4000a service

[Alwayslearningelec]

Ok, if I have a 4000A 3 phase 480v service can that feed all the following below? I don't know how to calculate it.

About 28 pumps totaling 2,300 amps all 480V
(1) 800 amp panel 220V
(2) 300 amp panels220V

Would I just total up the amps and that would equal 3,700 amps so I am under the 4000A so I am fine.... I know I will need a step down transformer for the 800a and 300a panels.

THanks a lot

 

[charlie b]

Would I just total up the amps and that would equal 3,700 amps so I am under the 4000A so I am fine?

No. You should never add amps to amps, and expect to get an answer in amps. Sounds funny, but it is true. That is because amps ain?t amps. There is a difference between ?amps on phase A? and ?amps on phase B,? and there is also a difference between ?amps at 480 volts? and ?amps at 220 volts.? Always do the math in terms of KVA, and convert back to amps only at the very end.

I can?t give you the correct math on your situation, because I don?t understand the 220 volt panels. Are they three phase or single phase? If three phase, why are they not 120/208? If single phase, why are they not 240?

I will give an example of the math, with the assumption that your panels are 120/208 volt, three phase. If they are something else, then you can do the corrections.

? 2300 amps (at 480 volts) times 480, then times 1.732 (the square root of 3) gives you 1,911 KVA.
? (800 + 300 + 300) (amps at 208 volts) times 208, then times 1.732 gives you 504 KVA.
? 1,911 + 504 = 2,415 KVA.
? 2,415 divided by 480, and divided again by 1.732, and multiplied by 1000 gives you 2,906 amps. That is less than your 4000 amp board?s rating, so you are OK.

 

[Alwayslearningelec]

No. You should never add amps to amps, and expect to get an answer in amps. Sounds funny, but it is true. That is because amps ain?t amps. There is a difference between ?amps on phase A? and ?amps on phase B,? and there is also a difference between ?amps at 480 volts? and ?amps at 220 volts.? Always do the math in terms of KVA, and convert back to amps only at the very end.

I can?t give you the correct math on your situation, because I don?t understand the 220 volt panels. Are they three phase or single phase? If three phase, why are they not 120/208? If single phase, why are they not 240?

I will give an example of the math, with the assumption that your panels are 120/208 volt, three phase. If they are something else, then you can do the corrections.

? 2300 amps (at 480 volts) times 480, then times 1.732 (the square root of 3) gives you 1,911 KVA.
? (800 + 300 + 300) (amps at 208 volts) times 208, then times 1.732 gives you 504 KVA.
? 1,911 + 504 = 2,415 KVA.
? 2,415 divided by 480, and divided again by 1.732, and multiplied by 1000 gives you 2,906 amps. That is less than your 4000 amp board?s rating, so you are OK.

Got it. Thanks very much.

 

[Lady Engineer]

Ok, if I have a 4000A 3 phase 480v service can that feed all the following below? I don't know how to calculate it.

About 28 pumps totaling 2,300 amps all 480V
(1) 800 amp panel 220V
(2) 300 amp panels220V

Would I just total up the amps and that would equal 3,700 amps so I am under the 4000A so I am fine.... I know I will need a step down transformer for the 800a and 300a panels.

THanks a lot

Charlie's right, you can't just add amps, because amps decrease as voltage increase, which is why power lines are 13KV and 26KV, so you can have smaller wires.

What you can do is if the panels are both three phase, you can divide 208V amps by 480V and get .4333 and multiply that by your amps at 208 and get your amps at 480V. Then you can add all the amps together, after you've found the amps at 480V, not 208V.

 

[Lady Engineer]

Oops...Charlie answered you, sorry I see you have the answers. My apologies...

 

[nakulak]

although we occasionally have to do load letters for services, I haven't done any for services over 800A - they have all been engineered. If you plan on designing services you might want to at least pick up Mike's CD on article 220 so you can have a chance at getting near the right number for sizing services. (just a suggestion)