Sizing Breaker for Motor

Bering C Sparky

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Mar 2014
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Hello all,

Well I am going to throw this out here just to get a general consensus of others thoughts of this subject.

We are going to do a Refrigeration Upgrade this Fall.
New Compressor Motors will be 300 HP 460v 3 phase.

This was supposed to be all planned out by the office already and contractors lined up to do the job. (Great No Worries for Me Right??)

Well today I get an email saying, "I think maybe we are going to have to increase the breaker and wire size for the new compressors, can you check this out and see what we need and if it will fit in our existing distribution"

Are you kidding me, "YOU THINK" here we go again, million dollar upgrades drawn out on ****tail napkins, being read through the ice in a scotch glass!!!

Anyway, since the only information I was given was the motors are 300 HP, I break out NFPA-70 turn to table 430.250.
300 hp motor, 3 phase, 460 volt = 361 FLA
Now I flip to 430.52 and the multiplier for Inverse Time Circuit Breaker is 250%.

361 x 2.50 = 902.5 amps
So 1000 amp breaker is needed for this application. (At least in my thinking)

Here is the kicker,
I look at an older 300 hp compressor we have on this vessel, it is on a 400 amp breaker.
I go over to another one of our ships that received this same Refer Upgrade a year ago and they put the Compressors on 500 amp breakers.

So where did someone come up with the sizing for these breakers?
I realize that they work and don't trip when the motor starts (they are on soft starts) but how did one come about calculating the size for them other than using 430 of the NEC?

I have found no exception or article that says you can/should use smaller breakers for motor circuits just because it is on a soft start.

Does anyone know of something I am missing here, or have an opinion on the subject.

Regards,
BCS
 
I am no electrician, but I think those calculations are for inrush. Seems like all new breakers have a setting to cover those calculations.
 
I'd consult the breaker supplier if you are uncertain. I think robo77 is correct, and a MCP (motor circuit protector) breaker has a time vs amps trip curve that accommodates inrush current. Your supplier should have a chart that cross references motor hp vs breaker selection. Most of these are for a standard Design B motor, so verify that as well.
 
I have always used this formula 125% of motor FLA and use the next higher breaker size

In your case 361 X 1.25 = 451 amp = 500 AMP breaker
That is only for a time delay motor protection breaker a standard breaker will need to be higher
I would call the breaker manufacture and run it by them.

Also confirm the motor FLA from the motor nameplate as the very a little between motors. Number of poles in the motor will change the speed and FLA as well as other factors.

If you can use a soft start it would be an advantage. It would limit the stating inrush. I would recommend one with a bypass contactor they extend the life of the soft start.
 
Soft starts will limit the inrush you'd get with DOL but that is usually managed by a motor rated circuit breaker anyway, rare though it is to start such a big motor DOL. What you instead can get with a soft start is a longer period of high starting current which may well (and has in my experience) tripped out a circuit breaker that would otherwise be fine for a motor.

What you're after though is a table to refer to which I doubt exists, but I would talk to the soft start supplier armed with info about the compressor load and estimated start up duty. They should be able to specify the correct breaker.
 
Easiest way - look in a Schneider catalogue. We use kW here so I cannot help with sizes. Schneider also list the contactor, soft starter and circuit breaker catalogue numbers - sometimes they are an MA type - magnetic only. These are illegal in Australia these days - not sure about over there. All components are sized to suit the application. Then there is Class 10 and Class 20 as well.
 
On a slightly different tack , while you are at the design stage , if this motor is drawing more than 25 % of your ship's generation , perhaps you should consider using an inverter instead . Although the initial cost is higher , reducing the compressor speed as well as the current unloading stages should result in fewer start / stops and the fluctuations in voltage and possibly frequency affecting other equipment.
Our company is in the stages of installing a spiral freezer with a 132 KW motor , and although shore based , are limited by the size of our substation , so have puchased a an inverter for exactly the reasons given above .

Paul
 
Quote:"Anyway, since the only information I was given was the motors are 300 HP, I break out NFPA-70 turn to table 430.250.
300 hp motor, 3 phase, 460 volt = 361 FLA
Now I flip to 430.52 and the multiplier for Inverse Time Circuit Breaker is 250%.

361 x 2.50 = 902.5 amps
So 1000 amp breaker is needed for this application. (At least in my thinking)"

Per NEC that is the maximum size you can use. Have a look at page 10 of this:http://literature.rockwellautomation.com/idc/groups/literature/documents/td/140g-td100_-en-p.pdf.
I provide general guidance on breaker selection.
You really need the complete nameplate data of the motors.
 
1HP=0.75kW, so 300HP = 225KW and at 500V FLC would be around 310 amps

Based on the attached document (albeit that the voltages are different, 500 rather than 460), I would have said that a 500A breaker would suffice.
 
Thanks for the feedback guys, I have a message into my Schneider rep and will review the documents you have posted.

I don't have any information other than the motors are 300 HP, but I suppose I could email another vessel and get them to snap a picture of the nameplate on one of their motors for me.

I am not an electrical engineer obviously but I do hold a Master Unlimited Contractors License, although when I was contracting it was 99% Residential and Small commercial applications, mostly revolving around HVACR.

