treedoctor
Member
Here is a link I found after researching the topic briefly. I found it to be easy to understand and answered all my questions. I hope you find it to be as helpful as I have! 
Here is an excerpt:
"You're on location, in a house built in the 1940s. But you're worried about blowing fuses or popping a circuit breaker with the bright and hot lights from the nice Arrilite kit you brought.
You need a way to tell in advance, how to know when the next light you plug in will blow the circuit. You're not an electrician, you heard there was a formula, by some Ohm guy, but you have no idea what it is.
No matter -- There is a simple way to figure this out, and keep out of hot water (and in the light) with the homeowner."
http://lenses-and-lights.blogspot.com/2008/03/how-many-lights-can-i-plug-into-this.html
Here is an excerpt:
"You're on location, in a house built in the 1940s. But you're worried about blowing fuses or popping a circuit breaker with the bright and hot lights from the nice Arrilite kit you brought.
You need a way to tell in advance, how to know when the next light you plug in will blow the circuit. You're not an electrician, you heard there was a formula, by some Ohm guy, but you have no idea what it is.
No matter -- There is a simple way to figure this out, and keep out of hot water (and in the light) with the homeowner."
http://lenses-and-lights.blogspot.com/2008/03/how-many-lights-can-i-plug-into-this.html