Basic electrical power consumption in Amps

If a single phase 240 Volt power pole transformer is rated at 25 KVA, that calculates out to roughly 104 amps maximum at 240V.

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But lets say that all the devices connected are 120V. Does this mean that the maximum amps at 120V is double that figure? In other words, the maximum amperage of combined 120V devices would be

208 Amps?

I believe this is right, but I'm double checking..... (Yea, I'm aware that the balance between the two legs of the centertapped 240 would need to fairly equal).

Reply to
tangerine3
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I wouldn't to run it at much more that 100 Amps as the I2R losses would predominate at both 120V and 240V. Regulation would of course be double that at 240V. If now running at 60Hz rather than 50Hz, regulation would further deteriorate proportionately.

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Mike Perkins
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Reply to
Mike Perkins

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I'm curious. Why would 60hz and 50hz be different?

My reason to post this is because of what happened yesterday in a nearby town. (See below)

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I dont believe this is a Memorial Day tradition, but a small rural town
had a big dinner cooking for the whole town.  For a reasonable price,
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Reply to
tangerine3

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Wait until they start plugging in their government mandated electric cars.

Reply to
tm

I can just imagine!!!!

How much power do this cars need during a recharge anyhow?

Reply to
tangerine3

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Reply to
John Fields

You lost me.....

First you said "or up to 104 amps out of each side of the center tap." That tells me that you'll get 208 amps of 120volt power.

Then you say "since each half-secondary can only supply 120V, the maximum power it can deliver into each load will be half of what can be transferred through the entire secondary feeding a 240V load."

This contradicts your previous statement. Or else I'm not understanding what you said.....

P 25000VA I = ----- = --------- ~ 208A E 120V

Of course one has to consider that this situation was using both 120v and 240v. All those roasters and coffee makers were pulling a very heavy 120v load, while the Air Conditioners, Walk in freezer, Well pump, and probably other things in that building and in nearby homes were using the 240v at the same time.

Reply to
tangerine3

You lost me.....

You can get 104 amps from each leg at 120V. And if the windings were in=20 parallel, you could get 208 amps. If the load is totally unbalanced, you = can=20 draw 208 amps at 120V and still not trip the primary fuse, because it = will=20 draw the current based on the 25 kVA (probably at 13 kV), or 2 amps. But =

with such an imbalanced load, the one set of secondary windings will be=20 overloaded by 2x, and will overheat. But for the same primary current, = you=20 can safely draw 104 amps on each leg, and remain within rating.

The line side breaker probably should have been rated at 100 or 125 = amps,=20 which would have prevented the problem - if it were the only breaker on = the=20 line. But apparently there were other customers on the same secondary, = and=20 they may also have had unbalanced loads. This is why it's important to = have=20 most high-power appliances on the 240V line, and the 120V circuits = should be=20 planned so as to balance the loads as equally as possible, perhaps by = having=20 every other outlet on opposite phases.

Even though houses may have the electrical service upgraded to 200 amps, = it=20 is usually assumed that each house will use, on average, maybe 1/4 to =

1/2=20 that much. But the utility company may not always be aware of such = upgrades,=20 and electricians may not always balance the loads, especially in older = homes=20 and facilities where circuits may be added based on convenience rather = than=20 optimal design practice.

And of course hot weather causes the additional A/C load which adds to = the=20 total.

I fixed your formula above:

Many of those loads are intermittent, and sizing of distribution depends = on=20 the duty cycles being small and randomness should assure they are not = all=20 running at the same time. But sometimes they just happen to synchronize, =

although this usually causes a very high peak current which will blow = the=20 fuse.

This was probably a "perfect storm" of coincidental conditions.

Paul

Reply to
P E Schoen

Yes, if you can use both halves for separate loads. If you only have one 120V load, you can only attach it to one side of the transformer, so you're limited to the 104A number. You can put two 104A 120V (or combinations adding to

104A) loads on the transformer, though.

Pesumably those 120V roasters are split evenly across both sides of the 240V line.

