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Old 11-30-2007, 11:06 AM   #1
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Calculating "load"

Hello, new guy here.
I have questions regarding how many lines I need to my breaker, wire gauge etc.

Here's what I need to do:
This is for my basement. I have a room where I have 12 x 35W halogen potlights. Also run 2 computers, scanner, fax, various other misc. electrical devices. An airbrush spraybooth, TV, and a couple other floor lamps. All in 1 room (approx. 15'x35').
My question, is there a way to calculate the gauge of wire I need to use and breakers etc. I have some open spots on my breaker, and I have an unused 2x15amp breaker switch for using 1 slot on the breaker (sorry if the terminology is way out of whack).

I've hooked up some lines previously so am somewhate familiar with this stuff.


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Old 11-30-2007, 06:07 PM   #2
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15 amp breakers use 14 gauge wire and 20 amp circuit use 12 gauge wire. Unless you have a extra long cable run there is no need to calculate anything regarding wire size.
The calculation you need is called a load calculation.


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Old 11-30-2007, 08:00 PM   #3
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Most likely if you put your lighting loads a 15 amp breaker and your cord and plug stuff on general purpose 20 amp branch circuit(s) you will be fine.

15 amp breaker = 14 awg copper wire

20 amp breaker = 12 awg copper wire.

Use 5-15r receptacles like these below for 15 amp circuits or 20 amp circuits the 5-20r is not required on a 20 amp branch circuit that serves more than one receptacle in the USA .....if in Canada I believe you must use 20 amp receptacles on 20 amp circuits and 15 amp receptacles on 15 amp circuits.

15 amp (5-15R) ... 20 amp ( 5-20R)

In the Usa and Canada....

A 15 amp branch circuit protected by a 15 amp circuit breaker will give you 1800 watts of power
A 20 amp branch circuit will give you 2400 watts of power.

It's becoming common to put entertainment equipment on its own 20 amp branch circuit.

Example.... You have 12 x 35watts = 420 watts... so you still have 1380 watts of power left on a 15 amp branch circuit. The idea here is if you approach 1800 watts of connected load and that load is all going to frequently be on at the same time then you need to design your circuits appropriately so you don't overload them. It is becoming more common for entertainment systems to require dedicated branch circuits because of the total load requirements. Anyway generally speaking lights on 15 and receptacles on 20 and you are usually fine.

Last edited by Stubbie; 11-30-2007 at 08:07 PM.
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Old 11-30-2007, 09:02 PM   #4
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I'm curious about the airbrush spraybooth. How many watts is that? I often run a seperate circuit for computers (20A) if it is in the budget. And what Stubbie said.

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