Motor Protection

[llewis]

I have a motor that is only protected by a Breaker with a switch, however the breaker is tripping, I believe the breaker is not able to handle the inrush current, it was originally sized at about 125% above the FLA. It is a QOB which is an inverse time type breaker and according to NEC Table 430.52 I am able to go 250% above. I had seen a similar comment recently and one moderator suggested even thogh the NEC would allow 250%, he suggested only 150%. Am I correct in looking at the code this way? is there any suggestions otherwise?

 

[augie47]

For most motors the 250% would be correct for the branch-circuit-short-circuit and ground-fault protection per 430.52.
Without added details there is concern when say "protected only by". How is overload protection addressed ?

 

[Aleman]

I had a motor with a high inrush that required a D curve breaker to keep from tripping at start up. And I went through 2 brands of D curve. Ended up with a Siemens
which was a little slower than the Eaton I tried. Sized at 125% FLA.

How large is your motor that is running on a switch?

 

[ActionDave]

I agree with others. More info about the motor is needed.

 

[llewis]

Motor Protection

This is an existing motor at a new customers facility, the only protection is the breaker, which as of now is a 20A QOB, it is a 2HP motor @ 230v single phase with an SFA of 14.6A it is a Capacitor start motor.

 

[ActionDave]

This is an existing motor at a new customers facility, the only protection is the breaker, which as of now is a 20A QOB, it is a 2HP motor @ 230v single phase with an SFA of 14.6A it is a Capacitor start motor.

A motor that size usually has an internal overload. If it does a bigger breaker would be okay. I would expect it to start on a 20A breaker though. What is the motor driving? Did anybody test amps?

 

[ActionDave]

...according to NEC Table 430.52 I am able to go 250% above. I had seen a similar comment recently and one moderator suggested even thogh the NEC would allow 250%, he suggested only 150%. Am I correct in looking at the code this way? is there any suggestions otherwise?

In regards to this question in a general sense- When a motor has proper thermal protection it is never a problem to go with the max short circuit/ground fault protection in 430.52.

430.32 values, on the other hand, are better followed at minimum allowed.

 

[Jraef]

If the motor has internal thermal protection, the nameplate must clearly state that. The term "clearly" is up to interpretation though. Some just use the letters "TP" for Thermally Protected, your mileage may vary.

The point is however that if your motor does NOT explicitly state that it has internal protection, you must assume it does not and use external means. That would limit your breaker to 125% of FLC, which often means it will not hold in during starting current or magnetic inrush. That's why you use a higher rated CB and an OL relay.

 

[rlundsrud]

If the motor has internal thermal protection, the nameplate must clearly state that. The term "clearly" is up to interpretation though. Some just use the letters "TP" for Thermally Protected, your mileage may vary.

The point is however that if your motor does NOT explicitly state that it has internal protection, you must assume it does not and use external means. That would limit your breaker to 125% of FLC, which often means it will not hold in during starting current or magnetic inrush. That's why you use a higher rated CB and an OL relay.

I am certain that I am misunderstanding you, but are you saying that you could run a non thermally protected motor without overloads if you limit the OCPD to 125%? I thought you could only skip TP if it was a fractional HP motor that was only started manually.

 

[kwired]

I am certain that I am misunderstanding you, but are you saying that you could run a non thermally protected motor without overloads if you limit the OCPD to 125%? I thought you could only skip TP if it was a fractional HP motor that was only started manually.

Yes you can, but most standard thermal magnetic breakers usually will give you startup issues if only sized at 125%.

I have a Square D sliding chart motor calculator and for the OP's 2 HP motor they suggest a 25 amp breaker. NEC would permit up to 30 amp breaker, but could be increased if that won't let it start.