# Basic electrical power consumption in Amps

T

#### [email protected]

Jan 1, 1970
0
If a single phase 240 Volt power pole transformer is rated at 25 KVA,
that calculates out to roughly 104 amps maximum at 240V.

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).

T

#### [email protected]

Jan 1, 1970
0
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.

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)

---
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,
you could get a complete meal with all the fixings and dessert too.

One problem. Many women in town converged on the town's community
center building, all equipped with an electric roaster filled with food.

They plugged in roaster after roaster, and had to move several of them
after tripping some breakers. They finally got them all working. There
were around 25 roasters total, but not all were plugged in at once.
However they did manage to plug in about 15 of them. Then they plugged
in a half dozen huge industrial sized coffee pots, cranked up the walk
in freezer for the ice cream, turned on all the lights in the large
dining room, and since it was a hot day, they turned on the air
conditioner.

For awhile, everything worked fine. Right when the people were coming
in to eat, the place went dark. No lights, all the roasters went off,
the air conditioning went off, the water stopped flowing from the sinks
(well pump), and everyone was eating in the dark. Minutes later the
fire department arrived with a large generator. The generator failed
after 3 minutes of use and although the gas engine still ran, it would
not put out even one volt of power. (They had hooked it to the
building's mains).

About that same time, the electrician arrived, several neighbors whose
homes are nearby came over and said they had no power in their homes.
OOPS!!!! The electrician confirmed that there was no power coming from
the pole to that building, nor to all the homes for a couple block area.

It was a hot day, as well, so lots of people were running Air
conditioners.

Yep, all those roasters and coffee pots and ACs and other stuff, roasted
the pole transformer.

It took the power company about an hour to arrive, and another 20
minutes to locate the drivers of cars that were parked in the way of
power company's trucks. (by that pole). They finally got to the top of
the pole, when one of the workers began to remove wires, he touched the
transformer with his bare hands and burned his fingers. (by this time
the power had been off for over one and a half hours, so it had cooled
down some).

They removed the 25 KVA transformer and replaced it with a 37.5 KVA
model.

When I got home, I did some figuring. Those roasters (most are old),
consume about 14 amps each. Those huge coffee pots are not much less.
So, lets just use round figures and say they averaged 12 amps (minimum)
per unit. 15 roasters, and 6 coffee makers is 21 devices. That works
out to a draw of 252 amps at 120V AC. Add to that the freezer, the AC,
the lights, and then add all the stuff that was running in homes along
that block.

(The building has a 200A main)

It's no surprise that the transformer failed.

T

#### tm

Jan 1, 1970
0
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)

---
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,
you could get a complete meal with all the fixings and dessert too.

One problem. Many women in town converged on the town's community
center building, all equipped with an electric roaster filled with food.

They plugged in roaster after roaster, and had to move several of them
after tripping some breakers. They finally got them all working. There
were around 25 roasters total, but not all were plugged in at once.
However they did manage to plug in about 15 of them. Then they plugged
in a half dozen huge industrial sized coffee pots, cranked up the walk
in freezer for the ice cream, turned on all the lights in the large
dining room, and since it was a hot day, they turned on the air
conditioner.

For awhile, everything worked fine. Right when the people were coming
in to eat, the place went dark. No lights, all the roasters went off,
the air conditioning went off, the water stopped flowing from the sinks
(well pump), and everyone was eating in the dark. Minutes later the
fire department arrived with a large generator. The generator failed
after 3 minutes of use and although the gas engine still ran, it would
not put out even one volt of power. (They had hooked it to the
building's mains).

About that same time, the electrician arrived, several neighbors whose
homes are nearby came over and said they had no power in their homes.
OOPS!!!! The electrician confirmed that there was no power coming from
the pole to that building, nor to all the homes for a couple block area.

It was a hot day, as well, so lots of people were running Air
conditioners.

Yep, all those roasters and coffee pots and ACs and other stuff, roasted
the pole transformer.

It took the power company about an hour to arrive, and another 20
minutes to locate the drivers of cars that were parked in the way of
power company's trucks. (by that pole). They finally got to the top of
the pole, when one of the workers began to remove wires, he touched the
transformer with his bare hands and burned his fingers. (by this time
the power had been off for over one and a half hours, so it had cooled
down some).

They removed the 25 KVA transformer and replaced it with a 37.5 KVA
model.