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Reply to
krw

I both halves of the center-tapped secondary can carry 104 amps, simultaneously, just one half could carry more. If the overall cooling of the can is the limit, one side loaded alone could carry 147 amps to have the same overall power dissipation and the same can surface temperature. In practice, some local heating effects might reduce the single-coil capacity to somewhere between 104 and 147 amps, probably towards the high end.

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John Larkin         Highland Technology, Inc

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Reply to
John Larkin

"Mike Perkins"

** Nonsense.

... Phil

Reply to
Phil Allison

Big loads, a/c and electric heat and clothes driers, will be 240 volt loads, so balance. 120 volt loads could be assumed to statistically sorta balance among multiple loads and households. I'm sure the math has been studied hard.

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John Larkin         Highland Technology, Inc

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Reply to
John Larkin

I was thinking leakage inductance where reactance is nominally proportional to frequency

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Mike Perkins
Video Solutions Ltd
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Reply to
Mike Perkins

No, that's wrong, because I forgot to account for the primary winding copper loss. The actual single-side current is about 170 amps for the same overall power dissipation, if I got the algebra right. I don't do much algebra these days.

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John Larkin                  Highland Technology Inc
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Reply to
John Larkin

Having never used one of those electric roasters, I'm only guessing that they turn on an off by a built in thermostat, to retain a fixed temperature (like a range oven). I only learned that after getting some information about them on the web, and found they have a temperature control. So, that would account for the transformer lasting as long as it did. I'd guess that when they reach their desired temperature they either draw no current, or draw a smaller amount.

But still, with all that stuff running, and probably a large number of air conditioners, it reached it's limit. I was unable to determine what is all on those wires, but there are two blocks of houses (8 to 12 of them). An entire trailer park with at least 24 trailers, a few store fronts (which were closed), and a large small engine repair garage (also closed). Plus this Community Center which is a large building that was built about 8 years ago. I'd bet that a lot of those trailer homes had their AC turned on, because trailers get real hot in summer.

The power company seemed to think the replacement 37.5 kwh transformer was now adequate. I heard them telling that to the electrician. Personally, that still seems inadequate for what is connected. I'd probably put all the houses on one transformer, have another one for the trailer park, and a separate one for the Community Center, if not adding yet another for the stores and engine shop. But this is not my job. I was told by a resident that this is not the first time there have been problems with this system. The fuse on the input side of the transformer blew recently and that 25kwh transformer was replaced last year, and they had a 15 kwh before that. They also said that 6 trailer home were recently added to that trailer park.

In my opinion, the power company is not checking what is hooked to their lines too well. You'd think they would install larger transformers BEFORE they burn out. Those things are not cheap!!! I guess this is why they keep raising our electric rates!!!

On a positive note, the dinner was excellent, even if the room was dark and the coffee wasn't exactly hot! The Fire Dept did manage to get a smaller generator and hook up a few halogen work lights halfway thru the dinner. Many people just ate outdoors, which was fine. They just needed a few more picnic tables!!!

Reply to
tangerine3

** Which is as I suspected.

But LI is normally very small, so at low frequencies ( like 50/60Hz) has almost no effect on transformer voltage regulation compared to that of resistance in the copper windings.

With transformers that have the secondary wound over the top of the primary, LI is of little effect until the frequency is many kHz and with toroidal types, this is true out to 50kHz or more.

... Phil

Reply to
Phil Allison

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YOU connect _any_ 120V device to 240V; be absolutely sure that you are standing very close so the results fry you as well.

Reply to
Robert Baer

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NO such thing, "one volt of power"-- #1: INCONSISTENT UNITS #2: Just because one volt is available does not mean _any_ power is being delivered or consumed #3: With one volt available, one could be using a gigaWATT of power. #4: What the hell happened to the circuit breakers?

Reply to
Robert Baer

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Bur..but..motorbutt..electricity is free, and that system "reduces pollution"..

Reply to
Robert Baer

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yeah if it's perfectly balanced you get 104A through one 120V half and then the same current through the other 120V half. for a total

208A worth of 120V loads
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Reply to
Jasen Betts

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