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#130099 - 02/09/06 06:11 PM Calculations  
Dnkldorf  Offline
Member
Joined: Dec 2004
Posts: 1,064
nowhere usa
I have a 120/240 3 phase, 4 wire (center tap) delta system (B stinger). My 3 phase load is 80 kva per phase (A, B, and C). My single phase load is 34 kva phase A and 41 kva phase C. What are the amps for each phase and how is this calculated? What size (amp rating) panel is needed?

This question was posted at Mike Holts site, and got alot of responses, but everyone there is saying the previous poster is wrong...

I thought you have 240kva load for the 80kva/phase, and then the 34kva and 41kva are figured at single phase 240V?

So how do you do it? I be confused...

Dnk...

[This message has been edited by Dnkldorf (edited 02-09-2006).]

[This message has been edited by Dnkldorf (edited 02-09-2006).]


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#130100 - 02/09/06 08:14 PM Re: Calculations  
Dnkldorf  Offline
Member
Joined: Dec 2004
Posts: 1,064
nowhere usa
Here's how I got it figured:

First the 240kva load would be 578a per phase.

Next I took the 34kva and the 41kva, that would be 34kva on a 240 single phase load, so I would have 141a additional on those perspective phases, leaving 7kva, on the 120v load.

So phase A is 578+141=719a
B is 578a
c is 578+141+58=777a

777a X 125%= 970A = 1000a Breaker for ocp


Shoot it down, and tell me where I am going wrong pls...


Dnk..


#130101 - 02/09/06 11:07 PM Re: Calculations  
Radar  Offline
Member
Joined: Apr 2004
Posts: 349
Los Angeles, CA
OK - here's my initial cut at this. Start with the 3Ø component - take 240,000 VA (80,000 VA per phase) divided by 240 * SqRt of 3, which comes out to around 578 amps.

Next I'd ignore the ØA load as it just counteracts the ØC load. The ØC load is 41,000 - divide that by 120V for another 342 amps (on the C leg).

All in all, 578 + 342 = 920 Amps <-- highest leg (which is ØC).

Taking 920 amps X 1.25 = 1,150 amps, I'd vote for a 1,200 amp OCP.

Tomorrow at work I'll plug the raw data into a panel schedule and see what it says.

Radar




[This message has been edited by Radar (edited 02-09-2006).]


There are 10 types of people. Those who know binary, and those who don't.

#130102 - 02/10/06 12:52 AM Re: Calculations  
Dave T  Offline
Member
Joined: Jan 2005
Posts: 155
Waukesha, WI, USA
Dnkldorf,
Isn't it getting too late for stuff like this? Look at the clock!! I was just going to shut the puter down and though I would take a peck and found this.
Now, back to shutting the puter down.
Good night!


#130103 - 02/10/06 06:41 AM Re: Calculations  
Dnkldorf  Offline
Member
Joined: Dec 2004
Posts: 1,064
nowhere usa
Dave, it's morning now. rise and shine.....

Turn on the puter and get them brain cells oscillating..


Dnk...


#130104 - 02/10/06 07:29 AM Re: Calculations  
Dnkldorf  Offline
Member
Joined: Dec 2004
Posts: 1,064
nowhere usa
I think I know where I went wrong...

The 34kva on A and C should be 68Kva load at 240. That would be 283A, not the 141A.

So..A phase would be 861A
B remains 578
c would be 919

So, the 919 x 125% would be a 1200A Ocp?

Does that sound better?

(Radar, you editted your post and I just saw it now)


Dnk...

[This message has been edited by Dnkldorf (edited 02-10-2006).]


#130105 - 02/10/06 08:14 AM Re: Calculations  
Dnkldorf  Offline
Member
Joined: Dec 2004
Posts: 1,064
nowhere usa
316 on the nuetral?

Dnk..


#130106 - 02/10/06 10:11 AM Re: Calculations  
Radar  Offline
Member
Joined: Apr 2004
Posts: 349
Los Angeles, CA
Dnk - I think the only current on the neutral is the unbalanced portion of the load between ØC & ØA. That is, the amount of ØC load that is not offset by the ØA load. This is 7,000 VA of ØC load (41KVA - 34KVA) divided by 120V = 58 amps.

Radar


There are 10 types of people. Those who know binary, and those who don't.

#130107 - 02/10/06 10:26 AM Re: Calculations  
pauluk  Offline
Member
Joined: Aug 2001
Posts: 7,520
Norfolk, England
Quote
I think the only current on the neutral is the unbalanced portion of the load between ØC & ØA.


Sounds right to me. The 3-ph load has no neutral component, the center-tap connection just acting a ground reference so that the phases sit at 120/208/120. Thus the neutral current will be just the difference between the extra single-phase load on A and C.


#130108 - 02/10/06 10:54 AM Re: Calculations  
JBD  Offline
Member
Joined: Jul 2001
Posts: 599
WI, USA
My brain hurts.
The description of your problem is poor.

Do you have a single 80KVA three phase load or (3) 80KVA single phase loads
or a single 240kVA three phase load?

Is your single phase loading connected A-N 34kVA @120V and C-N 41kVA @120V or is it a 34kVA @240 connected A-C and 7kVA C-N at 120V or 68kVA @240V connected A-N-C and 7kVA C-N at 120V?


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