciate it. 

The environment is a steel building in an industrial park where I have my 3,000 sq. ft. shop in which I maintain and modify the equipment that I use in my tree removal business, and also do woodworking and metal fabricating. My operation is not yet large enough to warrant having an electrician on the payroll.

I've done most of the wiring in my shop; the existing 3-phase compressor, a 3ph band saw, 3ph surface grinder, 3ph plasma cutter, 3ph chop saw, countless circuits, lighting, gas heaters, waste oil furnace, and even a coffee pot.

Yes, 25 HP is getting a little large. I need as much air as I can get for sandblasting, and I calculated that this was the biggest electric motor I could power. This compressor wil give me around 100 CFM, as opposed to the 23 CFM I'm getting out of my Quincy QE-5. I have 2 other conduits running to the main panel outside the building, so I can pull in 300 more amps if I need to in the future.

Thanks again for your cordial reply,

John D.


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## emolatur (Nov 8, 2010)

Oh I'm not denying that some people do have that kind of power in their homes. Like I said, I know a couple. It's just unusual. 

The latter, a friend's grandfather, runs (or ran - I think he's retired now) a body shop in a building that used to be something to do with a dairy farm, and his house is fed from that.


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## frenchelectrican (Apr 12, 2006)

John D in CT said:


> emolatur -
> 
> Thanks for the vote of confidence, I appreciate it.
> 
> ...


That is a commercal location and I think you better get busy deal with your electrician before you do anything many locations are getting strict with it.

I have see some building owner try to do themself and end up do more damage than just call in the electrician to deal with it.

The last one I deal that kind mess was change from 800 amp 240 volt corner ground delta to 800 amp 480Y277 system that took a while to change over everything while the shop is running { the final change over was sheduled during the shutdown }

Merci.
Marc


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## John D in CT (Nov 22, 2010)

emolatur - 

I do believe that every home in America should have 3-phase. As we're all forced to work harder and longer, I think the time has come for 3-phase coffee makers!

Marc - 

The label on my starter says that it's good for up to 30 HP at 208-230V. That got me wondering if I might have to increase the size of the heaters up from T57 since my motor will be drawing more amps than stated on the nameplate (63A) as has been pointed out to me. T57 = 64.0A (min)/70.3A (max); T58 = 70.4A/77.5A; T59 = 77.6A/84.9A.

Also, good call on my double-checking the voltage requirements of the coil. I think it'll be fine. The individual legs of my 3-phase vary from 121v to 123v, and as I said, the label on the starter speaks of 208-230V.

I'll keep checking this thread. If anyone else has any thoughts, please jump in.

Thanks.


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## John D in CT (Nov 22, 2010)

Marc - 

I trust the collective wisdom on this site more than that of any one electrician that I hire on to sign off on the job (which I will). Naturally I want it to be his a$$ and not mine, and naturally I'd like to avoid any personal injury or property damage. I just want to start ordering the materials that I'll need; essentially, wires and possibly heaters. 

Also; just read on the starter label, "This starter is furnished with a dual voltage coil pre-wired for operation on a 208-230 volt control circuit ..."
Check.

Would still like your views on the need for higher capacity heaters with a 63 amp/230 volt motor running on 212 volts. jbfan said that my motor at 208v would draw 74.8 amps; at 212V it's probably around 70 amps, which would put it right between a T57 and T58 heater. Do I need to go up one size, or will the existing T57's be OK?

Thanks.


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## frenchelectrican (Apr 12, 2006)

Just give me a day I will look up the specs on the heater of this unit and I have a strong feeling you will have to change the heater elements to comperised with lower voltage { 208 volts } 

As far for control circuit that should be fused anyway { for safety issue with starter like that big }

But once everthing is hook up and have a electrician do the current drawage to make sure it is not out of the limit of both heater and motour current drawage.

The major issue is the service entrance size IMO it pretty much smack right the limit with this motour alone depending on how often it start up.

Normally with this type of motour size Genrally I recommend that the service size at least 300 amp or larger service size so talk to your electrician before you get too far order the materals etc etc.

Merci.
Marc


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## kbsparky (Sep 11, 2008)

If that motor operates satisfactorily on 208 Volt supply, then those heaters may have to be sized a bit larger.

You should expect that motor to draw 350+ amps while starting. You might find your main tripping out on occasion.

The ground wire size is based on overcurrent value of the circuit. IF you keep the 100 Amp breaker, then a #8 is compliant. If you end up having to increase that breaker to handle the starting load, then you'll have to upsize the ground wire to a #6 if it's fed with up to a 200 Amp breaker....


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## John D in CT (Nov 22, 2010)

Marc and KB, thank you very much. I'll probably try the existing T57 heaters, and if they trip I'll go to T58, then T59 if they still trip.

I will get some other opinions on whether or not my existing service can even handle this motor.

My present 150-amp service panel has no main breaker; it's located outside with the meter hub, an inconvenient distance away. I need more room for breakers anyway, so I'll go with a 48-breaker service panel with a 150-amp main, and use my existing service panel as an auxiliary panel in another part of the shop. I'll still probably never use 150 amps at the same time, but I might just pull another 150 amps of service if I get more equipment. I do have my eye on a Weinig molding machine with five 7.5 amp 3ph motors on it that would definitely start pushing the limits of my 150 amps.

One more question: Is there a safety breaker equivalent of the old-time "slow-blow fuse" that will accomodate brief surges of power, like the momentary 350 amps this motor might draw on start-up, or are all safety breakers essentially "slow-blow"?

Thanks again.


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## emolatur (Nov 8, 2010)

As frenchelectrician stated earlier, you are allowed to use a breaker rated up to 250% of what is actually "correct" for the conductors used, in order to deal with the startup current.

You may not have to, if the startup is quick enough or unloaded. With a lower-than-spec voltage, I'm doubting it will be that quick, however.

When you do this, the breaker is no longer protecting the conductors (or motor) from overload or overheating. That's what the overloads in the starter are for. The breaker is only there in the event of a dead short, which pulls exactly three metric craptons of current regardless of conductor size.


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