Bus Bar Sizing And Circuit Breaker Sizing

[Sb1234]

As per NEC 430-24 to size a busbar for a switch board with all motor loads the following formula is to be used -

(25%of FLA of largest motor) + ( Sum of FLA of all motor )

Also as per NEC 430-52 and 53 and NEC -152 for incomer circuit breaker selection of the switchboard the following formula should be used .

(250% of FLA of largest motor) + ( Sum of FLA of all other motors )

From the above formula it seems that the bus bar rating of a system will be less than the required circuit breaker ratings in terms of ampere.

But it does not seem to be common. Is it okay to put an incomer circuit breaker with higher ampere rating than bus bar ampere rating.I generally see that circuit breaker ratings is switchboard are equal to bus bar rating.

 

Any comment is welcome .

 

 

Regards

[jraef]

As per NEC 430-24 to size a busbar for a switch board with all motor loads the following formula is to be used -

(25%of FLA of largest motor) + ( Sum of FLA of all motor )

Also as per NEC 430-52 and 53 and NEC -152 for incomer circuit breaker selection of the switchboard the following formula should be used .

(250% of FLA of largest motor) + ( Sum of FLA of all other motors )

From the above formula it seems that the bus bar rating of a system will be less than the required circuit breaker ratings in terms of ampere.

But it does not seem to be common. Is it okay to put an incomer circuit breaker with higher ampere rating than bus bar ampere rating.I generally see that circuit breaker ratings is switchboard are equal to bus bar rating.

 

Any comment is welcome .

 

Regards

Hi Sb1234,

 

First off, realize that 430.52 and 53 are referring to BRANCH protection devices, not Switchboards. Look at the title to .52, it is saying INDIVIDUAL motor circuits, the .53 says "GROUP" circuits on one BRANCH. So neither of those technically apply to a "Switchboard".

 

Secondly, getting away from the "Switchboard" semantics issue, you are still making a classic error. I'm not being condescending, what you are doing is the most common mistake I see made by people reading the NEC when trying to design components into a system.

 

The NEC is almost always speaking of MINIMUMS or MAXIMUMS, not BEST practices. It is still up to you to select the best practice for your system design within the limits of the NEC rules. Lets take an example to illustrate the point.

 

Say you have a motor lineup that is 820A FLC total, and the largest motor is 360FLA. Your conductor rating then must be at least 820 + (360 * .25), so 910A. Bus bar comes in certain sizes and although 1000A is going to be OK, it does not allow for any future expansion, so you jump to 1600A or even 2000A bus. This satisfies that MINIMUM requirement of the NEC, but in fact you could even jump to 3000A bus if you wanted to, so long as it is not less than 910A. Does that mean you automatically put a 1600A or 2000A circuit breaker on it if you used 1600 or 2000A bus though? Lets see.

 

In a multi-motor circuit (per 430.53), your breaker must be NO LARGER than 250% of largest motor load plus the remaining FLCs, so (820-360) + (360 * 2.5) = 1360A. If you put a 1600A or 2000A main breaker on that bus bar, it would violate the MAXIMUM rating required by the NEC. You would use a breaker with trip settings at 1300A. If on the other hand you did decide to use 1000A bus, you would not put in a 1300A main breaker, you would only be able to use a 1000A. The 1000A breaker would still satisfy the not more than 1360A rule. The 1000A breaker may trip on you if the 360A load is the last one started, but that is your problem. The NEC has been satisfied. So you see, it is still up to YOU to design the system within the rules. If the 1000A breaker is not going to work, you need to go to 1300A, which means you have to bump up to 1600A bus.

 

Now back to the "Switchboard" issue. Switchboards are distribution devices, so the sizing and ratings of main circuit protective devices are different than they are for "branch" circuits. If you are building a piece of distribution equipment (and it is not a "Motor Control Center"), then Articles 220 through 240 apply. So let's say you have a Switchboard that is feeding your entire plant. If the loads you are looking at in the above example are, let's say, 25% of the total plant load, your Switchboard would need to be roughly rated for 4000A. So you would obviously NOT put a 1300A main breaker on it. What you would do however, is put on a 4000A main breaker, then use a 1500A feeder breaker and conductors to feed your motor controller system. Then if it is in another building or out of sight of the Switchboard, you would need ANOTHER main breaker on that system, which you would then set at 1300A. If by chance your motor controller was in the same room as the switchboard, you could technically just have a 1300A feeder breaker and put a non-fused disconnect in your motor controller. And if you were in the same room and the Switchboard had only the 4000A Main breaker and 4 other feeders, you could technically not even have a main switch device on the motor controller at all! (That last part is called the "6 hand rule").

