600A service vs 1000A design

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PH Pumps

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Paso Robles, CA
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Hello All,

I've been a long time viewer of this forum but am a new member. We are a pump contractor in California and perform electrical work for some of the ag irrigation pumps we install. I have one project that is fairly simple, but I am scratching my head after trying to double check my design.

Project Details: Our customer has an existing PG&E OH service, 400A, 480V, 3PH, 4W that currently runs two 150 HP centrifugal pumps. They have drilled a new well are asking for us to provide a quote that includes a new 400 HP vertical well pump and forgo the two 150 HP pumps AND a quote for the new 400 HP capacity to run the 150 HP x 2 at the same time (starting all three seperately, of course).

For the 400 HP only option, my FLA is 447, so simply use 447 * 1.25 = 558.75, round up to a 600A service
Where I am getting a little lost is the feeder protection and if I round down, or keep at 600A?

For the 400HP + 150 HP x 2, I use 447 *1.25 + 180 + 180 = 918, round up to a 1000 A service
Do I keep my main disconnect (feeder protection) at 1000 A as well?

I did some reading on NEC 430 in attempt to answer my own question, but wound up more confused than when I started out.

Thank you for any help and I look forward to finally being a member here
 
P

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
PH Pumps said:
Hello All,

I've been a long time viewer of this forum but am a new member. We are a pump contractor in California and perform electrical work for some of the ag irrigation pumps we install. I have one project that is fairly simple, but I am scratching my head after trying to double check my design.

Project Details: Our customer has an existing PG&E OH service, 400A, 480V, 3PH, 4W that currently runs two 150 HP centrifugal pumps. They have drilled a new well are asking for us to provide a quote that includes a new 400 HP vertical well pump and forgo the two 150 HP pumps AND a quote for the new 400 HP capacity to run the 150 HP x 2 at the same time (starting all three seperately, of course).

For the 400 HP only option, my FLA is 447, so simply use 447 * 1.25 = 558.75, round up to a 600A service
Where I am getting a little lost is the feeder protection and if I round down, or keep at 600A?

For the 400HP + 150 HP x 2, I use 447 *1.25 + 180 + 180 = 918, round up to a 1000 A service
Do I keep my main disconnect (feeder protection) at 1000 A as well?

I did some reading on NEC 430 in attempt to answer my own question, but wound up more confused than when I started out.

Thank you for any help and I look forward to finally being a member here
Click to expand...
If the only motor you have on the 600 A feeder is the 400 HP motor, I would make it a 600 A breaker. You are allowed to go to 250% for the motor CB. Will you be using some kind of reduced voltage starter? I wonder if a 600 A CB might nuisance trip now and then.

I would make the 1000 A service breaker as large as possible too. otherwise nuisance trips might occur especially with ATL starting.
 
electrofelon

electrofelon

Senior Member
Location
Cherry Valley NY, Seattle, WA
petersonra said:
If the only motor you have on the 600 A feeder is the 400 HP motor, I would make it a 600 A breaker. You are allowed to go to 250% for the motor CB. Will you be using some kind of reduced voltage starter? I wonder if a 600 A CB might nuisance trip now and then.

I would make the 1000 A service breaker as large as possible too. otherwise nuisance trips might occur especially with ATL starting.
Click to expand...
I am certainly familiar with the 250% motor CB rule, however what are the allowances for sizing a breaker serving several motors, such a service disconnect for a service serving only motors?
 
david luchini

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
electrofelon said:
I am certainly familiar with the 250% motor CB rule, however what are the allowances for sizing a breaker serving several motors, such a service disconnect for a service serving only motors?
Click to expand...
The service ocpd could be not greater than the largest allowable ocpd for the largest motor plus the sum of the other motor fla's.
 
david luchini

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
petersonra said:
I think that is a calc for the smallest it can be using t he e smallest allowed wire. You can make it as big as you want if you increase the wire size
Click to expand...
For motor load only, you can use the maximum allowable ocpd size with the minimum allowable conductor size.
 
T

topgone

Senior Member
PH Pumps said:
Hello All,

I've been a long time viewer of this forum but am a new member. We are a pump contractor in California and perform electrical work for some of the ag irrigation pumps we install. I have one project that is fairly simple, but I am scratching my head after trying to double check my design.

Project Details: Our customer has an existing PG&E OH service, 400A, 480V, 3PH, 4W that currently runs two 150 HP centrifugal pumps. They have drilled a new well are asking for us to provide a quote that includes a new 400 HP vertical well pump and forgo the two 150 HP pumps AND a quote for the new 400 HP capacity to run the 150 HP x 2 at the same time (starting all three seperately, of course).

For the 400 HP only option, my FLA is 447, so simply use 447 * 1.25 = 558.75, round up to a 600A service
Where I am getting a little lost is the feeder protection and if I round down, or keep at 600A?

For the 400HP + 150 HP x 2, I use 447 *1.25 + 180 + 180 = 918, round up to a 1000 A service
Do I keep my main disconnect (feeder protection) at 1000 A as well?

I did some reading on NEC 430 in attempt to answer my own question, but wound up more confused than when I started out.

Thank you for any help and I look forward to finally being a member here
Click to expand...
Your 400HP is listed in the table as having a full load current of 477 A, not 447. The 150 HP motor has a full load amps of 180. The respective locked-rotor amps of your motors are 2,900 A and 1,085A, respectively. If these motors can possibly start at the same time (worst-case), you add their FLAs then you also sum up their LRAs. Choose a circuit breaker that will be able to run your motors at their full-loads and during starting (657 A FLA and 3,985 A LRA). The code says you should allow a 115% of FLA sum to be able to run your equipment without nuisance trips (430.110 (C)(2)
 
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