25 Horse 480V/3 Phase motor

[Transportation Guy]

Good afternoon fellow electrical comrads. I haven't done motor calculations in 10 years+ and need some help if possible. We have a 100' tower crane with a 3 Phase, soft start, 480V motor that is 25 horse. The manufacturer asked for a 60 AMP feed. I am looking through section 430 and extremely confused as I was years ago. I am trying to determine my overcurrent protection as well as fuse protection. I do not have the FLA of the motor but based on 430.250, it is 34 AMPs.

 

[petersonra]

Good afternoon fellow electrical comrads. I haven't done motor calculations in 10 years+ and need some help if possible. We have a 100' tower crane with a 3 Phase, soft start, 480V motor that is 25 horse. The manufacturer asked for a 60 AMP feed. I am looking through section 430 and extremely confused as I was years ago. I am trying to determine my overcurrent protection as well as fuse protection. I do not have the FLA of the motor but based on 430.250, it is 34 AMPs.

Fuses can be up to 175% of the FLA from the chart in the code (not from the motor nameplate). 60 Amp fuses would make sense. The conductors only have to have an ampacity of 125% of the FLA from the chart.

Over current protection is via the overload. It is set/selected using the FLA from the motor nameplate.

 

[Smart $]

For the 175% factor mentioned by Bob, see Table 430.52.

 

[Transportation Guy]

Fuses can be up to 175% of the FLA from the chart in the code (not from the motor nameplate). 60 Amp fuses would make sense. The conductors only have to have an ampacity of 125% of the FLA from the chart.

Over current protection is via the overload. It is set/selected using the FLA from the motor nameplate.

Thanks Petersonra.

What about 430.52 in regards to breakers?

 

[petersonra]

Thanks Petersonra.

What about 430.52 in regards to breakers?

according to the chart, you can normally go up to 250% if using a CB.

 

[Transportation Guy]

according to the chart, you can normally go up to 250% if using a CB.

Thanks Bob. We have a 480v panel we are feeding the disconnect out of. The breaker feeding the disconnect I had sized at 60AMP based on what customer originally wanted but I am concerned that will trip. I am going to go to the 250% based on 430.52.

 

[tom baker]

My handy Sq D motor data calculator calls for a 80 amp thermal magnetic breaker, 8 AWG conductor, 60 amp dual element time delay fuses

 

[Transportation Guy]

My handy Sq D motor data calculator calls for a 80 amp thermal magnetic breaker, 8 AWG conductor, 60 amp dual element time delay fuses

I had one of these Handy Dandy calcs but it walked away. Is there an electronic copy available? I will be checking that!

 

[jim dungar]

I had one of these Handy Dandy calcs but it walked away. Is there an electronic copy available? I will be checking that!

http://www.schneider-electric.us/si...-online-tools/nema-motor-data-calculator.page

I never have looked to see if there is an "app" version from Square D however, I am sure there are equivalents.

 

[Jraef]

Don't over think it. You have a soft starter, no need for concern, 60A is fine. If you bought a soft starter as a combination starter, it would have a 60A circuit breaker from the factory, assuming it can.

That is another issue. Some of them are NOT listed for use behind a circuit breaker, you MUST use a fused disconnect, so check the manual for your soft starter.

 

[kwired]

Don't over think it. You have a soft starter, no need for concern, 60A is fine. If you bought a soft starter as a combination starter, it would have a 60A circuit breaker from the factory, assuming it can.

That is another issue. Some of them are NOT listed for use behind a circuit breaker, you MUST use a fused disconnect, so check the manual for your soft starter.

Absolutely :thumbsup:

Higher fuse or breaker settings is only needed to handle the surge you have with across the line starting in first few cycles after energizing. Reduced voltage or soft starting methods are at least partly about eliminating the severity of that surge.