# 75 hp, 480v, 3 phase motor ocpd?



## carryyourbooks (Jan 13, 2010)

what size breaker do I need for this? please, forgive me, I don't have my code book in front of me.


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## nolabama (Oct 3, 2007)

Purty big one I suppose.


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## nolabama (Oct 3, 2007)

Master 'lectrican sitting next to me says 150 amp breaker.


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## bkmichael65 (Mar 25, 2013)

200 amp


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## bobelectric (Feb 24, 2007)

200 amp would do it but I'd look at going 150 amp.


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## carryyourbooks (Jan 13, 2010)

i haven't actually seen the motor, but someone told me it had a nameplate 96a rating. I agree with the 150a, too.


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## ponyboy (Nov 18, 2012)

I think 200 amp would be max since that's 250%. You might get by with 150 but...


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## Arc'n'Spark (Jul 21, 2011)

#1 Cu or 1/0 Al on a 125-225A breaker will suffice.


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## Ultrafault (Dec 16, 2012)

200.


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## 360max (Jun 10, 2011)

carryyourbooks said:


> i haven't actually seen the motor, but someone told me it had a nameplate 96a rating. I agree with the 150a, too.


you don't use nameplate to rate breaker. 200a breaker


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## don_resqcapt19 (Jul 18, 2010)

minimum wire size 1/0 copper, maximum thermal magnetic breaker 250 amps

minimum wire size should be #1 copper


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## JRaef (Mar 23, 2009)

I cheated.

I've got 3 motor starter slide charts in front of me, A-B, Siemens and Sq. D. All 3 say the recommended Thermal Mag CB is 175A for a 75HP motor, #1 Cu or 1/0 Al conductors. But to be honest, 175A CBs are rarely off-the-shelf items any more, and if it's a new Premium Efficiency motor, the mag trips may not adjust high enough. I'd go with 200A too. 150A will probably nuisance trip on you.


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## ampman (Apr 2, 2009)




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## Wirenuting (Sep 12, 2010)

JRaef said:


> I cheated.
> 
> I've got 3 motor starter slide charts in front of me, A-B, Siemens and Sq. D. All 3 say the recommended Thermal Mag CB is 175A for a 75HP motor, #1 Cu or 1/0 Al conductors. But to be honest, 175A CBs are rarely off-the-shelf items any more, and if it's a new Premium Efficiency motor, the mag trips may not adjust high enough. I'd go with 200A too. 150A will probably nuisance trip on you.


That ain't cheating, that's knowing we're to look


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## JRaef (Mar 23, 2009)

ampman said:


> View attachment 25128


What's that Ap?


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## nolabama (Oct 3, 2007)

JRaef said:


> What's that Ap?


X2 here


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## ampman (Apr 2, 2009)

JRaef said:


> What's that Ap?


electrician helper


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## nolabama (Oct 3, 2007)

ampman said:


> electrician helper


This one ?


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## ampman (Apr 2, 2009)




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## ampman (Apr 2, 2009)

nolabama said:


> This one ?
> 
> View attachment 25129


yes but i need to update


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## nolabama (Oct 3, 2007)

Cost extra to update ?


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## ampman (Apr 2, 2009)

nolabama said:


> Cost extra to update ?


no but i have to link to wifi because the download is so large


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## nolabama (Oct 3, 2007)

That's good to know. My little iPhone is about out of memory. I don't do enough new work to get that app tho. All I would use it for is to look smart here....


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## micromind (Aug 11, 2007)

The operating voltage wasn't stated. It makes a rather huge difference.


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## carryyourbooks (Jan 13, 2010)

ampman said:


> View attachment 25128


how can this be?

given:

480v
3 phase
96a nameplate rating
75 hp
135' run of conduit

how can you use a 200a breaker with #1 cu wire?

The entire job is a meter loop for this motor and a 15 kva 480/240 1 phase xfmr.

my thinking is to bring in 200a, 480v, 3 ph service using 4/0 cu.
use a 175a breaker to feed 3 phase motor using 2/0 cu.
40a breaker to feed 15kva xfmr.
xfmr will feed 70a mlo load center....the sole purpose of the low voltage panel is to install 2 LED motion fixtures (50 watt total for all lamps) and a plug for the camera system.
install 200a, nf disconnect at motor. 



am i missing anything?


