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Main Circuit Breaker Selection

Main Circuit Breaker Selection

I have a customer asking for assistance with selecting the correct main circuit breaker for a 10HP 460V triplex vacuum system.  Depending on the demand, only one unit could be in operation or all three could be operating at the same time.  Motor FLA is 14 each (w/0 1.15 SF).

Cutler Hammer Motor Circuit Selector says the following:

1.  Instantaneous trip circuit breaker GMPC/HMCP: 30 AMPS and
2.  Circuit breaker trip rating: 35 AMPS

A.) Can someone provide a briefly explain the difference between #'s 1 and 2? and
B.) Should the breaker be sized based on all three units in operation (10HP x 3 = 30HP)?

Just trying to help a guy out!



RE: Main Circuit Breaker Selection

I felt that this was not the apprpriate place to post my question so I reposted it in Electric Power Engineering.



RE: Main Circuit Breaker Selection

Hello acfm.

This was probably close enough since it is sort of motor control.

Just red flag these posts and say you miss posted it and it will be taken care of.  Not a problem.

Keith Cress
Flamin Systems, Inc.- <>

RE: Main Circuit Breaker Selection

You CANNOT use version 1, the Instantaneous trip (Magnetic only) breaker as the main for the entire triplex panel, you must have a Thermal Mag breaker as the main. Mag. only breakers, a.k.a. Motor Circuit Protectors or MCPs, can only be used on an individual motor circuit where there is also an overload relay in addition, and even then only when they have been specifically tested and lited by UL in that combination (unless UL listing is unimportant to you).

The Main CB must protect the entire panel load, so the sizing would be total possible FLA x 1.25, which is also how you would size the incoming cable to it. So, 14FLA x 1.15 = 16.1A x 3 pumps = 48.3A x 1.25 = 60A assuming there is nothing else in the panel (small CPT may be OK).

Eng-Tips: Help for your job, not for your homework  Read FAQ731-376

RE: Main Circuit Breaker Selection

Review NEC 430. Look up overload and short circuit protecton for muuti-motor loads.

Essentially it says if it is a factory wired assembly and it has factory applied lable/intructions then follow that.

If you have to design, then again follow NEC 430. For multimotor feeder breaker sizing refer to 430.62 which requires a feeder breaker rating sized for largest motor plus the FLA of other motors, that is 250% of the FLA of largest motor plus sum of the FLA of the other motors (for a thermal magnetic breaker as main device.)

Condcutors are to be sized per 430.24, 125% of the largest motor FLA plus sum of FLA of the rest of the motors.

RE: Main Circuit Breaker Selection

rbulsara is correct, but just wanted to clarify that the 250% of largest motor FLA plus sum of other FLA gives you the **largest** allowed breaker size.  The NEC would allow a smaller breaker.  

RE: Main Circuit Breaker Selection

And the breaker size must match with the conductor selection. Maximum sizing is not always best.

Eng-Tips: Help for your job, not for your homework  Read FAQ731-376

RE: Main Circuit Breaker Selection


Motor circuiting is one of the exceptions where conductor amapcity can be less than the breaker rating.

RE: Main Circuit Breaker Selection

Only on the specific motor branch circuit, because the Overload Relay is providing long term thermal protection. The original question was for a MAIN circuit breaker in a panel feeding 3 separate motor starters. That exception will not apply.

Eng-Tips: Help for your job, not for your homework  Read FAQ731-376

RE: Main Circuit Breaker Selection

If it is a feeder feeding all multi-motor load, it still applies, that is what 430.24 and 430.62 says..

Each of the motors still have their O/L protection.

RE: Main Circuit Breaker Selection

I agree with almost all of your posts jraef. However in this case may I politely suggest that you re-check NEC section 430 and the examples in chapter 9.
Unless there have been serious changes in the last code edition, in which case I will tender my appologies.

