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Old 11-21-2010, 10:54 PM   #16
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Quote:
Originally Posted by John D in CT View Post
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.
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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Old 11-21-2010, 11:14 PM   #17
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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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Old 11-21-2010, 11:25 PM   #18
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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.

Last edited by John D in CT; 11-21-2010 at 11:38 PM.
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Old 11-21-2010, 11:45 PM   #19
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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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Old 11-21-2010, 11:49 PM   #20
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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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Old 11-22-2010, 12:13 AM   #21
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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.

Last edited by John D in CT; 11-22-2010 at 12:24 AM.
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Old 11-22-2010, 09:41 AM   #22
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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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