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Old 05-28-2012, 06:20 PM   #1
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Load calculations


When doing load calcs for a 20A circuit how close to 20A is allowed by code?
Am I using the formula P=EI?
This circuit is for kitchen/dining area lighting which will include approx. 11 recessed cans, ceiling fan w/4 lights, fixture over island and under counter lighting along approx 18' of counter.

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Old 05-28-2012, 06:23 PM   #2
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Residential lighting is not considered a continuous load you you can go all the way to 20 amps. Many would limit themselves to 80% or 16 amps.

Remember to calculate the maximum wattage of the recessed, not just the bulb wattage used.

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Old 05-28-2012, 06:33 PM   #3
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Residential lighting is not considered a continuous load you you can go all the way to 20 amps. Many would limit themselves to 80% or 16 amps.

Remember to calculate the maximum wattage of the recessed, not just the bulb wattage used.
I am approximating at this time a 2KW load as the absolute max. Am I using 110V as my reference? And if so I am at 18A. Would you leave it on the one 20A circuit?
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Old 05-28-2012, 06:39 PM   #4
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For calculations the correct voltage is 120, not 110.

At 17 amps I would leave it as one circuit.
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Old 05-28-2012, 06:47 PM   #5
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Originally Posted by Jim Port View Post
For calculations the correct voltage is 120, not 110.

At 17 amps I would leave it as one circuit.
I am surprised to learn 120 is for calculations. I have not measured in this house but I seem to remember seeing 110v or maybe even a bit less in a previous residence so I assumed 110v would be the reference.
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Old 05-28-2012, 06:57 PM   #6
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Quote:
Originally Posted by elmaur View Post
I am surprised to learn 120 is for calculations. I have not measured in this house but I seem to remember seeing 110v or maybe even a bit less in a previous residence so I assumed 110v would be the reference.
I can cite the NEC section if you want, but Jim is correct.

Here:

220.5 Calculations.
(A) Voltages. Unless other voltages are specified, for purposes
of calculating branch-circuit and feeder loads, nominal
system voltages of 120, 120/240, 208Y/120, 240, 347,
480Y/277, 480, 600Y/347, and 600 volts shall be used.

Last edited by Code05; 05-28-2012 at 06:59 PM. Reason: add code section
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Old 05-28-2012, 07:01 PM   #7
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Quote:
Originally Posted by elmaur View Post
I am surprised to learn 120 is for calculations. I have not measured in this house but I seem to remember seeing 110v or maybe even a bit less in a previous residence so I assumed 110v would be the reference.
110, 115, and 117 are all previous numbers used, we now use 120 as a nominal number in the US.
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Old 05-28-2012, 07:03 PM   #8
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Load calculations


Quote:
Originally Posted by elmaur View Post
When doing load calcs for a 20A circuit how close to 20A is allowed by code?

2400 watts per 20 amp circuit is acceptable....
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Old 05-28-2012, 07:08 PM   #9
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2400 watts per 20 amp circuit is acceptable....

Yes, but usually I do my calcs in VA, I do not always know my PF.
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Old 05-28-2012, 07:13 PM   #10
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Yes, but usually I do my calcs in VA, I do not always know my PF.
lol... good idea.
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Old 05-28-2012, 07:21 PM   #11
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Yes, but usually I do my calcs in VA, I do not always know my PF.
What is PF?
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Old 05-28-2012, 07:28 PM   #12
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Quote:
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What is PF?

Power Factor, forget about it, it is an electrician thing.

I was just teasing Stickboy.
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Old 05-28-2012, 07:28 PM   #13
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What is PF?
Power Factor
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Old 05-28-2012, 07:31 PM   #14
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Actually, you should know one thing: if anyone tries to sell you a device saying that it will correct yours-run like hell, it is a scam.
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Old 05-28-2012, 07:43 PM   #15
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Originally Posted by Code05 View Post
Power Factor, forget about it, it is an electrician thing.

I was just teasing Stickboy.
Actually it's an electrical theory thing. I studied electrical/electronics technology in school many years ago. I do know there is a difference in calculating DC and AC values. I need to brush up on it.
BTW P=EI is the same as P= VA


Last edited by elmaur; 05-28-2012 at 07:45 PM.
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