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05-28-2012, 06:20 PM   #1
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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.

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.

__________________
Answers based on the National Electrical Code. Local amendments may apply. Check with your local building officials.

 The Following User Says Thank You to Jim Port For This Useful Post: Code05 (05-28-2012)
05-28-2012, 06:33 PM   #3
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Quote:
 Originally Posted by Jim Port 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?

 05-28-2012, 06:39 PM #4 Licensed electrician   Join Date: Sep 2007 Location: Maryland Posts: 10,861 Rewards Points: 1,876 For calculations the correct voltage is 120, not 110. At 17 amps I would leave it as one circuit. __________________ Answers based on the National Electrical Code. Local amendments may apply. Check with your local building officials.
 The Following User Says Thank You to Jim Port For This Useful Post: elmaur (05-28-2012)
05-28-2012, 06:47 PM   #5
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Quote:
 Originally Posted by Jim Port 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.

05-28-2012, 06:57 PM   #6
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Quote:
 Originally Posted by elmaur 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

 The Following User Says Thank You to Code05 For This Useful Post: elmaur (05-28-2012)
05-28-2012, 07:01 PM   #7
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Quote:
 Originally Posted by elmaur 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.

 The Following User Says Thank You to stickboy1375 For This Useful Post: elmaur (05-28-2012)
05-28-2012, 07:03 PM   #8
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Quote:
 Originally Posted by elmaur When doing load calcs for a 20A circuit how close to 20A is allowed by code?

2400 watts per 20 amp circuit is acceptable....

05-28-2012, 07:08 PM   #9
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Quote:
 Originally Posted by stickboy1375 2400 watts per 20 amp circuit is acceptable....

Yes, but usually I do my calcs in VA, I do not always know my PF.

05-28-2012, 07:13 PM   #10
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Quote:
 Originally Posted by Code05 Yes, but usually I do my calcs in VA, I do not always know my PF.
lol... good idea.

05-28-2012, 07:21 PM   #11
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Quote:
 Originally Posted by Code05 Yes, but usually I do my calcs in VA, I do not always know my PF.
What is PF?

05-28-2012, 07:28 PM   #12
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Quote:
 Originally Posted by elmaur What is PF?

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

I was just teasing Stickboy.

05-28-2012, 07:28 PM   #13
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Quote:
 Originally Posted by elmaur What is PF?
Power Factor

 05-28-2012, 07:31 PM #14 Member     Join Date: May 2009 Location: Easton MD Posts: 1,893 Rewards Points: 1,104 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.
05-28-2012, 07:43 PM   #15
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Quote:
 Originally Posted by Code05 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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