Struggling with NEC on Thermal Mag Breaker adjustable Settings

[MrJLH]

So I need some help with Thermal Mag breakers

NEC 430.25 states that a thermal mag should be sized to 250% of FLA.

How does the adjustable magnetic setting come into this to meet code?

My motor FLA is 144A with a 300A ITE JL type breaker. The setting is turned all the way up and creates a conflict with coordination on the upstream switch rack.
Thanks

 

[Ingenieur]

not sure about code implications but this is what I do

get starting kva code from mtor
calc starting current, add 20%
set it there
start motor a few times (get it up to operating temp)
if it reliably starts good to go
if not, bump it up a notch 10% and repeat

 

[Jraef]

So I need some help with Thermal Mag breakers

NEC 430.25 states that a thermal mag should be sized to 250% of FLA.

How does the adjustable magnetic setting come into this to meet code?

My motor FLA is 144A with a 300A ITE JL type breaker. The setting is turned all the way up and creates a conflict with coordination on the upstream switch rack.
Thanks

The mag can be adjusted to 1100% of the motor FLA, or if necessary, 1300% (most new motors will not hold in at 1100%), or if it is an Energy Efficient motor and 1300% is proven to not be enough to hold in, 1700% max.

You have a 300A breaker, so the mag trips will be adjustable up to 3000A. Your motor is 144A, so 1300% would be 1872A, 1700% would be 2448A. If it is cranked all the way up to 3000A, that's a Code violation.

 

[kingpb]

So I need some help with Thermal Mag breakers

NEC 430.25 states that a thermal mag should be sized to 250% of FLA.

How does the adjustable magnetic setting come into this to meet code?

My motor FLA is 144A with a 300A ITE JL type breaker. The setting is turned all the way up and creates a conflict with coordination on the upstream switch rack.
Thanks

It would be helpful if you could provide more detail on the "conflict" in question, otherwise speculation will prevail and Alice's rabbit hole will be visited once again.

 

[MrJLH]

The mag can be adjusted to 1100% of the motor FLA, or if necessary, 1300% (most new motors will not hold in at 1100%), or if it is an Energy Efficient motor and 1300% is proven to not be enough to hold in, 1700% max.

You have a 300A breaker, so the mag trips will be adjustable up to 3000A. Your motor is 144A, so 1300% would be 1872A, 1700% would be 2448A. If it is cranked all the way up to 3000A, that's a Code violation.

My understanding that applies to Instantaneous trip circuit breakers only

 

[MrJLH]

It would be helpful if you could provide more detail on the "conflict" in question, otherwise speculation will prevail and Alice's rabbit hole will be visited once again.

I'm just not sure about the adjustable setting on thermal mag breakers to meet NEC 430.52.

I understand that the max rating for a thermal mag breaker for a motor is 250% of FLA, but there seems to be no mention of the max adjustable setting.

 

[Jraef]

I'm just not sure about the adjustable setting on thermal mag breakers to meet NEC 430.52.

I understand that the max rating for a thermal mag breaker for a motor is 250% of FLA, but there seems to be no mention of the max adjustable setting.

That's because not all T-M breakers have adjustable max trips.

Most people just accept that when adjustable mag trips are available, the same rules for I-T breakers can apply. The basic concept remains the same.

 

[Ingenieur]

table 430.52 gives the initial maximum setting based on motor type
squirrel cage (non B eff) 800% MAXIMUM

you can only adjust up by exception, ie, won't hold
then in increments up to 1300%

settings above 800% must be made after engineering evaluation


get the lr kva code
say 125 hp code J 7.1-7.99 kva/hp, fla 156
use mid range 7.5
7500 x 125 / (sqrt3 x 480) = 1130 A x 20% = 1350
use 1200 or 1500 as you starting point
1200 A = 770%, 1500 A = 960%, call it 800% and 1000%
so start at 1200 A/800%, if it won't run bump it up 1 step