ECN Forum Forums Code Related Discussion NEC & other Code issues sizing service and service feeder

Wonder if anyone would mind helping me size this service. I'm going to have to tell the owner that there's going to be a drastic change in his plans of machinery if my calculations are right. A lot of the machinery he has will have to be changed to 480 volts in order for the existing 120/240 service to handle just a portion of the machines he has. What I have is an existing 120/208 service, which I may have to have the power company to retap to make it 120/240, since his name plates specify 230/460 volts.
I have a 200 amp. 42 cir. 3 phase main breaker panel, with a main lug sub panel fed out of the side of it.(someone else done the existing wiring) There is a 100 amp. breaker ahead of the sub panel.( This is a Seimans "plug in type" ) I have a total of 7 pieces of machinery, with a total of 115 amps.including the 125% for the largest motor My supplier has informed me that the 100amp. breaker is the biggest 3 phase "plug in type" that I can get. The only way I can see that this will work, is that the "authority having jurisdiction" ok it, because some of the motors may not be used at the same time. Is my figuring right so far??
Next the total load on the building consist of: "industrial" application
11,700 sq. ft. X 2 wts./ sq. ft. = 23,400 v/a
Since this is an industrial establishment, according to table 220-11, there is no demand factor that can be applied. It is taken at 100%
I have about (35) 120 volt outlets. If I figure these at 180 v/a I get 6,300 v/a
Since there is no demand factor that can be given to the lighting circuit, I must take these at 100% also.
I have:
(1) 10 hp motor at 230 volts = 28 amps
(1) motor with ? HP = 18.2 amps
(1) more motor with = 18.2 amps
(1) machine with 2 motors
2 hp each (6.8amps/motor) = 13.6 amps
(1) more with the same as above= 13.6 amps
(1) 5 hp motor at 230 volts 15.2 amps
(1) 5 hp motor (same as above) 15.2 amps
Note: all above motors are 3 phase
(1) 3 hp single phase 230volt =17 amps
I have:
2 window a/c and heat units (230 volt
single phase )18.8 x 2 = 37.6 amps
(1) 1.6 hp 115v gas heater motor = 4.4 amps

I get a total of 74848 va / 360 = 207.9 amps
Wonder if this anywhere right, if you care to help? Thanks ..Steve

This is not counting at least 94 amps of other 3 phase machinery he wants to put on this same service. Thanks again for the help. It's been a while since I've had to do these calculations, so I've probably missed something somewhere.

Hi,
There is a big problem with your design..

YOU HAVE NO ROOM FOR FUTURE EXPANSION.

This is required by the code..

Save yourself and your client a lot of time and worry and just install a 400 or 600A service. You will be glad you did!

Now if this is just a practice exercise then it doesnt matter.

Why try to pin it down to the very least? That is not code nor is it good designing.

What will you do if he grows a lot in the next six months and wants to add 5 more machines?


good luck

-regards

greg



[This message has been edited by mustangelectric (edited 03-17-2005).]

I once worked in a machine shop where there were hundreds of machines wired up but we only used 2 or 3 at a time.

You should be able to figure actual demand and size the service for that.

greg, I'm only trying to calculate according to what he already has. I will be adding a 277/480 volt service on the other end of the building to take care of other motors. I just want to make sure that my calculations are right. If this was a new installation, I would definitly put in at least a 400 amp. 120/240 service. That is my point in asking, whether to tell the owner that he will have to change the existing service or not. That will bring up questions also, because he is under the assumption that the service is capable of handling the machines, because someone from the inspection office, ( he assumes ), says that the service is alright. I have my doubts about that though, since I don't think he gave them all the necessary information.
According to what I have come up with on my calculations, I take 125% of the largest motor, and add the total amps of the other motors for the feeder for the sub panel, then for the service size, I take the lighting loads and recept.loads per tables 220-3(a) and table 220-11 and other loads, then divide by 360?? Does this sound right. He has other motors he wanted to put on this service, but the way I see it they will have to be changed to 460 volts and taken out of the other service panel that I am to add, if
he can get by with the minimal machines on the exisiting 120/208 service. Since there are no prints for this job, and there is quiet a bit of confusion between the owner and inspection office, and ME, I'm trying to go the simpliest way and the safest way possible under the circumstances. Like I said, if this was a new installation, I would do it quiet different, but we're trying to make what is available work in as much as possible. As last resort, I may have to tell him that the existing service will have to be changed, but I want to make sure I am telling him right. This is a big expense other than the expense he is already taking.Thanks.. Steve ... Also the 460 volt service was already there on the other end of the building, only the previous owner took the panels with him, and only left the parralled service conductors in a 2 X 2 juncion box. As a matter of note here, he must have disconnected them "hot", seeing that there was still power on the wires. Thankfully he did leave them taped up.... I had to get the local power company to kill the power on them Like I said, I'm preparing for what to tell him ...

