Sizing Main Breaker for Motor Control Panel

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scottmarston

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I'm kind of getting sick of going back and forth with UL (I realize they could care less about NEC) on this topic and I just wanted to get something set in stone...for peace of mind. I'm sizing a main inverse-time circuit breaker inside a 480VAC motor control panel...here is my methodology and how I interpreted NEC:
Given:
  • (2) 1HP Motor Starters ? 3FLA/motor
  • (2) 25HP Motor Starters ? 34FLA/motor
  • (2) 1HP VFDs ? 2.4FLA/drive (line side)
430.53(C)(4) ? The branch circuit shall be protected by fuses or inverse time circuit breakers having a rating not exceeding that specified in 430.52 (250%) for the highest rated motor (34FLA) connected to the branch circuit plus an amount equal to the sum of the full-load current ratings of all other motors and he ratings of other loads connected to the circuit....

So breaker calc,
(34*2.5)+34+3+3+2.4+2.4 = 129.2A Maximum

409.20 ? The size of the industrial control panel supply conductor shall have an ampacity not less than 125% of the FLA rating of all resistance heating loads plus 125% of the FLA of the highest rated motor (34FLA) plus the sum of the full-load current ratings of all other connected motors and apparatus based on their duty cycle that may be in operation at the same time.

Wire size calc,
(34*1.25)+34+3+3+2.4+2.4 = 86.7A ? round up to #2 (115A Max)

The FLA of the system is 78.8A so the 80% rated breaker must be higher than 98.5A

98.5A < Breaker Size < 115A

So final breaker size can be 100A

Does anybody see ANY problem with this methodology??
 
I don't know where you get 3 amps from for a 1 HP motor 480 volts? Edit to add: a 1.5HP motor is a 3 amp motor, maybe that is what you actually have?

There are two things you are looking at here and they are not really all that related to each other.

First is minimum ampacity of the supply conductor, which will basically be table 430.250 values for the motors and the nameplate ratings of the drives. You must have a conductor with ampacity of 125% of largest motor plus 430.250 values for all other motors and 100% of the drive ratings.

So I get 34x1.25=42.5 for the largest motor plus 34.0+(2.1x2) other motors plus (2.4x2) drives for a total of 85.5 rounded to 86 amps is your minimum feeder ampacity.

Second thing is short circuit and ground fault protective device setting. There is no minimum. If a 50 amp breaker would happen to hold while starting/running these loads there is no NEC violation, but you will likely want at very least a 90 amp breaker simply because of the 86 amps minimum ampacity required of the supply conductors. The 250% of largest motor plus all other loads is the maximum inverse time circuit breaker allowed, with exceptions if it does not allow for starting.
 
are you trying to size the main CB on a UL508a control panel?
if so, you have to do it according to the rules in UL508a and not the NEC, although they are pretty close.
 
petersonra said:
are you trying to size the main CB on a UL508a control panel?
if so, you have to do it according to the rules in UL508a and not the NEC, although they are pretty close.
Click to expand...

I based my response on NEC, but good point. I also believe unless OP is qualified to build UL508a panels he is likely stuck with NEC being the standard.

If he is qualified to build UL508a panels he likely would not have come here with the questions he has either.
 
kwired said:
I based my response on NEC, but good point. I also believe unless OP is qualified to build UL508a panels he is likely stuck with NEC being the standard.

If he is qualified to build UL508a panels he likely would not have come here with the questions he has either.
Click to expand...

he did mention he has some kind of beef with UL which suggested some kind of UL standard was in play.

as a practical matter you can make the feeder breaker as big as you want as long as the conductor ampacity matches the breaker.
 
scottmarston said:
I'm kind of getting sick of going back and forth with UL (I realize they could care less about NEC) ...

430.53(C)(4) – The branch circuit shall be protected by fuses or inverse time circuit breakers having a rating not exceeding that specified in 430.52 (250%) for the highest rated motor (34FLA) connected to the branch circuit plus an amount equal to the sum of the full-load current ratings of all other motors and he ratings of other loads connected to the circuit....
Click to expand...

Its a semantics issue, there is no conflict. You are misinterpreting the term "branch" in the context of this application. The BRANCH breaker is the LAST breaker before the load. The main breaker in a control panel is not the branch breaker for the motors, unless you are somehow getting away with not having an OCPD for each separate motor, which UL would not allow you to do in a control panel anyway. So technically the Main of a control panel is a FEEDER, although not really, because it is not going to protect the up stream conductors, so they will need a circuit breaker anyway and THAT one is technically the Feeder. In fact because of that, UL and the NEC do not require that you have a Main breaker inside of the control panel at all. That's up to you, so sizing is up to you as well.

