Motor protection

[Dustin Foelber]

Gentlemen
I have a single phase 240v motor with an FLA of 27 amps feeding a stream circulating system. It is fed by a 2 pole 40 with solid 10 awg for about a 100'. At the motor there is a double pole 30 amp rated toggle switch. My gut feeling is that the 2 pole 40 is the right breaker size if it is properly sized to the 125% overcurrent protection for motors but the conductors are only rated at 30 amps. That is problem 1.
Problem to is starter requirements. Are starters (or localized protection) required on the motors or are they primarily used for the control circuit function. I am rusty on my motor codes. I appreciate any feedback.
Thank You
DUstin

 

[Strahan]

Gentlemen
I have a single phase 240v motor with an FLA of 27 amps feeding a stream circulating system. It is fed by a 2 pole 40 with solid 10 awg for about a 100'. At the motor there is a double pole 30 amp rated toggle switch. My gut feeling is that the 2 pole 40 is the right breaker size if it is properly sized to the 125% overcurrent protection for motors but the conductors are only rated at 30 amps. That is problem 1.
Problem to is starter requirements. Are starters (or localized protection) required on the motors or are they primarily used for the control circuit function. I am rusty on my motor codes. I appreciate any feedback.
Thank You
DUstin

Article 430 for motors. 430.247-250 for FLC ratings to be used for conductor sizing. Not sure how you are starting the motor but usually the overloads protect the conductors. Where did you find 125% for OCPD? That may be where you got confused. I do not have my code book at this time but I believe the 125% goes for conductor sizing (for biggest motor plus the sum of the rest).

 

[raider1]

Is this motor a 5 HP motor?

When sizing the branch circuit conductors you start with the table values for full load current from Table 430.248 for single phase motors and not the nameplate FLC.

I found that a 5 HP 230 volt single phase motor has a FLC rating of 28 amps.

So taking the 28 amps and applying the 125% for continuous loads I come up with an ampacity of 35 amps. So we need to find a conductor that has an ampacity of 35 amps or greater.

Now provided that the #10 AWG conductors are not part of an NM or SER cable or are TW and all the terminations are rated for 75 degrees C then you can use the 75 degree column of Table 310.16 for ampacity.

#10 Copper has an ampacity of 35 amps at 75 degrees C so you are permitted to use #10.

Now keep in mind that with a length of 100 amps voltage drop may be a problem and a larger wire may be desired.

As for protection at the motor, the motor requires overload protection, this can be provided by a starter or be integral to the motor such as a thermally protected motor.

Also a disconnecting means is required within sight of the motor.

Chris

 

[Dustin Foelber]

Does this motor need to have another ocpd then the breaker. It has a 2 pole switch for service?
10 at 75 C can go on a 40 amp breaker??

 

[infinity]

Does this motor need to have another ocpd then the breaker. It has a 2 pole switch for service?
10 at 75 C can go on a 40 amp breaker??

Yes, the OCPD is not protecting the #10 wire from overload, it's only providing short circuit and ground fault protection. The motor overloads will protect the condcutors. In this case you could use an even larger CB.

 

[Strahan]

Article 430 for motors. 430.247-250 for FLC ratings to be used for conductor sizing. Not sure how you are starting the motor but usually the overloads protect the conductors. Where did you find 125% for OCPD? That may be where you got confused. I do not have my code book at this time but I believe the 125% goes for conductor sizing (for biggest motor plus the sum of the rest).

In a nut shell

 

[Dustin Foelber]

Thanks for your input. I am still trying to find what code says there must be another type of motor protection other then breaker. Any thoughts?

 

[raider1]

Thanks for your input. I am still trying to find what code says there must be another type of motor protection other then breaker. Any thoughts?

Check out Part III of Article 430. This section is Titled Motor and branch circuit overload protection.

Normally a breaker will provide a circuit with overload, short circuit and ground fault protection, but in the case of motor circuits we separate out the overload protection and have separate overload protection.

So normally the breaker in a motor circuit will provide the short circuit and ground fault protection and a separate overload device will provide the overload protection.

Check out Part IV of Article 430 for the requirements for motor branch circuit short circuit and ground fault protection.

Chris

 

[jim dungar]

Thanks for your input. I am still trying to find what code says there must be another type of motor protection other then breaker. Any thoughts?

There isn't one.

The NEC says you must provide 2 types of protection: oveload and short circuit.

The NEC allows you to use a single protective device (i.e. your breaker) or to provide 2 separate devices.

Because the NEC allows a lot more flexibility if 2 separate devices are used, this has become the industry norm.

 

[kwired]

circuit breakers often will not provide overload protection because they will trip too soon while starting the motor if they are sized to provide 125% overload protection.

if you look at Table 430.52 and the exceptions you could have a possible 400% of full load current protective device in some cases to allow for the motor to start. If a motor was allowed to run at this current level it would not last long at all before the smoke is let out.

many single phase motors have built in thermal overload if yours does you only need to worry about short circuit and ground faults. If this is a 5hp motor as suggested it may be many breaker manufacturers have recommended sizes for different horsepower and 60 amp breaker is common for 5 hp 1 ph.