3 Phase Motor Breaker Sizing

Status
Not open for further replies.
USMC1302

USMC1302

Senior Member
Location
NW Indiana
The branch circuit breaker trips intermittently for a 3 phase motor circuit. It is a 5 Hp, 208 V motor FLA 12.4 SF 1.15. I believe it is a severe duty rated motor. The branch breaker is 30 A, and the contractor has megged the wiring, changed the motor and the breaker, and the problem persists. I don't have access to NEC tables, but understand they must be used for conductor sizing and SC/GF protection. I suspect the breaker is sized too tight for in-rush, and a 35 A breaker is allowed. I'm trying to learn Art 430, so could someone verify if I'm heading in the right direction.
 
USMC1302 said:
The branch circuit breaker trips intermittently for a 3 phase motor circuit. It is a 5 Hp, 208 V motor FLA 12.4 SF 1.15. I believe it is a severe duty rated motor. The branch breaker is 30 A, and the contractor has megged the wiring, changed the motor and the breaker, and the problem persists. I don't have access to NEC tables, but understand they must be used for conductor sizing and SC/GF protection. I suspect the breaker is sized too tight for in-rush, and a 35 A breaker is allowed. I'm trying to learn Art 430, so could someone verify if I'm heading in the right direction.
Click to expand...

See table 430.250 and get the NEC full load current which is 16.7. Then look at 430.52 and you will see that an inverse time breaker can be 250% which is 41.75. Next size up is allowed here so you could use a 45 amp breaker. The conductors must be sized for 16.7 X 125% for ampacity of 21.
Note that the name plate current is not relevant for the branch circuit short circuit/ground fault protection. Name plate current is only used to size the OL protection.
 
Last edited:
It's hard if you only have a circuit breaker with fixed instantaneous trip settings! What type of circuit breaker are you using, if i may ask?
 
If you are dealing with a " High Intertia " load, you are in trip class 20 to 30 range for your starter. I would be curious to see the inrush on a recording meter such as Fluke 87 and also see where the current goes when it ramps up. VFDs have helped me in this case.
 
When is it tripping, during startup or when it has been running for some time?

During startup - many cases a 30 amp breaker likely holds here even though code does allow higher setting. Mostly has to do with total circuit impedance (including source impedance as well as conductors) limiting the starting current. If you have a really stout source and a pretty short circuit length, you may have some issues with a 30 amp breaker.

If it is tripping after some run time has passed, 30 amps is well over motor full load rating and it should not trip here, that means something else is happening like possibly a bad connection either on output terminals or on the bus to breaker connection is causing excess heating and eventually causing thermal trip to occur, or other internal breaker failure causing excess heating.
 
It is a Square-D breaker, although I have not been out to see what type/style. I tried to ask as many questions as possible to try and get better info on what the cause might be, and it appears this occurs during first use. The breaker has been changed once already. I came up with the same conclusion Texie provided. I'm going out tomorrow to gather more info, and will report back.
 
USMC1302 said:
It is a Square-D breaker, although I have not been out to see what type/style. I tried to ask as many questions as possible to try and get better info on what the cause might be, and it appears this occurs during first use. The breaker has been changed once already. I came up with the same conclusion Texie provided. I'm going out tomorrow to gather more info, and will report back.
Click to expand...

Normally, you have around 1 second trip time window, when your motor inrush is around 6 times the rated amps (120 - 180A). An FA 30 circuit breaker will be better at about 5 seconds trip delay.
 
Update....I did not get to review on-site, but was able to find drawings. Another lesson to find out what was SUPPOSED to be there in the first place. Surprise, 35A breaker was called out. Thanks everyone.
 
USMC1302 said:
Update....I did not get to review on-site, but was able to find drawings. Another lesson to find out what was SUPPOSED to be there in the first place. Surprise, 35A breaker was called out. Thanks everyone.
Click to expand...

35 amp breaker is what is suggested on Square D motor calculator (slide chart). NEC allows 45 amp breaker before it has to fail to start then you can possibly increase it some.

That said many cases a 20 or 25 might hold, all depends on conditions of your install and how much starting inrush current you are actually experiencing. Stout source and short circuit length, will tend to need higher breaker setting than weaker source and/or longer circuit length as those items alone will have current limiting effects on how much starting current you will see.
 
If you are dealing with a " High Intertia " load, you are in trip class 20 to 30 range for your starter. I would be curious to see the inrush on a recording meter such as Fluke 87 and also see where the current goes when it ramps up. VFDs have helped me in this case.




mybkexperience
 
Last edited:
Erin7798 said:
If you are dealing with a " High Intertia " load, you are in trip class 20 to 30 range for your starter. I would be curious to see the inrush on a recording meter such as Fluke 87 and also see where the current goes when it ramps up. VFDs have helped me in this case.
Click to expand...

You cannot increase trip class without a motor specifically built for it. Trip class is 20 under NEMA unless they name plate says otherwise. IEC the default is 10. The curve is slightly different but close. Low values are for things like hermetically sealed motors. I’ve only seen class 30 on crusher duty motors.
 
For 3-phase loads, you divide the VA by the nominal voltage and by the square root of three (approximately 1.732). If your total 3-phase load in a 480V system is 50,000VA, what size breaker do you need? 50,000VA ÷ (480V × 1.732) = 60.2A. The next size up is 70A
 
paulengr said:
You cannot increase trip class without a motor specifically built for it. Trip class is 20 under NEMA unless they name plate says otherwise. IEC the default is 10. The curve is slightly different but close. Low values are for things like hermetically sealed motors. I’ve only seen class 30 on crusher duty motors.
Click to expand...
Yes, but that’s for the overload relay. He said it is the breaker that is tripping.

We still don’t know if it trips on startup or after running for a while. If it trips within a second on startup, it’s the instantaneous trips of the breaker. If this is a QO panelboard breaker, some of those have fixed magnetic trips set at just 6-7x the rating, as opposed to the typical 10x for everyone else. That makes them more likely to nuisance trip, especially if not sized at the maximum allowable by the NEC.

If the BREAKER is tripping after the motor has been running more than a few seconds, then there is something wrong with the motor.
 
rosenik said:
For 3-phase loads, you divide the VA by the nominal voltage and by the square root of three (approximately 1.732). If your total 3-phase load in a 480V system is 50,000VA, what size breaker do you need? 50,000VA ÷ (480V × 1.732) = 60.2A. The next size up is 70A
Click to expand...
Hmmm, not sure this would allow for your 1.25 factor. It definitely would not account for motor inrush.
Might want to re-think this.
 
rosenik said:
For 3-phase loads, you divide the VA by the nominal voltage and by the square root of three (approximately 1.732). If your total 3-phase load in a 480V system is 50,000VA, what size breaker do you need? 50,000VA ÷ (480V × 1.732) = 60.2A. The next size up is 70A.
redacted (mod note: removed link that although probably not malware was at least unrelated and did not have proper notice given.)
Click to expand...
 
Last edited by a moderator:
Status
Not open for further replies.
Top