 

By the way, if it is an official "Motor Control Center", then it is covered under 430.92. However you will see a relative consistency in the rules in all of these places.

 

Motor loads just provide for an exception to the rules in 220-240 because they acknowledge that it takes extra current for a brief time to start them. Article 430 establishes the limits within that can be taken into consideration.

[Sb1234]

Hi Sb1234,

 

First off, realize that 430.52 and 53 are referring to BRANCH protection devices, not Switchboards. Look at the title to .52, it is saying INDIVIDUAL motor circuits, the .53 says "GROUP" circuits on one BRANCH. So neither of those technically apply to a "Switchboard".

 

Secondly, getting away from the "Switchboard" semantics issue, you are still making a classic error. I'm not being condescending, what you are doing is the most common mistake I see made by people reading the NEC when trying to design components into a system.

 

The NEC is almost always speaking of MINIMUMS or MAXIMUMS, not BEST practices. It is still up to you to select the best practice for your system design within the limits of the NEC rules. Lets take an example to illustrate the point.

 

Say you have a motor lineup that is 820A FLC total, and the largest motor is 360FLA. Your conductor rating then must be at least 820 + (360 * .25), so 910A. Bus bar comes in certain sizes and although 1000A is going to be OK, it does not allow for any future expansion, so you jump to 1600A or even 2000A bus. This satisfies that MINIMUM requirement of the NEC, but in fact you could even jump to 3000A bus if you wanted to, so long as it is not less than 910A. Does that mean you automatically put a 1600A or 2000A circuit breaker on it if you used 1600 or 2000A bus though? Lets see.

 

In a multi-motor circuit (per 430.53), your breaker must be NO LARGER than 250% of largest motor load plus the remaining FLCs, so (820-360) + (360 * 2.5) = 1360A. If you put a 1600A or 2000A main breaker on that bus bar, it would violate the MAXIMUM rating required by the NEC. You would use a breaker with trip settings at 1300A. If on the other hand you did decide to use 1000A bus, you would not put in a 1300A main breaker, you would only be able to use a 1000A. The 1000A breaker would still satisfy the not more than 1360A rule. The 1000A breaker may trip on you if the 360A load is the last one started, but that is your problem. The NEC has been satisfied. So you see, it is still up to YOU to design the system within the rules. If the 1000A breaker is not going to work, you need to go to 1300A, which means you have to bump up to 1600A bus.

 

Now back to the "Switchboard" issue. Switchboards are distribution devices, so the sizing and ratings of main circuit protective devices are different than they are for "branch" circuits. If you are building a piece of distribution equipment (and it is not a "Motor Control Center"), then Articles 220 through 240 apply. So let's say you have a Switchboard that is feeding your entire plant. If the loads you are looking at in the above example are, let's say, 25% of the total plant load, your Switchboard would need to be roughly rated for 4000A. So you would obviously NOT put a 1300A main breaker on it. What you would do however, is put on a 4000A main breaker, then use a 1500A feeder breaker and conductors to feed your motor controller system. Then if it is in another building or out of sight of the Switchboard, you would need ANOTHER main breaker on that system, which you would then set at 1300A. If by chance your motor controller was in the same room as the switchboard, you could technically just have a 1300A feeder breaker and put a non-fused disconnect in your motor controller. And if you were in the same room and the Switchboard had only the 4000A Main breaker and 4 other feeders, you could technically not even have a main switch device on the motor controller at all! (That last part is called the "6 hand rule").

 

By the way, if it is an official "Motor Control Center", then it is covered under 430.92. However you will see a relative consistency in the rules in all of these places.

 

Motor loads just provide for an exception to the rules in 220-240 because they acknowledge that it takes extra current for a brief time to start them. Article 430 establishes the limits within that can be taken into consideration.

 

Hi jraef

 

Thanks for providing the detailed explanation.

 

Regards