[This message has been edited by sparkync (edited 03-17-2005).]

[This message has been edited by sparkync (edited 03-17-2005).]

Hi,
Oh I see...you are trying to calculate the existing load...

thats easy!

get a copy of ALL of his electric bills for the last year or so..

no better way than that!

good luck

greg

Hi,
Geo...you said: "I once worked in a machine shop where there were hundreds of machines wired up but we only used 2 or 3 at a time.
You should be able to figure actual demand and size the service for that."
-----------------------------------------------

can you really calculate a load in a industrial building that has hundreds of peices of equipment that are online by using a couple of them to determine the demand factor and calculate the se load/sizing?

that just doesnt sound right to me. i have never heard of that method...what part of the code is that?

would you use intermittent use or non continuous or what?

just curious..

i always thought you had total up all the defined loads including future expansion?

1600 watts on a 20 amp circuit...thats the max right? so if I have two hundred hair dryers in a salon and they say I only get two cuustomners a day...can i install 2 20a circuits?

what if I have an apartment building that has 200 units...not all of them will be used at the same time...the demand factor is what determines the size fo the service...not everyone will be using the electric range at the same moment BUT the service HAS to be sized for a certain percentage of them being in use..

if i used your method...i could just figure willy nilly that oh say 25 apartments will be cooking at the same time and be within code?

please educate us on this if you dont mind..

-regards

greg

[This message has been edited by mustangelectric (edited 03-17-2005).]

No greg, I'm not trying to figure his electric bill, I'm trying to see if someone can verify that my calculation procedure is right for the load that I described. Thanks

If I understand you clearly you have a 120/208 three phase service at 200 amperes. You are installing a three phase 480/277 service in the plant as well. If the power utility will supply a 120/240 volt delta service then you can run some of the three phase motor load off of the existing panel. Your own calculations indicate that that panel would be over capacity with the loads you already know about and the owner has several more loads he wants supplied. I suspect that you will end up with no motor loads on the 120/208 volt service and that the power company will decline to provide a delta 120/240 volt three phase delta high leg service. If you plan for all of the motors to run at 480 volts; 460 being a nominal rating; then you will only have to size the new service for the motor loads and you can run all of your receptacles and lighting from the existing 120/208 service.

You need the answers to a couple of questions. What size are the existing paralleled service entry conductors on the 480/277 service? Will the POCO provide 120/240 volt high leg delta service? Can the existing panel be used in a high leg delta configuration supply?

Your preferred switch gear supplier can provide a lot of support in verifying your calculations and suggesting ways to deliver the needed power. Keep in mind that every circuit that you can build at the 480/277 voltage will be cheaper to build then delivering the same power at a lower voltage. Your best approach may be to run every motor that can be tapped for it at the 480 volt level.
--
Tom H

mustangelectric ---

Perhaps I misspoke. He should not base the service on the number of machines we used. He should base the service size on the number of machines that the customer will be using at one time. That is usually not hard to do.

And as someone else pointed out using the prior electric bills would give a reasonable estimate of usage.

Hi,
Sparky-There is NO BETTER WAY TO TELL WHAT THE LOAD IS FOR EXISTING INSTALLATIONS THAN A REVIEW OF THE POWER CONSUMPTION over a period of time.

There are tons of ways to calculate the size of a service..

In todays age and time..why even bother with a pencil?

You said: "I'm trying to see if someone can verify that my calculation procedure is right for the load that I described."

No it is incorrect.

good luck..

-regards

greg