Your method is fine for determining a minimum size, but you can go much larger depending on what size conductors you are willing to use, plus the ratings and listings of the components.
 
kwired said:
I don't know where you get 3 amps from for a 1 HP motor 480 volts? Edit to add: a 1.5HP motor is a 3 amp motor, maybe that is what you actually have?

There are two things you are looking at here and they are not really all that related to each other.

First is minimum ampacity of the supply conductor, which will basically be table 430.250 values for the motors and the nameplate ratings of the drives. You must have a conductor with ampacity of 125% of largest motor plus 430.250 values for all other motors and 100% of the drive ratings.

So I get 34x1.25=42.5 for the largest motor plus 34.0+(2.1x2) other motors plus (2.4x2) drives for a total of 85.5 rounded to 86 amps is your minimum feeder ampacity.

Second thing is short circuit and ground fault protective device setting. There is no minimum. If a 50 amp breaker would happen to hold while starting/running these loads there is no NEC violation, but you will likely want at very least a 90 amp breaker simply because of the 86 amps minimum ampacity required of the supply conductors. The 250% of largest motor plus all other loads is the maximum inverse time circuit breaker allowed, with exceptions if it does not allow for starting.
Click to expand...

Sorry...Should have been 1.5HP motors on the motor starter.
 
petersonra said:
he did mention he has some kind of beef with UL which suggested some kind of UL standard was in play.

as a practical matter you can make the feeder breaker as big as you want as long as the conductor ampacity matches the breaker.
Click to expand...

Doesn't 430.52(C)(1) prevent you from making the panel breaker, "as big as you want"?
 
scottmarston said:
Doesn't 430.52(C)(1) prevent you from making the panel breaker, "as big as you want"?
Click to expand...
I will add one more thing to what has already been said. 430.52 is titled "Rating or Setting for Individual Motor Circuit". I think the title alone disqualifies that section from applying to a device supplying a multiple motor installation.
 
If I may ask what are you trying to protect with this breaker? Would each motor have its own combination motor starter or motor starter?
Then in what manor is power distributed to those starters? Shouldn't that be sized to carry the load for those starters? Wouldn't it be safe to assume that the main breaker would be sized to protect you means of distribution?
I would then just consider the current carrying conductors between the main breakers to where the power is distributed to the starters.
 
The missing piece of this puzzle is the branch circuit short circuit protection for the individual motors. None of the branch circuits leaving the panel can be protected by the 100A MCB unless they are #3 or larger.

Jraef is right. You need a separate fuse or breaker for each motor. The starter overloads don't provide the short-circuit and ground-fault protection required by 430.52.
 
JoeStillman said:
The missing piece of this puzzle is the branch circuit short circuit protection for the individual motors. None of the branch circuits leaving the panel can be protected by the 100A MCB unless they are #3 or larger.

Jraef is right. You need a separate fuse or breaker for each motor. The starter overloads don't provide the short-circuit and ground-fault protection required by 430.52.
Click to expand...

The short-circuit and ground-fault protection is integrated in with my Allen Bradley 190S Compact Combination Starters. Each one comes with a 140M Motor Circuit Protector built in unless I am mistaken?
 
templdl said:
If I may ask what are you trying to protect with this breaker? Would each motor have its own combination motor starter or motor starter?
Then in what manor is power distributed to those starters? Shouldn't that be sized to carry the load for those starters? Wouldn't it be safe to assume that the main breaker would be sized to protect you means of distribution?
I would then just consider the current carrying conductors between the main breakers to where the power is distributed to the starters.
Click to expand...

The load side of the feeder circuit breaker (located inside the panel) is connected to a power distribution block, so I'm attempting to protect those wires. The load wires of the distribution block feed the VFDs and combination motor starters (what each motor has that isn't equipped with a VFD) and fall within the 10ft. tap-rule.
 
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kwired said:
I will add one more thing to what has already been said. 430.52 is titled "Rating or Setting for Individual Motor Circuit". I think the title alone disqualifies that section from applying to a device supplying a multiple motor installation.
Click to expand...

430.53(C)(4) Several Motors or Loads on One Branch Circuit - Other Group Installations, tells you to refer to 430.52 to size the highest rated motor plus the sum of the rest of the motors...unless I'm misunderstanding what you're meaning?
 
scottmarston said:
430.53(C)(4) Several Motors or Loads on One Branch Circuit - Other Group Installations, tells you to refer to 430.52 to size the highest rated motor plus the sum of the rest of the motors...unless I'm misunderstanding what you're meaning?
Click to expand...
I did not go back and re-read what has been posted so forgive me if I'm off base. 430.53 if for multi motors and applies to feeders, but each individual motor circuit is covered by 430.52. Additional protection will usually be required for each individual motor with the exception of cases where multiple motors can all be on the same branch circuit, usually going to be cases of fractional HP motors.
 
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