RE: Main Circuit Breaker Selection

I don't know. I read that over and over and I don't see my error. I also don't see where it allows the breaker to be sized for 250% as a branch feeder protection device.

I generally go by UL508 rules, NEC historically had to do with installations, not panel assemblies. That has of course been blurred in the past few changes to the NEC. If you ask me, the way they wrote it is very confusing, requiring jumping back and forth from one section to another and having too read all the exceptions to determine if it allpies to you. UL is much cleaner.

Eng-Tips: Help for your job, not for your homework  Read FAQ731-376

RE: Main Circuit Breaker Selection

I agree with the comments re; confusing writing.
My interpretation is:
It does not allow the feeder breaker to be sized at 250% of the feeder size.
It allows the feeder breaker to be sized at the same size as the over current protection for the largest motor plus the sum of the current of the other motors.
You can re-state the 125% conductor size rule as; "The ampacity of the feeder conductors shall be equal to the sum of the motor currents plus 25% of the current of the largest motor."

I believe that you can accurately restate the 250% breaker size rule as; "The ampacity of the feeder circuit breaker shall be equal to the sum of the motor currents plus 150% of the current rating of the largest motor."

The way it is written, I understand it to mean the maximum allowable, but if someone claimed that it was based on the size of the protection actually installed for the largest motor rather than the largest allowable the argument may be interminable.
Is this a reasonable explanation, rbulsara and dpc?

RE: Main Circuit Breaker Selection

Yes, i did not say 250% of the feeder.

I will double check exact wording when in the office. But the logic in sizing breaker for a motor feeder is simple. It needs to be sized to accomodate starting of the largest motor when rest of the motors are running. Hence the overcurrent rating needs to be one required for the largest motor plus the sum of FLA of the rest. If the OCPD is a breaker, is can be upto 250% of the largest motor FLA plus sum of FLA of the rest.

Again overload of the feeder is taken care of by OL protection at the motor. The condutos sizing is 125% of largest plus sum of the rest.

RE: Main Circuit Breaker Selection

Let's get back to the OP.
Does this sound right freinds?
One motor @ 14 amps
Conductor @ 125% = 17.5 amps, #12 AWG
Breaker @ 250% + 35 amp or next larger standard. (Is 35 Amps considered a standard size?)

Three motors, feeder;
Conductor ampacity (125% of 14) + 14 + 14 = 45.5, #8 AWG
Breaker ampacity (250% of 14) + 14 + 14 =63 amps or next smaller size. 60 amps.
If this will not allow the motor(s) to start, the feeder size will have to be increased and the breaker sized to the feeder ampacity

RE: Main Circuit Breaker Selection


You understanding is correct. I beleive you can get a 35A breaker.

As for the feeder #8 is good only for 40A (this is a totally different discussion). For 45.5A, #6 has to be used per NEC.

60A breakers sounds fine. In fact in this particular case, as it turns out #6 is good up to 60A. But that is just a good coinidence.

RE: Main Circuit Breaker Selection

hey start from the basics and work the calculation. There is some info missing and must be known for a proper conductor,heaters,ocpd,branch circuit, feeder conductor and feeder ocp.So (motor data plate would be help) but lets make a
1. table 430.250 select fla (14 amps)
2. table 430.32 heater selection sf 1.15xfla=16.1 amps low  end 1.40x14=19.6amps high end.
3. table 430.22 conductor selection 1.25xfla=17.5 amps (TW      14awg)
4. table 430.52 OCPD (pick you flavor) nontime delay fuse (%300) dual element(%250) instantanious cb (%800) inverse time (%250). code permits going up to the next size (240.6)
5.table 430.24 feeder conductor size, largest motor x 1.25 and add the rest of the flc connected motors. table 310.16 for feeder conductor selection (14x1.25+14+14=45.5amps)thhw     8awg (50amps).
6. table 430.62 feeder ocpd, select largest branch circuit
and add all other flc (connected to the same line). step 4 pick your flavor and add flc. i.e. Inverse time %250x14.0=35+14+14=63amps (code select down on feeder ocpd)
60 amp ocpd. can be what ever ocpd in 430.52.

hope that has helped, this is all in the 05 nec.           

