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Old 11-21-2010, 06:36 PM   #1
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Wiring for a 3-phase 25 HP motor


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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Old 11-21-2010, 06:52 PM   #2
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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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Old 11-21-2010, 07:31 PM   #3
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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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Old 11-21-2010, 07:47 PM   #4
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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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Old 11-21-2010, 08:16 PM   #5
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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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Old 11-21-2010, 09:00 PM   #6
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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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Old 11-21-2010, 09:08 PM   #7
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"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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Old 11-21-2010, 09:31 PM   #8
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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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Old 11-21-2010, 09:34 PM   #9
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The issue you will run into is the conductor you need minum of 25mm˛ {#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

Last edited by frenchelectrican; 11-21-2010 at 09:41 PM. Reason: add a note
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Old 11-21-2010, 10:19 PM   #10
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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.

Last edited by John D in CT; 11-22-2010 at 12:19 AM. Reason: Additional info
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Old 11-21-2010, 10:26 PM   #11
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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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Old 11-21-2010, 10:31 PM   #12
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Quote:
Originally Posted by John D in CT View Post
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.

Quote:
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 }

Quote:
To all:

Unanswered questions on the table:

Can a #4 black conductor be re-identified as a blue conductor.
Oui, not a issue.

Quote:
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.
Quote:
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.
16mm˛ { #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 25mm˛ { #4 AWG } for grounding conductors

Merci.
Marc

Last edited by frenchelectrican; 11-21-2010 at 10:34 PM.
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Old 11-21-2010, 10:33 PM   #13
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Quote:
Originally Posted by emolatur View Post
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!
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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Old 11-21-2010, 10:47 PM   #14
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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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Old 11-21-2010, 10:49 PM   #15
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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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