I have recently been asked by strip mall if their 700KVA transformer will handle 800 more amps of fused pull outs/4 more 200 amp panels, for perspective tenant. All units are 120/208 single phase and current tenants have an equivalant of 2000amps of max capability through a combine panel rating and SES fused pull outs. What is the formula? Do I need more information to calculate this? or should I just tell him "I an electrician not an elctrical engineer, and I show my paycheck to prove it" :grin:
How to calc max load on 700KVA trans
- Thread starter Aledrell
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Johnmcca
Senior Member
- Location
- Mid-Hudson Valley, NY
120/208 single phase?
cadpoint
Senior Member
- Location
- Durham, NC
How much U pay'n the Code allows use to calculate the Load of the Application...
Richardrlr
Member
- Location
- spring,tx
formula
formula
700x1000=? / 208 =
formula
700x1000=? / 208 =
my mistake 3 phase 120/208 no high leg.
I don't know the draw or usage calcs? For enigneers I assume this is a common question and that usage could be assumed approximately 40%-60% max or whatever generic calulation is used. Not really sure I understood your comment though.
Power Tech
Senior Member
- Location
- Lebec, California
First I would get the customers account number and call the service planner for the POCO in your area.
Get the peak demand for the customer and use that for the building permit.
I attach a copy with the plans being submitted to the building department.
Get the peak demand for the customer and use that for the building permit.
I attach a copy with the plans being submitted to the building department.
erickench
Senior Member
- Location
- Brooklyn, NY
You've given us the KVA rating of the transformer but you have not told us if it's 3-phase and what the secondary voltage rating is.
Your 700 kva transformer is good for only 1445 amps total. If this is the utilities transformer, why not let them decide. You need to record the max demand. The NEC allows you to do this. Suggest you get the utility involved.
They may can provide the kw demand for the present customers.
The mall is only thinking about the idea in the early prelimary stages, if they just want to know if having the utility change the transformer is a definate yes if this tenant moves in, a no, or a probably. Thought I could help give them one of those three answers. Where I work I have yet to be asked to track the average usage, I believe utility does that already. Thanks for your input.
May be I'm out of my element here but I thought I said it's 120/120/120 and any two phases give 208. That's what I meant when I said 3 phase 120/208 no high leg. Don't they refer to this as a 120/208 "Y" config? Not understanding what else I can provide. But some of the other posts have calc the trans at about 1400 therefor I believe that there exists a strong probability utility will want to change transformer. Please let me know if I misuderstood.
Load???
Load???
it's all about the load. because the panel or associated fused disc. is rated at 200A with a 200A fuse I highly doubt that the load is close, the best approach would be (as others have already stated) to get the peak max demand for the xfmr. from the utility to determine if the xfmr. can handle it. also what is the new space going restaurant, retail? what potential load will you be adding? There are many cases where panels are filled with breakers with minimal load when the reading is done due to diversity of the space.
Load???
it's all about the load. because the panel or associated fused disc. is rated at 200A with a 200A fuse I highly doubt that the load is close, the best approach would be (as others have already stated) to get the peak max demand for the xfmr. from the utility to determine if the xfmr. can handle it. also what is the new space going restaurant, retail? what potential load will you be adding? There are many cases where panels are filled with breakers with minimal load when the reading is done due to diversity of the space.
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
Yes. That is a standard three phase system. The total rating of the transformer is 1,944 amps, as has already been stated. You get that by taking the KVA rating of 700,000, dividing that by the 208 volt secondary rating, and dividing again by 1.732 (the square root of 3). Some people combine the two division steps by dividing once, by 360. It's the same thing.
But to answer your question, you can never add load to an existing building without proving that the new load will not overload the system. One way to do that is to add up each and every single load that is connected. That is a bit inconvenient, and is seldom the method used. Another way is to measure the peak load. You can do that with utility records, if they can give you a years worth of peak load measurements. Otherwise, you can connect a load meter that takes measurements every 15 minutes for a total of 30 days.
You determine if there is capacity for the new load by taking the peak load, adding 25%, then adding the proposed new load, and comparing it to the 1944 amp capacity.
Reference: NEC 220.87
No fault with your information.
My post was in response to erickench's post #12.
I was just drawing his attention to the fact that you had provided that information and where he could find it.
You should really only be worried about the size of the switchboard(s) per the other posts about demand. If the utility needs to change the transformer (and possibly the pad) then they will make that decision. And they may reduce or eliminate the fees due to the added revenue. Of course you would still have to charge for your part of the work of trenching/grounding etc. <G>
Like they've said a demand reading from the utility on existing customers is needed.
Also yes, 1944 amps is the fully loaded amperage to get 700 kva.
Now consider that most transformers will never see the 2000 amps of connected load. Also consider that utility transformers are routinely expected to exceed their rating to 150% of it, during seasonal peaks.
So to add another 800 amps of "connected load" is pushing it right out to the extreme. Real loads run in the 40-60 % of "connected load" as stated before. So 2800 amps of connected load could turn out to be only 1400 amps of demand - still within the rating of the transformer, without pushing into 150% range.
I think you'd be ok but you need to see if the demands are reasonable to go forward. You don't want any surprises.
Also yes, 1944 amps is the fully loaded amperage to get 700 kva.
Now consider that most transformers will never see the 2000 amps of connected load. Also consider that utility transformers are routinely expected to exceed their rating to 150% of it, during seasonal peaks.
So to add another 800 amps of "connected load" is pushing it right out to the extreme. Real loads run in the 40-60 % of "connected load" as stated before. So 2800 amps of connected load could turn out to be only 1400 amps of demand - still within the rating of the transformer, without pushing into 150% range.
I think you'd be ok but you need to see if the demands are reasonable to go forward. You don't want any surprises.
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