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Old 07-24-2012, 02:54 PM   #1
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A HO asks you


if he or she is getting the full power delivery capability of his/her 200A residential service so you rent a 48 kw jobsite heater and head over.

With no load on the panel you read 240.0 v and with the heater on you read 216.0 v.

In answer to the HO's question, what say you?

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Old 07-24-2012, 03:08 PM   #2
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What is the question?

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Old 07-24-2012, 03:18 PM   #3
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What say I is... Where did you plug the 48KW heater in at?
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Old 07-24-2012, 05:17 PM   #4
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I'm not an electrician, but I like puzzles, so here is my take.

If it's a 48kW, 240V heater, it's designed to pull 48000/240=200 A, so it's resistance is 240/200=1.2 ohms. Since there is only 216 V through it, it means it only pulls 216/1.2=180 A. At these currents and resistances, wire resistance is not negligible. So in fact we have a circuit with a 1.2 ohm machine and some wire with resistance R connected in line. We know (assume) that voltage is 240 V at the transformer, so we have that total resistance of the circuit is 240/180=1.333 ohms. That means that total wire (plus connections, etc) resistance is 0.133 ohm. Plausible to me. So I would say, it's normal.

That said, I'm also wondering where he/she plugged it in, given the fact that it's pulling 180A. Operating that beast costs $5/hr, wow.
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Old 07-24-2012, 06:48 PM   #5
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Quote:
Originally Posted by Julius793 View Post
What is the question?
...if he or she is getting the full power delivery capability of his/her 200A residential service
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Old 07-24-2012, 07:18 PM   #6
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Quote:
Originally Posted by Yoyizit View Post
...if he or she is getting the full power delivery capability of his/her 200A residential service
Ask the POCO.
If you think I am going to go out and rent a 48kW heater and hook it up to the main service you're NUTS!

Where the hell do you rent a 48kW electric heater anyway???
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Old 07-24-2012, 07:21 PM   #7
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Is the 48kw heater assumed to be accurately spec'd and labeled?
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Old 07-24-2012, 07:25 PM   #8
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What would be the point of this?
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Old 07-24-2012, 07:27 PM   #9
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Quote:
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What would be the point of this?
To melt a transformer?
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Old 07-24-2012, 07:29 PM   #10
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MOST power company transformers are undersized. It is quite common to have several houses connected to the same 25 or 50 kVA transformer. It's also common to have an overhead wire that appears to be undersized, such as a #4 aluminum ACSR serving a 200 Amp service.

25kVA is just over 100 Amps, and is usually just fine to serve a couple of houses with 200 amp services. They can operate at twice the rated capacity for a short duration if necessary. Power companies apply demand factors to their design of the grid to keep infrastructure costs to a reasonable level.

The problem is when you max out a residential panel by switching on a 48 kW load, you can discover the limits of a transformer, the connectors, wires, etc. If one wanted to permanently connect such a load, the POCO would certainly need to upgrade some of their local grid to handle the load(s), and possibly pass on some of the costs to those who require the unusually heavy loads.

Some folks who install electric on-demand water heaters find out this the hard way....
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Old 07-24-2012, 07:32 PM   #11
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I say no. 10% voltage drop is unacceptable.
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Old 07-25-2012, 10:08 AM   #12
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Thanks especially to KBSparky and Mpoulton for understanding the letter and the spirit of my question and answering it.

http://www.google.com/search?client=...w=1093&bih=741
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Old 07-25-2012, 06:02 PM   #13
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Quote:
Originally Posted by Yoyizit View Post
Thanks especially to KBSparky and Mpoulton for understanding the letter and the spirit of my question and answering it.

http://www.google.com/search?client=...w=1093&bih=741
You're welcome.
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Old 07-26-2012, 12:50 PM   #14
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I'm happy that everyone is satisfied, but my curiosity won't let me rest. So do you guys say that 0.13 ohm resistance for the circuit is impossibly high? Or something was wrong with my calculations?

Just to clarify basically what I said was the following: every time you operate an electronic device, you connect the device in series with the wires. Everything in a series circuit gets a voltage that is proportional to its resistance. If the wires have 0.13 ohm resistance, that is usually negligible, because almost everything has much higher resistance in the circuit, like a 60 W bulb has 120 ohms resistance. But this monster heater has only 1.2 ohms resistance, so the wires are not negligible, so they eat up some of the voltage (and heat up in the process).

So either I messed up something or you say the circuit can't have 0.13 ohm resistance.
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Old 07-26-2012, 01:41 PM   #15
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Quote:
Originally Posted by csab_ View Post
I'm happy that everyone is satisfied, but my curiosity won't let me rest. So do you guys say that 0.13 ohm resistance for the circuit is impossibly high? Or something was wrong with my calculations?
{snip}
So either I messed up something or you say the circuit can't have 0.13 ohm resistance.
0.13 ohm resistance for the service (which is what's being measured, not the branch circuit's resistance) is too high. 10% voltage drop is unacceptable. If it's a REAL 200A service that can actually be used for 200A total load, then it would have to have low enough voltage drop at full load to stay within reasonable voltage specifications. That's typically 5% or less, but 3% would be better.

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