Easy Inverse time breaker question

[W@ttson]

I am familiar with table 430.52 and its requirements and the differences between the types of short circuit protection. This question is a matter of true life real world application:

Motor FLA = 48A
This is an AC Polyphase motor other than wound rotor motor therefore:
Max MCCB (inverse time breaker) setting/size = 2.5*48 = 120A.

I now go to the breaker store and ask the guy for a catalog he gives me the attached chart. With out thinking I look to choose 125A size because it is the next size up from my calculation. But then I notice, this particular MCCB has options for adjustable thermal (inverse time) and adjustable magnetic (instantaneous) trips. The range for the Mag trip is 625A - 1250A. If I look at my FLA and multiply by 800% which is under the Instantaneous (MCP breaker) column of 430.52 I get a maximum rating of 384A. That gets me wondering. Should I just buy the Max continuous amps of 70 breaker since the mag trip adjustment is in the range for of the maximum allowed instantaneous setting?

I would never have second guessed the 125A frame selection if I didn't see the Mag trip range.

 

[Ingenieur]

No
the 800% is for mag only
you have an adjustable it

even on a fixed it
125 rating
8xfla ~ 400 A
or 400/125 ~ 300% x in
this is no where near the avg instant range of 10-13 x in of most it cb's
it is in the thermal range
my guess trip time is 10-20 sec or more, far from instant

 

[W@ttson]

I told you it was an easy one!

No
the 800% is for mag only
you have an adjustable it

even on a fixed it
125 rating
8xfla ~ 400 A
or 400/125 ~ 300% x in
this is no where near the avg instant range of 10-13 x in of most it cb's
it is in the thermal range
my guess trip time is 10-20 sec or more, far from instant

 

[Jraef]

But in reality, you started off at the MAXIMUM size breaker you CAN use for a motor with an FLA of 48A. The NEC does not say you MUST use the maximum size...

I would generally use a smaller breaker for that, so that the mag trip adjustment range falls into the range you want for that motor: 800% to a maximum (if demonstrated to be nuisance tripping) of 1300%, then if it is an Energy Efficient motor, 1700%. If it is not an EE motor, I would have picked the 70A breaker from that chart, of if it is a EE motor, the 80A.

 

[Ingenieur]

My selection would be based on the starting method: vfd, fvnr, etc
and the LOAD profile
motor kva may factor in to a lesser degree

the cb is more or less for the feeder protection
the ol for the motor

the starter itself may have a cb or motor protection
with a fvnr you may have nuisance tripping with the 70
say the starting current is 800% or 400 A
set instant at 500 or 1000%
400/70 = ~600%
it may thermal trip in a sec or two, an issue with a high inertia load that takes some time to get spooled up

guess that is why it's called engineering judgement

 

[W@ttson]

Yes I agree. This will actually be for a high intertial load and will be across the line started. I like jaref's answer too, it makes me see that my 70A questioning had validity. I'll look through it a bit deeper.

My selection would be based on the starting method: vfd, fvnr, etc
and the LOAD profile
motor kva may factor in to a lesser degree

the cb is more or less for the feeder protection
the ol for the motor

the starter itself may have a cb or motor protection
with a fvnr you may have nuisance tripping with the 70
say the starting current is 800% or 400 A
set instant at 500 or 1000%
400/70 = ~600%
it may thermal trip in a sec or two, an issue with a high inertia load that takes some time to get spooled up

guess that is why it's called engineering judgement

 

[adamscb]

At the plant I work at, we always like to choose the biggest breaker we can while still being kosher with code. This way we don't have to change out the circuit breaker when we do upgrade to a NEMA Energy Efficient motor. After install we measure the inrush using an ammeter, and set it appropriately.