I received the following email recently and felt that is was so good I had to post it. The author wishes to remain anonymous. For the uninitiated: A = amp, AWG or G = American Wire Gauge/Gauge (where smaller numbers equal thicker wires), W = watt; the names and their abbreviations are used interchangeably here.
I have a bit of an electrical background… I dont remember the specifics of what you said [in your book], but it was something to the effect of [to figure approximately] 5 amps per 600W light, and [don’t run more than three 600W ballasts] on a 20A circuit.
According to the 2011 NEC – AWG 14 @ 15amps, AWG12 @ 25A, and 10G @ 30A (solid copper). A lot of houses aren’t properly wired for whatever breaker it has, also we havent taken into account the % of efficiency of the ballast/light fixture. I worry and I have seen it – a 20amp breaker or 30A breaker on a strand of 14G wiring. Then some numb-nuts tossing a whole operation on one circuit… Poised for failure.
I always, when doing wiring, give an appliance (the ballast in this case) 90% efficiency (even though I know some are better and some worse). What this means is for a 1000W you’ll be bumping 10% on top of the number, and for the 600W +60W (1100W/660W, respectively).
That makes a 600W @ 5.5amp and a 1000W @ 9.166amp.
One of the other big deals is how long the wire run is. Also how many other things you have running on the wire–fans, water pumps, air pumps, ambient lighting, etc. All of these factors make it very difficult to [offer] “one-size-fits-all” [instructions] for wiring situations.
The main problem I have with the wiring is the ‘safety’ factor. Yes a 14AWG wire is technically rated at 20A – but you NEVER want to run a wire over 75% [of its] rated [load]. It can and will get hot and potentially burn up.
Figure: I used to run 2 x 1000W [ballasts] and, for the argument, they equal about 18amps (technically under the 20A limit of 14G) – however, you can damn near melt the insulation off of them with the heat that they’re making. 12G isn’t much better. I run 10G on a 25-30A breaker to run 2 x 1000W’s. (The wires stay nice and cool and I have no worries about burning my home down.) It is, however, a bit more expensive, but I feel it’s worth the extra $20-$30 to make sure I’m not homeless.
Superlative advice, folks. And remember: When in doubt, call in a pro.