Motor breaker sizing

[bdrwirenut]

If I do a motor calculation for a 50 hp motor @ 460 volt the branch circuit overcurrent protection device is calculated by flc 65a times 250 %= 162.5a which would be a 175a standard breaker size correct? If you use the square d slide chart for motors they call for a 100a breaker for this motor where do they get the amp rating from. This question has come to my attention because several local panel builders use the 100a rating in their lift station control panels and I was wondering where the calculation came from.The same panel builders seem to apply step #6 of the motor calculation to the feeder breaker size but use the slide chart breaker size for the branch circuit breaker instead of step #4 could someone please explain this to me? Thank you for any help with this.

 

[LarryFine]

. . . several local panel builders use the 100a rating in their lift station control panels . . .

I would ask if they're having any starting issues with the 100a breakers.

 

[jim dungar]

If I do a motor calculation for a 50 hp motor @ 460 volt the branch circuit overcurrent protection device is calculated by flc 65a times 250 %= 162.5a which would be a 175a standard breaker size correct? If you use the square d slide chart for motors they call for a 100a breaker for this motor where do they get the amp rating from. This question has come to my attention because several local panel builders use the 100a rating in their lift station control panels and I was wondering where the calculation came from.The same panel builders seem to apply step #6 of the motor calculation to the feeder breaker size but use the slide chart breaker size for the branch circuit breaker instead of step #4 could someone please explain this to me? Thank you for any help with this.

Where does it say that you must use 250% for sizing a breaker?

The Square D slide-chart is based on the FLA from the NEC.

 

[bdrwirenut]

You are correct in asking where it says you must use 250% but it does state in 430-152 that 250% is the maximum you can use. I understand that the square d slide chart is based on the nec fla chart but where is the calculation they use to determine the breaker sizes they use on their chart, does some guy just randomly pick a number and say that should work? I also understand by them using a smaller breaker than if calculated using 250% you actually get better protection but who regulates what they use in their product? I guess my point was on a test if you were asked this same question ( What size is the branch circuit overcurrent device for a 50 hp 460 v 3 ph motor? Would you would be marked wrong for using a 100 a breaker?) I need to see the math that is used to figure out their calculation.

 

[jim dungar]

You are correct in asking where it says you must use 250% but it does state in 430-152 that 250% is the maximum you can use. I understand that the square d slide chart is based on the nec fla chart but where is the calculation they use to determine the breaker sizes they use on their chart, does some guy just randomly pick a number and say that should work? I also understand by them using a smaller breaker than if calculated using 250% you actually get better protection but who regulates what they use in their product? I guess my point was on a test if you were asked this same question ( What size is the branch circuit overcurrent device for a 50 hp 460 v 3 ph motor? Would you would be marked wrong for using a 100 a breaker?) I need to see the math that is used to figure out their calculation.

There is no math that you can see.

The breaker needs to be able to hold the inrush of the motor during starting. Typically a motor has about 8-10 times inrush (the actual range can be found on the motor by referring to its starting code letter). So the 100A breaker needs to hold 520-650A instaneously. According to their Digest this breaker will not trip instantly until the current is over 900A which is good for this motor. Now we need to look at the breaker trip curve versus the overload trip element. A standard NEMA overload realy is 6X current for 20 sec (there are otherpossibilities). The trip curve for this FA breaker shows it will hold 390A for about 8 secs, so it looks like there could be a selectivity conflict between the overlod relay and this breaker. When I plot a standard 65A overload relay with this FA breaker the actual conflict occurs at 318A or 4.9 times FLA. The end result is that except for the selectivity at true locked rotor conditions this 100A breaker should be useable with a 50HP 480V motor.

 

[Jraef]

There is no math that you can see.

The breaker needs to be able to hold the inrush of the motor during starting. Typically a motor has about 8-10 times inrush (the actual range can be found on the motor by referring to its starting code letter). So the 100A breaker needs to hold 520-650A instantaneously. According to their Digest this breaker will not trip instantly until the current is over 900A which is good for this motor. Now we need to look at the breaker trip curve versus the overload trip element. A standard NEMA overload realy is 6X current for 20 sec (there are otherpossibilities). The trip curve for this FA breaker shows it will hold 390A for about 8 secs, so it looks like there could be a selectivity conflict between the overlod relay and this breaker. When I plot a standard 65A overload relay with this FA breaker the actual conflict occurs at 318A or 4.9 times FLA. The end result is that except for the selectivity at true locked rotor conditions this 100A breaker should be useable with a 50HP 480V motor.

That's of course the right way to do it, although most of us don't bother. When you have a motor starter with an OL relay AND a thermal-magnetic (inverse time) CB, the CB need only be the short circuit protection device; the OL relay becomes the long time over current protection. So the thermal trip elements of the CB become essentially irrelevant, other than having the potential for nuisance tripping as mentioned above. But most thermal-mag breakers don't come with adjustable (or widely adjustable) magnetic trips, so getting one that will hold in on startup means having to use a larger breaker. That's why starter manufacturers use mag-only breakers; they can get the higher trip settings without having to buy a bigger breaker. But we in the field can't use mag-only breakers; we have to use thermal-mag. So what most of us do is to select a thermal-mag breaker based on the magnetic trips only, while still fitting within the 400% (300% if over 100A) allowed in exception 2 of 430.52.C.

 

[jim dungar]

In the example we are discussing Square D is saying that a thermal-magnetic 100A FA breaker will work with a 50HP 480V motor. They have been recommending this combination for at least 30yrs so I am sure that is does work. One reason they do this is the frame size of this breaker. Until recentrly Square D would have required a 250A frame breaker housing if they used a breaker larger than 100A trip. A 250A frame takes up a lot more real estate than does a 100A frame so the starter enclosure would need to be bigger.

 

[hillbilly]

Very informative post

steve