Sizing motor feeder wires correctly

[mario8449]

I am confused when it comes to sizing feeder cables going out to a motor from a starter. I have always done 125% of whatever is full load amps on the motor but just recently I was told that I should actually size the motor feeders based on size of the MCP. I will list an example below. The system I am considering for the example below is an AB MCC with a bucket composed of a MCP and starter.

100 A MCP (instantaneous is the only thing that can be changed)
AB Size 3 starter
30 HP Motor (say 36 FLA for this example)

36A x 1.25 = 45 A

Therefore, does my feeder going to the motor from the starter need to protect for 45 A or does it need to protect for 100 A?

Thanks for the help in advance.

 

[david luchini]

I am confused when it comes to sizing feeder cables going out to a motor from a starter. I have always done 125% of whatever is full load amps on the motor but just recently I was told that I should actually size the motor feeders based on size of the MCP. I will list an example below. The system I am considering for the example below is an AB MCC with a bucket composed of a MCP and starter.

100 A MCP (instantaneous is the only thing that can be changed)
AB Size 3 starter
30 HP Motor (say 36 FLA for this example)

36A x 1.25 = 45 A

Therefore, does my feeder going to the motor from the starter need to protect for 45 A or does it need to protect for 100 A?

Thanks for the help in advance.

The conductor size can be sized based on 125% of the motor full load current, per 430.22(A).

However, the full-load current should be from the Table value (eg, T430.250) rather than the nameplate value. So for a 30HP, 460V motor, you would need conductors with an ampacity of 40*1.25= 50A.

 

[Jraef]

I am confused when it comes to sizing feeder cables going out to a motor from a starter. I have always done 125% of whatever is full load amps on the motor but just recently I was told that I should actually size the motor feeders based on size of the MCP. I will list an example below. The system I am considering for the example below is an AB MCC with a bucket composed of a MCP and starter.

100 A MCP (instantaneous is the only thing that can be changed)
AB Size 3 starter
30 HP Motor (say 36 FLA for this example)

36A x 1.25 = 45 A

Therefore, does my feeder going to the motor from the starter need to protect for 45 A or does it need to protect for 100 A?

Thanks for the help in advance.

Being that you are in the US, motor conductor sizing is not negotiable, it is clearly outlined in article 430 of the NEC, deviations are only permissible by the stated exceptions.

So to be clear, it is NOT 125% of the motor nameplate FLA, it is 125% of the FLA as per the motor nameplate HP, taken from Table 430.250 (or other applicable table). So for your 30HP example, assuming 460V, the FLA in table 430.250 is 40A. So the conductors must be sized at no LESS than 40 x 1.25 or 60A. From there you can increase the size as appropriate for voltage drop or if you like, for future anticipated motor increase, but sizing for 60A is a minimum. it does NOT matter than the nameplate of your motor says 36A.

The concept of sizing per the size of the circuit breaker is just wrong, whomever told you that is mistaken. In a FVNR starter bucket of an MCC, the CB is usually a "Magnetic Only" or "Instantaneous Trip" breaker, it has no thermal over current protection, because it is the overload relay in the STARTER that takes care of that. So in your Size 3 bucket, that "100A" breaker is not the same as a 100A CB in a panelboard for instance. it can ONLY be used in CONJUNCTION with an overload relay, and the "100A" rating is simply the maximum continuous current rating of the contacts inside. The only trip function it will have will adjust from 400-1000A because it is intended to protect ONLY against a short circuit. Sizing the conductors based on that is just wasteful.

 

[david luchini]

BSo for your 30HP example, assuming 460V, the FLA in table 430.250 is 40A. So the conductors must be sized at no LESS than 40 x 1.25 or 60A. From there you can increase the size as appropriate for voltage drop or if you like, for future anticipated motor increase, but sizing for 60A is a minimum. it does NOT matter than the nameplate of your motor says 36A.

Is this the new Common Core math? I'm so out of touch.

 

[augie47]

Being that you are in the US, motor conductor sizing is not negotiable, it is clearly outlined in article 430 of the NEC, deviations are only permissible by the stated exceptions.

So to be clear, it is NOT 125% of the motor nameplate FLA, it is 125% of the FLA as per the motor nameplate HP, taken from Table 430.250 (or other applicable table). So for your 30HP example, assuming 460V, the FLA in table 430.250 is 40A. So the conductors must be sized at no LESS than 40 x 1.25 or 60A. From there you can increase the size as appropriate for voltage drop or if you like, for future anticipated motor increase, but sizing for 60A is a minimum. it does NOT matter than the nameplate of your motor says 36A.

The concept of sizing per the size of the circuit breaker is just wrong, whomever told you that is mistaken. In a FVNR starter bucket of an MCC, the CB is usually a "Magnetic Only" or "Instantaneous Trip" breaker, it has no thermal over current protection, because it is the overload relay in the STARTER that takes care of that. So in your Size 3 bucket, that "100A" breaker is not the same as a 100A CB in a panelboard for instance. it can ONLY be used in CONJUNCTION with an overload relay, and the "100A" rating is simply the maximum continuous current rating of the contacts inside. The only trip function it will have will adjust from 400-1000A because it is intended to protect ONLY against a short circuit. Sizing the conductors based on that is just wasteful.

I agree with all of Jraef's comments however I think the 50 amp figure david quotes in post #2 is the correct number. Jraef's "60" I believe si a typo.

 

[mario8449]

Thank you very much for clarifying this topic for me. Have a great day.

 

[Jraef]

Is this the new Common Core math? I'm so out of touch.

Not a typo unfortunately, I was thinking 25% of 40 was 20...

You know, I looked at that several times and decided, in my haste, that YOU had made the error... :ashamed1:

I'm multi-tasking, trying to figure out how to make a VFD not trip on an error when a transfer switch opens and re-closes, killing an Ethernet communication link to a remote PLC. I do this sort of stuff in between as a diversion when my head hurts.