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## micromind (Aug 11, 2007)

The code minimum CU wire size for a 75HP 3ø motor operating at 480 volts is #1. 

The code maximum breaker size for said motor is 225 amp. 

So it's perfectly legal to have a 225 amp breaker with #1 CU.


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## carryyourbooks (Jan 13, 2010)

micromind said:


> The code minimum CU wire size for a 75HP 3ø motor operating at 480 volts is #1.
> 
> The code maximum breaker size for said motor is 225 amp.
> 
> So it's perfectly legal to have a 225 amp breaker with #1 CU.


please, excuse my ignorance. if #1 cu is good for 130 amps, is the breaker only to protect the motor and not the wire? please, explain your calculation.


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## carryyourbooks (Jan 13, 2010)

carryyourbooks said:


> please, excuse my ignorance. if #1 cu is good for 130 amps, is the breaker only to protect the motor and not the wire? please, explain your calculation.


i think i have found the answer to my own question. i had forgotten the second step with motors is to size the conductors and then the ocpd.


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## frenchelectrican (Mar 15, 2007)

carryyourbooks said:


> please, excuse my ignorance. if #1 cu is good for 130 amps, is the breaker only to protect the motor and not the wire? please, explain your calculation.


 
I will make it sweet and short for ya .,

Basically the #1 is sized for running amps on the 75CV ( HP ) Moteur however the breaker is sized maxuim of 250% of running amps for backup protection in case conventail starter fail to trip. The breakers will serve as back up for the overload but primary for short circuit protection.

Merci,
Marc


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## bkmichael65 (Mar 25, 2013)

Conductors are sized at 125% of fla from 430.250 and ocpd is sized at 250% per 430.52


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## carryyourbooks (Jan 13, 2010)

bkmichael65 said:


> Conductors are sized at 125% of fla from 430.250 and ocpd is sized at 250% per 430.52


this being said, what size service should be brought in? i wanted to bring in a 200a meter loop service, but that wouldn't work would it?


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## frenchelectrican (Mar 15, 2007)

carryyourbooks said:


> this being said, what size service should be brought in? i wanted to bring in a 200a meter loop service, but that wouldn't work would it?


I will not bother that small a service size due I do not know what else you have on the load so something like 300 or 400 amp service is more better suited for this purpose.

And you may want to let your POCO know that you have 75CV moteur in there so they can able sized up the transfomer properly.

Merci,
Marc


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## bkmichael65 (Mar 25, 2013)

carryyourbooks said:


> this being said, what size service should be brought in? i wanted to bring in a 200a meter loop service, but that wouldn't work would it?


I'm pretty sure you would be okay with that. Just depends what else you would be putting on that service


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## carryyourbooks (Jan 13, 2010)

bkmichael65 said:


> I'm pretty sure you would be okay with that. Just depends what else you would be putting on that service


just a plug for a dvr and a 4 flood fixtures rated at 68 total watts. its in the middle of huge field.


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## frenchelectrican (Mar 15, 2007)

carryyourbooks said:


> just a plug for a dvr and a 4 flood fixtures rated at 68 total watts. its in the middle of huge field.


Now that is very light load there beside the moteur but the question is how often it will be started ?

I don't know if the 200 amp service can be ok with it unless you put in a soft start then it can get by without issue.

Merci,
Marc


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## Ultrafault (Dec 16, 2012)

I completly agree ask the poco how much you can save on a 200 amp service with softstart or vfd vs. Direct on line starting. It may save you a ton.


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## micromind (Aug 11, 2007)

carryyourbooks said:


> please, excuse my ignorance. if #1 cu is good for 130 amps, is the breaker only to protect the motor and not the wire? please, explain your calculation.


The breaker is there for short circuit and ground fault protection. The starter will protect both the motor and wire against overcurrent. But the starter cannot react fast enough to protect against short circuits and ground faults. A breaker is quicker. 

If the breaker is sized to the wire, it will very likely trip on the motor starting surge current, so it is allowed to be larger.


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