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#46851 - 01/04/05 12:30 PM Residential Transformer size
srjc69 Offline
Junior Member

Registered: 01/03/05
Posts: 1
Loc: san francisco, ca
I have 200 amp service to my home. Connected to this is two a/c units, an air compressor, small welder, stove, and other stuff. Now the transformer on the pole is a 15kva (because theres a big '1 5' on the side of it). now sometimes my power usage can be up to 125 amps for a while. would the main on my subpanel break before anything in the transformer? what is that 15kva rating anyways? is that like a duty cycle rating? anyones brain i can pick?

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#46852 - 01/10/05 05:52 PM Re: Residential Transformer size
circuit man Offline

Registered: 11/05/01
Posts: 287
Loc: saluda,s.c.
KVA = kilovolt the transformer would be 15,000 this would be 125 amps@120volt 0r 62.5@240volts. the transformer will deliver this on a continous basis. but it also has a built in overload capacity as the power company tech said but i don't recall how much.the breaker in the house before the breaker on the transformer would trip or the primary fuse would blow if it happened to have one externally. some of the other guys can proably go into more detail.hope this helps!

#46853 - 01/11/05 09:23 AM Re: Residential Transformer size
dereckbc Offline

Registered: 10/08/03
Posts: 158
Loc: Tulsa, OK
KVA as already explained is kilovolt-ampere, or the apparent power in wattage the transformer is capable of delivering on a continuos basis. Since your service is rated @ 200-mps, it is theoretically possible you could demand 48KVA from a 15 KVA transformer, which does not appear to make since. It is like writing a $48,000 check with only $15,000 in the account. However a utility company will never size a residential transformer to the size of the service. The transformer does seem undersized, as I would suspect to see at least something along the lines of 20-to-30 KVA for a 200-amp service.

From a safety point of view, you are not in any danger. If you overload the transformer, the transformer fuse will operate and put you in the dark. Your 200-amp service (common size for medium sized homes) was selected based on the size of your house and the computed load that is possible. If during the summer months you notice your light staying dim while both AC units and electric stove are on, report it to the utility and they will investigate and determine if they need to upsize your transformer.

#46854 - 12/23/05 02:30 AM Re: Residential Transformer size

Registered: 12/08/05
Posts: 862
Loc: Titirangi, Akld, New Zealand
We replace a lot of older 100 kVA 3Ø transformers with 200 kVA or 300 kVA after dataloggers were installed.
The tranny's have been taking hughe overloads for years without any major problems. Last week we had a 100kVA blow up and loose all its oil after been running at 384 Amps almost continuous on 3 Ø. ( Inom is 139 Amps Ø )
We found that out when the temp. 200 kVA generator stalled about 3 times and needed to be replaced with a 325 kVA genny.
HT fuses are usually overrated, because they need to supply the backfeed line if a fault occurs via the open point. Often a
56 Amps 11kV HRC fuse is the 11 kV protection fuse for the primary side of the tranny ( Inom 5.5 Amps ) LT fuses non existent or double 200 Amps NZI's or 5 way 160 Amps DIN panels.
This is for 11000/230/400volts system.
The product of rotation, excitation and flux produces electricty.

#46855 - 12/23/05 06:22 AM Re: Residential Transformer size
WFO Offline

Registered: 09/03/05
Posts: 206
Loc: Cat Spring, TX
Transformers are traditionally undersized due to a diversity factor. You may have all the items you listed, but how many are running simultaneously?
As noted above, their rating is for continuous duty...temporary overloads are factored into the equation. Unless they remain on for extended periods of time (causing the transformer to overheat), they won't hurt a thing.

We fuse our transformers for 100% overload. So assuming a 7200 volt primary, we would stick in a 5 amp fuse (it calculates out to something like a 4.16 fuse, but that is not a standard size).
That would get you up to 36 Kva (150 amps on your side) before the fuse "theoretically" should start to melt.

But primary fuses like the standard S&C fuses we use don't actually start to open (referred to as "minimum melt") until they hit approximately twice their rating. So the 5 amp fuse mentioned above won't start to melt until it approaches 10 amps (which doubles the Kva again).
So pulling 125 amps intermittently won't hurt a thing, unless you leave it on forever. But be cognizant of the possibilities, and check with your POCO if you're worried.

[This message has been edited by WFO (edited 12-23-2005).]

#46856 - 12/23/05 01:17 PM Re: Residential Transformer size
mxslick Offline

Registered: 10/24/04
Posts: 785
Loc: Atomic City, ID USA
Another thing I didn't see mentioned here, but depending on your location and POCO's standard operating practices, there may be more than one transformer feeding the lines to your house.

Look at the wiring at the pole where your drop attaches. If there is a triplex or three wires leading off to other poles, then chances are you'll see more tx's hooked uo to that line (commonly called a lateral).

The idea is the diversity of loads on the multiple homes won't exceed the long-term capacity of all the tx's serving that lateral.

In some rural areas they will place a tx at each house.

A 100% overload short-term doesn't seem to harm most pole-mount transformers.
Stupid should be painful.

#46857 - 12/24/05 03:41 AM Re: Residential Transformer size
iwire Offline

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
We can forget anything we have learned from the NEC when it comes to utility company transformers.

They simply will not ever seem large enough to someone trying to apply the NEC to them.

The POCOs primary fuse will not protect the transformer at all. All it will do is open if the transformer has a catastrophic failure and at that point the POCO wants that transformer off line so the rest of that circuit can stay on.

The POCO has a long history of what works and does not work, it would be extremely wasteful for the POCO to install the same size transformers we would. All that lost power running large transformers when smaller ones will do the job.

As has been mentioned the oil filled transformers on poles can take repeated short term overloads with no damage.

If eventually the continuous load exceeds the rating then you will see a failure.

[This message has been edited by iwire (edited 12-24-2005).]
Bob Badger
Construction & Maintenance Electrician

#46858 - 12/30/05 08:04 PM Re: Residential Transformer size
gobblerhuntr Offline

Registered: 05/26/05
Posts: 23
Loc: TN
I have sized many, I mean many for residential dwellings. Right know myself and my neighbor are running off of a 15 kva and we have about 2300 to 2500 square foot houses. You have to be careful when sizing, I have seen a mobile home have to have a 25 kva on a 200 amp service (poor insulation, thin wall, etc.) and have seen many 200 amp houses run on 10 kva pots forever. Diversity is how we get away with the small size pots, also the pots we use have a 150% overload capacity, as well as most in the nation. We don't mind running our residential pots a little hot but our commercial and industrial we limit to 100%.

#46859 - 12/31/05 12:37 AM Re: Residential Transformer size
Trumpy Offline


Registered: 07/05/02
Posts: 8532
Loc: SI,New Zealand
RODALCO speaks volumes from a PoCo perspective as far as us workers in New Zealand go.
We had a guy ring us just after Xmas to tell us "the box on your pole" had smoke coming out of it.
What he failed to tell us was the fact that the bottom had rusted out of it and there was a nasty black oil patch under the transformer.
I rang the Fire Service upon getting there.
I've never in my life heard that sound come out of any Electrical Equipment.
500kVa transformer slowly cooking, low fat style.
Guy also paid me out for spraying shingle on his newly laid driveway.
Not my fault he had such a long spur line that I had to get to the end of.
Earlier warning could have saved that.

[This message has been edited by Trumpy (edited 12-31-2005).]
Let's face it, these days if you're not young, you're old - Red Green grin


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