When I got home, I did some figuring. Those roasters (most are old),
consume about 14 amps each. Those huge coffee pots are not much less.
So, lets just use round figures and say they averaged 12 amps (minimum)
per unit. 15 roasters, and 6 coffee makers is 21 devices. That works
out to a draw of 252 amps at 120V AC. Add to that the freezer, the AC,
the lights, and then add all the stuff that was running in homes along
that block.

(The building has a 200A main)

It's no surprise that the transformer failed.

---
Comment - Personally, I think that 37.5 transformer should be dedicated
ONLY to the Community Center, and another one (or several) should be
added for the nearby homes. "Women with electric roasters are dangerous
to the electrical system ".

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

T

#### [email protected]

Jan 1, 1970
0
Wait until they start plugging in their government mandated electric cars.

I can just imagine!!!!

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

T

#### [email protected]

Jan 1, 1970
0
---
Sorry, but it's wrong.

If you have a transformer with a 240V secondary rated to supply 25kVa

P 25000VA
I = ----- = --------- ~ 104A
E 240V

A transformer with a center-tapped 240V secondary rated at 25kVA into
a resistive load(s) is the same beast, but with a lead brought out at
the point where there are an equal number of turns on both sides of
the point where it's connected to the secondary.

Therefore, for the same I²R losses in the secondary, you can get up to
104A across the whole secondary, or up to 104 amps out of each side of
the center tap.

The rub is that 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.

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."

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.

P

#### P E Schoen

Jan 1, 1970
0
tangerine3 wrote in message

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.

You can get 104 amps from each leg at 120V. And if the windings were in
parallel, you could get 208 amps. If the load is totally unbalanced, you can
draw 208 amps at 120V and still not trip the primary fuse, because it will
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
overloaded by 2x, and will overheat. But for the same primary current, you
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,
which would have prevented the problem - if it were the only breaker on the
line. But apparently there were other customers on the same secondary, and
they may also have had unbalanced loads. This is why it's important to have
most high-power appliances on the 240V line, and the 120V circuits should be
planned so as to balance the loads as equally as possible, perhaps by having
every other outlet on opposite phases.

Even though houses may have the electrical service upgraded to 200 amps, it
is usually assumed that each house will use, on average, maybe 1/4 to 1/2
that much. But the utility company may not always be aware of such upgrades,
and electricians may not always balance the loads, especially in older homes
and facilities where circuits may be added based on convenience rather than
optimal design practice.

total.
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."
Or else I'm not understanding what you said.....

P 25000VA
I = ----- = --------- ~ 104A
E 2*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.

Many of those loads are intermittent, and sizing of distribution depends on
the duty cycles being small and randomness should assure they are not all
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
fuse.

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

Paul

K

#### [email protected]

Jan 1, 1970
0
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.

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.
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."

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.

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

P

#### Phil Allison

Jan 1, 1970
0
"Mike Perkins"
If now running at 60Hz rather than 50Hz, regulation would further
deteriorate proportionately.

** Nonsense.

.... Phil

T

#### [email protected]

Jan 1, 1970
0
Many of those loads are intermittent, and sizing of distribution depends on
the duty cycles being small and randomness should assure they are not all
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
fuse.

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

Paul

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!!!

P

#### Phil Allison

Jan 1, 1970
0
"Mike Perkins"
I was thinking leakage inductance where reactance is nominally
proportional to frequency

** 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

J

#### Jasen Betts

Jan 1, 1970
0
If a single phase 240 Volt power pole transformer is rated at 25 KVA,
that calculates out to roughly 104 amps maximum at 240V.

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).

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

J

#### Jasen Betts

Jan 1, 1970
0
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!!!

If it burned out and didn't just go into some sort of thermal shutdown.
I reckon it's probably up a different pole now.

J

#### Jamie

Jan 1, 1970
0
Robert said:
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?
What circuit breakers? You mean those annoying switch looking thing that
keeps turning itself off? Just wedge a screw in the switch gear, it'll
solve those pesky problems!

Jamie

T

#### [email protected]

Jan 1, 1970
0
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

I doubt any system would ever be *perfectly* balanced, (especially with
thermostatically controlled devices), but if things are wired properly,
I could see it being close to a balanced situation. However, in the
case described here, I'd have my doubts of any real balance. Like I
said, trailer homes were added later, that large community center was
built in the last decade and was built where a few old houses were
demolished, when they likely just hooked it to the same pole without
giving much thought to the transformer size..... The engine repair shop
was built in the last year or two, and was wired to that same pole....
and so on....

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