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How Many Amps From The Xfrmr?

 
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Old 11-28-2009, 03:00 PM   #1
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How Many Amps From The Xfrmr?


Driving by a recently renovated building in town today got me wondering...

This building was converted to 4 apartments, so there are five meters on the side.

Assuming that each service is 100A, the building could, in theory, pull 500A from the pole.

So, what's the max amperage one of these standard sized pole transformers will supply?
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Old 11-28-2009, 03:24 PM   #2
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Re: How Many Amps From The Xfrmr?


Tree Fiddy

Oops-wrong thread just kidding!

Actually I am curious too!

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Old 11-28-2009, 03:38 PM   #3
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Re: How Many Amps From The Xfrmr?


Dependent more on the size of the wire to them, the voltage to them, etc.

The size of the transformers are just a function of the number turns that the wire makes in the transformer. At least, that's what I recall from physics.

For example, a 20kv line leading into a transformer that turns it down to 230 volt will have more 'turns' in the transformer than one that converts 10kv down to 230v. Higher amperage can be obtained at the users end with the same size cables between the PUD & the transformer by running higher voltage into the transformer, and then just larger wire from the transformer to the meter bases.

So...I think the question comes to - what's the largest practical wire size to run between the transformer and the meter base, based on a realistic length between the two places? Copper 1000 wire is rated to 545 amps @ 75 deg. Celsius. This may be the largest wire commonly available, based on the chart I'm looking at.
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Old 11-28-2009, 04:46 PM   #4
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Re: How Many Amps From The Xfrmr?


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Tree Fiddy
Maybe Five Fiddy?
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Old 11-28-2009, 04:51 PM   #5
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Re: How Many Amps From The Xfrmr?


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Originally Posted by piercekiltoff View Post
So...I think the question comes to - what's the largest practical wire size to run between the transformer and the meter base, based on a realistic length between the two places?
Valid point there.

I'm just thinking that there must be a Model XYZ transformer that is typically installed in residential usage. How many amps can that supply?
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Old 11-28-2009, 05:36 PM   #6
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Re: How Many Amps From The Xfrmr?


First we need to know if the service is single-phase or three-phase.

Second, we need to know square footage of the building and some of the appliances that'll be used so we can define a calculated load.

Last but not least, (5) 100 amp service disconnects does not constitute a 500 amp load. In fact, you could have as many disconnects rated at 100 amps as you'd like but after (6) of them you'll need (1) main disconnect so the fire department can quickly disconnect power to the building in the event of a fire.
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Old 11-28-2009, 06:41 PM   #7
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Re: How Many Amps From The Xfrmr?


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First we need to know if the service is single-phase or three-phase.

Second, we need to know square footage of the building and some of the appliances that'll be used so we can define a calculated load.

Oh damn...this is getting complicated.

I thought I had a simple question that someone was going to answer "Geez dumbass...you can only get X amps out of that".

I'm not looking to do a load calc on the apartment, I just want to know how many amps you can pull from a typical residential pole transformer before it starts to melt.

Quote:
Originally Posted by Magnettica View Post

Last but not least, (5) 100 amp service disconnects does not constitute a 500 amp load.
But, it could draw 500A in our hypothetical world, correct?
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Old 11-28-2009, 06:57 PM   #8
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Re: How Many Amps From The Xfrmr?


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Old 11-28-2009, 09:22 PM   #9
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Re: How Many Amps From The Xfrmr?


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Originally Posted by DetailHandyman View Post
Valid point there.

I'm just thinking that there must be a Model XYZ transformer that is typically installed in residential usage. How many amps can that supply?

22,000.

No joke. That's probably the short-circuit current rating.
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Old 11-28-2009, 09:32 PM   #10
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Re: How Many Amps From The Xfrmr?


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22,000.

No joke. That's probably the short-circuit current rating.



I believe.
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Old 11-28-2009, 10:16 PM   #11
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Re: How Many Amps From The Xfrmr?


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I believe.

Especially when you've witnessed the Big Blue Zot.
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Old 12-02-2009, 12:31 AM   #12
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Re: How Many Amps From The Xfrmr?


Amps..
+ YouTube Video
ERROR: If you can see this, then YouTube is down or you don't have Flash installed.
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Old 12-02-2009, 09:16 PM   #13
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Re: How Many Amps From The Xfrmr?


Quote:
Originally Posted by DetailHandyman View Post

I'm not looking to do a load calc on the apartment, I just want to know how many amps you can pull from a typical residential pole transformer before it starts to melt.



But, it could draw 500A in our hypothetical world, correct?

Well, it depends... but let's say it is single phase.

At 240 volts, a 50kva transformer can provide 208 amps.

A 75kva can give you 312.

Ohm's law = P/ E = I or watts divided by volts equals amps.

The type they use on the street vary dependent upon the actual connected load. Transformers also look different from state to state, believe me, I know because I look at them.

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Old 12-04-2009, 03:53 PM   #14
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Re: How Many Amps From The Xfrmr?


Quote:
Originally Posted by piercekiltoff View Post
Amps..
+ YouTube Video
ERROR: If you can see this, then YouTube is down or you don't have Flash installed.
That was amazing!
I saw a back-hoe grab 13.2 once on a job-site near a pad-mount transformer. The machine had every joint welded together by the time power was shut down, like a huge sculpture.
Best one I ever saw was a phase-to-phase fault on a 2000 kva 2-phase transformer. It burned back from a connector and puddled the conductors, creating an arc furnace in a duct under 16th street in Philadelphia. It was spewing molten copper out an uncovered end-box. The faces of the firemen who responded were completely WHITE because the copper was spraying onto a bank of gas meters.

Anyway, available short circuit current is also a function of tansformer impedance. You can get hellish currents for short periods.

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