RE: Main Circuit Breaker Selection

Maybe my semantics are wrong, but kaukau's post illustrates the point I was trying to make earlier. If, by Item 5, you select the feeder conductor size to be rated for 50A, then the feeder OCPD CANNOT be 60A as indicated in Item 6 regardless of what the sizing rules say that the maximum can be. It must be rated for the feeder conductor size, which in this case is 50A! If you want to, you can use the maximum rule to select a 60A OCPD first, then go the other way and select 60A rated conductors, but you can't have 50A rated conductors protected by a 60A breaker.

Eng-Tips: Help for your job, not for your homework  Read FAQ731-376

RE: Main Circuit Breaker Selection

I believe in a different ckt(other than motors and xfmrs ect.) you would be required to match ocpd to conductor rating . The code is addressing these calculations for motors by giving "demand factor or %%% for starts. Example would be a %800 on a instantanouis cb. This is to carry over the inrush in a start. Example %800x14=112amps.  

check out MIKE HOLT,  this is a good question and if we had the data plate and installation conditions we could be on the money for specing a install. there are a lot of other things like distance of feeders,# of conductors ect......

RE: Main Circuit Breaker Selection


With all due respect, and  I mean it, trust me on this.

What kaukau said is no different tha what waross said, and I would still have the same comment as i made in my last post to kaukau as well.

#8 is only good for 40A. Below 100A or #1 AWG conductor you have to use 60 deg.C column of NEC310-16. This is a differnet requirement in NEC.

RE: Main Circuit Breaker Selection

If the electrics for this vacuum system are in an MCC then jraef is likely right. It could still be argued that the feeder to the MCC is only supplying motor loads and therefore falls under the rules for motors but you may lose this argument with an inspector.

If they controls are in one big box then the others are right. The code does allow the breaker to be sized larger than the current capability of the wire.

Be prepared for the inspector to want more appropriate protection for each individual motor too. Code does give the rules for multiple motors on one feeder but I've still run into cases where the inspector wanted protection for each motor.

If I'm building a panel with the breaker I have to follow UL508A. UL508A is very restrictive on multiple motors with one overcurrent protection means. It's basically not allowed except for some special cases. I put the main breaker with external handle for the panel and then a fuse block or smaller breaker for each individual motor.

RE: Main Circuit Breaker Selection

OCPD for Motor Control Centers are covered by NEC 430.94 and its has to be in accordance with article 240.

This is different than a multi-motor load (a combinaiton of motors, but one assembly). Do not confuse the two.

Look up NEC 430.7, 430.24, 430.25 and 430.62. It may be invloved but not still clear enough.

I have enough of this thread now. I beleive.

RE: Main Circuit Breaker Selection

My point is that you really do not know how the electrical controls for this vacuum are configured. Everyone is just assuming that since it's a "triplex vacuum system" that all the motor controls are in one box.

RE: Main Circuit Breaker Selection

Section 240-3 contains twelve rules that modify the general requirement and permit the conductors not to be protected in accordance with their ampacities, they include:

   1. Power Loss Hazard
   2. Devices Rated 800 Amperes or Less
   3. Tap Conductors
   4. Motor-Operated Appliance Circuit Conductors
   5. Motor and Motor-Control Circuit Conductors
   6. Phase Converter Supply Conductors
   7. Air-Conditioning and Refrigeration Equipment Circuit Conductors
   8. Transformer Secondary Conductors
   9. Capacitor Circuit Conductors
  10. Electric Welder Circuit Conductors
  11. Remote-Control, Signaling, and Power-Limited Circuit Conductors
  12. Fire Alarm System Circuit Conductors

RE: Main Circuit Breaker Selection

rbulsara (Electrical)

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