kw's

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Carultch

Senior Member
Location
Massachusetts
if I'm told I need a 60 kw service, does that mean I need a 250 amp service? What is the formula?
60,000 / 240? or 60,000 / 120?


For 120/240V single phase, common in residential applications, to calculate the current that goes with 60kw is 60000 W / 240A = 250A.

The reason why you divide by the 240V instead of the 120V, is that the grid configuration has two live circuits that are 120V to ground with a common neutral, and 180 degrees out of phase (in otherwords, opposite). The difference between them is 240V. Large loads like an oven would be connected across the two live wires, at 240V.

Distribute the remaining 120V loads in a balanced manner, and you can treat the big picture like it is a 240V load. You do need to consider the worst case current, if there is an imbalance of the 120V loads between the two lines.

For 120/208 three phase common in light commercial applications, to calculate the current that goes with 60kw is 60000 W / 208A / sqrt(3) = 167A.

For 277/480 three phase common in larger commercial applications, to calculate the current that goes with 60kw is 60000 W / 480A / sqrt(3) = 72A.


Just because you are working with 60kW, doesn't necessarily mean that your service has the amperes that directly corresponds to the kW. It depends on the nature of the loads, continuous vs non-continuous, and power factors. There can be additional safety factors and diversity factors that apply to some loads (and not to others) per the NEC.
 

DarylH

Member
Location
San Marcos, CA
Also wanted to point out a unit error in an earlier post. Amps were indicated where volts should have been. That is, 60000 W / 240 V = 250 A, not 60000 W / 240 A = 250 A. Same applies to the remainder of the examples in the earlier post.

I = P/V
 
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Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
...
Also wanted to point out a unit error in an earlier post. Amps were indicated where volts should have been. That is, 60000 W / 240 V = 250 A, not 60000 W / 240 A = 250 A. Same applies to the remainder of the examples in the earlier post.

I = P/V

Hmmm... actually, not really 100% complete.

I = P/V when PF = 1. But what's the PF of the load? If someone told him he needs a 60kW service, I'd be concerned it is an inductive load and the equipment supplier is pawning off the actual kVA calculations to the ELECTRICAL CONTRACTOR CAPABLE OF SIZING THE SERVICE. So if it has motors, what percentage of that 60kW is represented by the motors then? 1%? 10%? 100%? 60kW that is 100% motors at a .8 power factor is going to be 75kVA, / 240V = 312.5A. A 250A service is too small.

Also, why is everyone ASSuming this is a single phase service? Because he asked if it was 60kW at 240V or 60kW at 120V? What if the load is 3 phase and he is trying to size a single phase service for it? What if it is a 120/240V 3 phase 4 wire service?

I have all the respect in the world for people who chose to run pipe and pull wire for a living but never received training on how to determine a service size. In fact thank GOODNESS there are people willing to do that so that I don't have to. But if that person is not QUALIFIED to determine a service size, then he is not, and he SHOULD not take on that project. Maybe I was too harsh in using the term "electrician" too strongly, call me a purist I guess. But an "Electrical Construction Worker" is not, in MY opinion, the same as a trained and competent electrician.

BTW, I WAS a J-man a long time ago, but I have not kept up my license because I no longer do that for a living. But in MY day, I was not called an "electrician" until I had passed a test, which included material such as would be necessary to determine a service size calculation. I'll also note that when I went back to college to get my degree, that information actually never came up as it specifically would relate to NEC requirements!
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
I would like to ask the members not to question the members about their qualifications. If you have a question then report the post and have one of the mods look at it. When you all question the member you tend to scare them off and no need for that. We don't know the circumstances of the question and we all had to learn at some point. Thanks
 

Carultch

Senior Member
Location
Massachusetts
I would like to ask the members not to question the members about their qualifications. If you have a question then report the post and have one of the mods look at it. When you all question the member you tend to scare them off and no need for that. We don't know the circumstances of the question and we all had to learn at some point. Thanks

I completely agree. Question the scenario for clarification, not the qualifications of the asker.
 

JRW 70

Senior Member
Location
Eastern Central Missouri
Occupation
Testing and Engineer
60 kW

60 kW

60kW is really on the upper limit of what
utilities like to serve as single phase. The
excellent post above showing the different
amperages at certain available supply voltages
shows this question could be more satisfactorily
answered by what it is going to be used for.
Transformer ratings are given in kVA and phase
configuration so the "amps" @ 60kW could have
a few different, yet true answers.

Maybe the poster could clarify the situation, and get
an answer that can be both relevant now and help
understanding if this scenario comes up in a future.

Peace

JR
 

kwired

Electron manager
Location
NE Nebraska
60kW is really on the upper limit of what
utilities like to serve as single phase. The
excellent post above showing the different
amperages at certain available supply voltages
shows this question could be more satisfactorily
answered by what it is going to be used for.
Transformer ratings are given in kVA and phase
configuration so the "amps" @ 60kW could have
a few different, yet true answers.

Maybe the poster could clarify the situation, and get
an answer that can be both relevant now and help
understanding if this scenario comes up in a future.

Peace

JR
Maybe in some areas. I have seen many single phase POCO transformers supplying single phase service that are 75, 100 and even 167 kVA.
 

iwire

Moderator
Staff member
Location
Massachusetts
Gentlemen, I have chopped the heck out of this thread. Post that were questiong the OPs quilifications and off the OPs topic were all pulled.

Below is a 'sticky' from the NEC forum http://forums.mikeholt.com/showthread.php?t=81328 and I hope we can follow it.

To all forum members, I am formally requesting that you no longer publicly question new members qualifications for posting here.

Keep in mind that all new member posts have already been approved by a forum moderator before they show up on the forum.

If you feel a new member is over their head please exercise one of two options.

1) Notify one of the Moderators, I suggest you send a PM to one of us with a description of the problem you see with a post. We will gladly discuss it with you.

2) Do nothing, ignore the post and move on.

The reasons for this should be self evident, we are going to chase away new members if they feel they have to justify their qualifications with the entire membership at large.

Not to mention many times it is just plain rude.

Bob
2007 Chief Moderator Mike Holt Forum
 

Besoeker

Senior Member
Location
UK
And I'm formally stating that I didn't question his qualifications despite having been accused of doing so.
In fact, I pointed out to another member here that his profile states that his profile states that he is a journeyman.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
I don't think I have ever heard a service size described as being a certain kW before.

The NEC has a procedure embedded in it for calculating what size service is needed. It always ends up in Amps.
 

kwired

Electron manager
Location
NE Nebraska
I don't think I have ever heard a service size described as being a certain kW before.

The NEC has a procedure embedded in it for calculating what size service is needed. It always ends up in Amps.
Actually we find ourselves calculating in VA for quite a few general purpose and lighting load calculations and then converting that to amps depending on voltage and number of phases.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Actually we find ourselves calculating in VA for quite a few general purpose and lighting load calculations and then converting that to amps depending on voltage and number of phases.

true, but in the end, the result is in Amps.

I suspect that in context what the OP was told made some kind of sense, but since no one here is aware of the context in which such a statement may have been made, it is pretty difficult to answer.

It is somewhat analogous to asking how many blueberries does it take to make a cherry pie to some random people on the Internet with no context. Such a question might make sense in context, but without that context it is just wild guessing as to what the question itself might mean.
 
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