When sizing for motors we would use the 125% rule for the CONDUCTOR sizing (continuous loads) but the breaker for the motor would always be much larger than this. ( and actually 90% of the equipment we used was already engineered and had a minimum circuit ampacity on the nameplate (which was 125% of FLA) and a Min and Max Breaker Size right on the nameplate and this breaker size was always more than 125%)
(I can remember newbies comenting all the time on things like "How can you put #10 AWG on a 45 amp breaker its not legal", but it is when you are talking motors)(If you have an older condensing unit (A/C) on your home go take a look at the nameplate and you will see what I mean).

There is evidence all over the NEC that states the same thing.
210- Branch Circuit Over Current Protection for Continuous Loads. (states 125% for the conductors caring continuous loads and also for the breakers)
230 - Feeders for Continuous Loads.(states 125% for the conductors caring continuous loads and also for the breakers)
(But these have nothing to do with motors, at the beginning of each of these articles it says see 430 for motor loads).

430- Motors (this article also goes into detail that the Conductors will be sized at 125% and for sizing the breaker you use 430.52, which yes is the MAX RATING but without engineering the job it would be just a WAG as to how close or how far away from the max should you be.
This is not something you want to get wrong, 50 million dollars are on the line the following season that this installation will work. If not..... I don't want to think about it.

I am not arguing any points you have made and I appreciate your input.
In fact I think you are right on the money with most of this, I spent some time researching industrial breakers last night, you can get adjustable breakers and the options and choices for them are endless so I can believe that there are breakers Engineered to perform just as you are saying.
(But picking the correct one out of a hat will be tricky)

Will let you know what I find out.

But in truth I should not be finding out anything, this job should be Engineered specifically for this application and drawn out from start to finish.

Not sketched out on a bar napkin and then thrown into my lap last minute to figure it out.
I guess I am just venting, Not sure why I really expected anything different.

Thanks again for all your input and advice.
BCS
 
Last edited:
Here's what you REALLY need to know...

If you are MAKING the motor controller for this, or if you are FEEDING the motor controller for this. That's the first and critical aspect at this point. You can get yourself all caught up in sizing issues that can end up NOT being correct if you don't know this.

If you are MAKING the controller for this, then the rules in 430 apply. If the compressor is COMING to you WITH a controller already, then all you are doing is selecting a FEEDER for the circuit, slightly different rules apply.

Also, don't forget that the NEC is not a DESIGN guide, it is a set of MINIMUM or MAXIMUM requirements. So for example the 250% cited earlier is not a minimum, that is a MAXIMUM, and only under the specific conditions outlined.

And by the way, what you may or may not see inside of a factory built controller on another unit is not a good way top judge this, because the rules are different for FACTORY built combination starters vs FIELD assembled units. Many many many people are woefully unaware that you are NOT allowed to use "MCP" (mag-only) circuit breakers to build your own controllers in the field, they are ONLY allowed to be used in FACTORY assembled and listed combination starter assemblies, and even then, only Across-the-Line versions now. So again, not appropriate for field design decision purposes.

So start with step one, determine EXACTLY what they mean when telling you this; are you sizing a FEEDER, or a breaker as part of the CONTROLLER (Branch)?
 
Thanks for the input Jraef,

Well I did not think I was going to be involved in this at all, I have planned to play with my laptop and PLC equipment during this tie-up and catch up on some much needed maintenance in my factories. (but I am guessing the office has other plans for me now)

But seriously, I have little information at this point, (typical).
I do know that there will be a soft starter of some sorts supplied, if this comes as part of the compressor unit skid or is built up by some other 3rd party vendor...at this point, I don't know.

I do know that they have done it both ways in past installations.
(we are the last ship to be converted in out fleet, so you THINK they would have this down to a science by now....but not!)

I just sent out an email to the other vessels to get them to take some pictures for me of the motor nameplates and soft starter information, so I have something other than..."It's 300 hp".... to present to the Schneider Engineer when I talk with him.

(By the way we are putting 4 of these units onboard, and not one bit of load calculation has been done for the overall vessel load from the distribution I would bet)
(Fishing is Fun) :)

Regards,
BCS
 
...
I do know that there will be a soft starter of some sorts supplied, if this comes as part of the compressor unit skid or is built up by some other 3rd party vendor...at this point, I don't know.

If you get some input to this, have a talk with the folks at RAM INDUSTRIES (now owned by Schneider). They make an awesome soft starter and have great tech support. They also make motors for refrigeration compressors.

It has been a few years since I worked with them and assuming Schneider did not break them up, I can say that they made my job much easier due to their expertise at sizing and quoting soft starters for this industry.
 
Thanks Okie,

I don't want to get too involved if I don't have to, really don't want any part of what I am sure will be a grand, 3 ring event, under the big top. :)

I have heard nothing from Schneider yet, hopefully will hear something on Monday.

Enjoy your weekend.

BCS
 
You are correct in using the NEMA FLA for the motor current rather than the actual nameplate FLA. Code specifies using the NEMA value for sizing motor branch circuit conductors. If you get a chance to participate in the softstarter selection, you may wish to look at those made by Benshaw. Great reputation and very helpful people. Also, rather than use a circuit breaker, you may want to specify the softstarter as a combination starter (fused disconnect input). The softstarter manufacturer will do the sizing for you.
 

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