Just need confirmation for Motor feeder protection calculations, please.

[Davisteam]

Hi, I have a Lift station with 2, 3hp, 240vac, 3 phase pumps.

I need to replace the 60A Fused, 3-pole, double-throw, safety switch with 6, 60A fuses, which feeds two 20A IT breakers. Each of these breakers feeds a size 0 motor starter with overload protection. I have added a drawing of what I have as an attachment. I just need someone to check my calculations to make sure I am upgrading to code.

Thank You,
Mike

 

[ptonsparky]

Do your math again. I can see what you possibly did, but explain why.

 

[augie47]

My math came up with a Feeder SCGF protection of 35 amps. The SCGF on one motor plus the FLA of the 2nd motor 430.62

 

[Davisteam]

My math came up with a Feeder SCGF protection of 35 amps. The SCGF on one motor plus the FLA of the 2nd motor 430.62

--The 25A ckt. breaker in each leg was chosen because each pump has a FLC of 9.6A. So, 9.6A x 250% (Table 430.52, squirrel cage, Inverse Time Breaker)=24A. Next standard size is 25A Breaker.

--I chose #10 wire, for feeder conductors, instead of the allowable #12 simply for preference.

--I chose 30A Dual Element Fuses for Feeder OCP because my largest breaker is 25A. 25A + 9.6FLC = 34.6A. In order not to exceed this value I chose 30A.

 

[Davisteam]

Do your math again. I can see what you possibly did, but explain why.

--The 25A ckt. breaker in each leg was chosen because each pump has a FLC of 9.6A. So, 9.6A x 250% (Table 430.52, squirrel cage, Inverse Time Breaker)=24A. Next standard size is 25A Breaker.

--I chose #10 wire, for feeder conductors, instead of the allowable #12 simply for preference.

--I chose 30A Dual Element Fuses for Feeder OCP because my largest breaker is 25A. 25A + 9.6FLC = 34.6A. In order not to exceed this value I chose 30A.

 

[augie47]

If the 30 amp fuses were noted on the diagram originally, I missed it and zeroed in on the 60 amp switch. With 30 amp fuses, I see no problem.

 

[Jraef]

Your full load amps is just the sum of the two motors, 19.2A. The 25A is not a load, it’s just the rating of the breakers on the same circuit as each motor.

 

[ptonsparky]

Your full load amps is just the sum of the two motors, 19.2A. The 25A is not a load, it’s just the rating of the breakers on the same circuit as each motor.

:thumbsup:

 

[ptonsparky]

A 30 amp double throw is adequate, plus you won’t have to look for 60-30 fuse reducers.

edit- most likely adequate. Are both motors started at the same time?

 

[Davisteam]

A 30 amp double throw is adequate, plus you won’t have to look for 60-30 fuse reducers.

edit- most likely adequate. Are both motors started at the same time?

The pumps don’t start at the same time, they alternate each cycle and the second pump only comes on in a high level situation.

 

[Jraef]

A 30A switch is fine.
Is this a control panel or are the starters each in their own separate enclosures as factory built combo starters? Assuming all separate parts, I would use a stand alone main breaker rated 30A, then feed that to a non-fused manual transfer switch (double throw safety switch). Then feed the two combo starters (through a gutter with a distribution block). Using a fused MTS means having 6 fuses, which means 6 ways of single phasing those motors. The generator likely has its own breaker anyway, and I don’t think you can get a SUSE label on a double throw safety switch (I might be totally wrong on that though).

If this is a control panel, you can’t use IT breakers on your own, they are only legal in factory built and tested/listed assemblies. Those breakers would need to be Thermal-Mag.

 

[petersonra]

It looks a little goofy to me but also looks code compliant.

i would get rid of the fuses. If you are going to the trouble of allowing for a generator connection it seems like reliability is important and it is possible the fuses could blow and there be no spares, where with a CB you can just reset it.

I don't think you need to protect the generator line as the genset will have its own protection.

 

[petersonra]

A 30A switch is fine.
Is this a control panel or are the starters each in their own separate enclosures as factory built combo starters? Assuming all separate parts, I would use a stand alone main breaker rated 30A, then feed that to a non-fused manual transfer switch (double throw safety switch). Then feed the two combo starters (through a gutter with a distribution block). Using a fused MTS means having 6 fuses, which means 6 ways of single phasing those motors. The generator likely has its own breaker anyway, and I don’t think you can get a SUSE label on a double throw safety switch (I might be totally wrong on that though).

If this is a control panel, you can’t use IT breakers on your own, they are only legal in factory built and tested/listed assemblies. Those breakers would need to be Thermal-Mag.

in a later post the OP stated IT = inverse time, not instantaneous.

 

[Jraef]

in a later post the OP stated IT = inverse time, not instantaneous.

Oops, missed that. Thanks.

So in the immortal words of Rosanne Rodannadanna,
“Oh, never mind...”

 

[Davisteam]

A 30A switch is fine.
Is this a control panel or are the starters each in their own separate enclosures as factory built combo starters? Assuming all separate parts, I would use a stand alone main breaker rated 30A, then feed that to a non-fused manual transfer switch (double throw safety switch). Then feed the two combo starters (through a gutter with a distribution block). Using a fused MTS means having 6 fuses, which means 6 ways of single phasing those motors. The generator likely has its own breaker anyway, and I don’t think you can get a SUSE label on a double throw safety switch (I might be totally wrong on that though).

If this is a control panel, you can’t use IT breakers on your own, they are only legal in factory built and tested/listed assemblies. Those breakers would need to be Thermal-Mag.

My situation is the latter (it's a control panel with both starters). And as far as the breakers go, I got confused after looking at the Table 430.52 about the IT breakers. I'll go with TM. Thanks.

 

[Davisteam]

It looks a little goofy to me but also looks code compliant.

i would get rid of the fuses. If you are going to the trouble of allowing for a generator connection it seems like reliability is important and it is possible the fuses could blow and there be no spares, where with a CB you can just reset it.

I don't think you need to protect the generator line as the genset will have its own protection.

Great idea.