calculation 3 phase power

[jmlelectric1@yahoo.com]

I am working in a huge warehouse and i have to quote some major work. They are looking to add 90 retractable cord reels that will be drawing an average somewhere from 7 to 17 amps a peice. No more than 60 reels will be in operation at one time.
While I was on the site visit I recommended adding a subpanel to accomadate the new wiring. I was told there was an abandoned 3phase 100 amp (3 wire with ground B/O/Y) wiring near the desired location. Do u think that this will be sufficeint for this application. Please help

 

[charlie b]

I sincerely doubt this will work. Taking 60 loads, each using the minimum 7 amps, gives you over 400 amps. If the reels are single phase, then the total amps spread out over all three phase would still be well over 200.

In order to calculate this out, you will need to have a clearer notion of the loads being drawn. Absent any specific details, I would design this under the presumption that I would have 60 reels, each drawing 17 amps. That is over 1000 amps.

 

[Jljohnson]

Just noticed that there will be 90 total reels, not 60. all calcs below would need to be adjusted accordingly. The system needs to be designed to handle all 90 reels, unless there is some sort of interlock to lock 30 of them out and guarantee that only 60 could be used at a time.



Assuming that the B*O*Y configuration that you mentioned is 480 volt 3 phase, you calc would look like this if all loads are assumed to be 17 amps
17 x 120 = 2040 watts for each drop
2040 x 60 = 122400 total system watts
122400/(480x 1.732)= 148 amps at 480 volts

Not enough power available at the present location. Now, if you could get better information forom the customer, your calculation COULD look like this:
7 x 120 = 840 watts for each drop
840 x 60= 50400 total system watts
50400/(480 x 1.732)= 61 amps at 480 volts

Quite a difference between the 7 and 17 amps that you were given by the customer as far as system design goes. I would design to the worst case unless they could show me that the load would be closer to the 7 than the 17 amps. Both scenarios involve a step down transformer to 120/208 volts AND I assumed that the cord drops were to be 120 volts even though the OP did not specify voltage on the cord reels.

 

[charlie b]

Jim, why are you using 120 volts as the voltage of the single phase load, and 480 volts as the voltage of the three phase supply? :-?

Also, what does "B*O*Y" mean, in this context?

Oh, and a question for JML: Are the reels single phase loads?

 

[Jljohnson]

Charlie, the B*O*Y would indicate brown, orange, yellow and lead me to assume that the existing 100 amp circuit is 480 volts. The calculation of the total load wattage is done @ 120 volts because that is the voltage at the usage point. It will, however, be connected back to a 3 phase 480 volt supply and a step down 3 phase transformer so the total load would be calculated at 3 phase/480 volt for feeder size, etc. The only thing that the 120 volts is used for in the calculation is determining total wattage.

 

[charlie b]

That's a fair number of assumptions. I would like the OP to chime in with a confirmation or with new information. Specifically, since the OP said he had suggested adding a new subpanel, I would have assumed that was going to be the point at which branch circuits (120 volt, single phase, if your assumption is correct) would originate. The existing 100 amp panel would not suffice for that application. That was the basis of my original comment. If the incoming power is 480 (another assumption that I would like to hear the OP confirm), and if a step down transformer will be used to get the necessary 120 volts to serve the reels, then all that is needed at the 480 volt level would be a single breaker. Thus, the existing 100 amp panel would not be of use for that application either. So I am still thinking that the 100 amp panel will not be a player in this installation.

 

[BJ Conner]

I am working in a huge warehouse and i have to quote some major work. They are looking to add 90 retractable cord reels that will be drawing an average somewhere from 7 to 17 amps a peice. No more than 60 reels will be in operation at one time.
While I was on the site visit I recommended adding a subpanel to accomadate the new wiring. I was told there was an abandoned 3phase 100 amp (3 wire with ground B/O/Y) wiring near the desired location. Do u think that this will be sufficeint for this application. Please help

Calculate your load in VA not watts.
90 reels x (7+17)/2) Average current
Load = 90 reels x 12 amps/reel x 120 volts = 129,600 VA

Apply the diversity factor they have given you. Be careful with this especialy if there all motors. Some AHJs will want to know how no more than 60 will be running at once.

MAx load =129,600 x(60/90) = 86,400 VA
The load on a 3 Phase 480 volt system is
I= 86,400/(480*1.732)= 103 amps (If all are ballanced). The existing 480 volt supply looks shakey.
A 75 KVA 480-208/120 xfmr is not big enough.
The next size up is a 112.5 KVA
OR
you can use 2-45 kVA or 3 30 KVA transformers.
If you have 90 reels I think your going to need 90 breaker spaces, thats 3 panels.
I would look at the layout if you can optimize the locations of the three 30 kva xfmrs you can save on branch circuit length.
IF the average of 7-17 amps is nearer to 17 then every thing goes up. IF the reels are 208 3 phase every thing goes up. KVA and circuit breaker space requirements)

 

[Electric-Light]

If it's 480v and power at point of use is 120v, you'll need a 480Delta-208Y/120 transformer and have to factor in less than perfect efficiency.

 

[jmlelectric1@yahoo.com]

yes they are single phase

 

[jmlelectric1@yahoo.com]

confuzed

confuzed

Thanks all for the replies. what do you think is the best plan of attack? im a 3 man electrical company and dont want to get in over my head. I could prob get them to reduce quantitys down to maybe 30-40 reels, But i am still gonna need atransformer if its 480. There has to be 120/208 wiring in warehouse somewhere. So if i use the calculations suggested for vending machines that need to be plugged in to be serviced. So this is a compressor is the element of electrical needs.
They also want to add 2-30 amp 208v circuits, as well as 12 t-5 lights
What am I going to have find to accomadate all these needs. I am wondering if they have enough main power just to suffice these new developments (800 amp straight 3 phase)