How many lights per circuit

[Bob Cooper]

When calculating the number of fluorescent fixtures per circuit how much wattage should I assume the ballast consumes. There are four 34watt bulbs/fixture -- the standard ones found at the big box stores.

[Nick Lazz]

Not an electrician here (but I play one all the time ) but I did my shop based on some online research and questions at my local electrical supply. If you multiply your volts and amps you get a wattage per circuit. Ex: 120 x 20 = 2400. It is recommended (or so I have read) that if the circuit is going to be used significantly use an 80% rule. 2400 x .80 = 1920 watts.

Find out what your ballasts / bulbs are pulling and you should be able to figure this out with that. Real electricians can chime in.

I found out - its a lot. I had no problem wiring all my lights on one circuit (20amp) I have 9 fixtures that carry 3-4' T8 bulbs. I can't remember my wattage, but I had room to spare.

Edit: Go to your local electrical supply for the fixtures. You will likely get good advice and much better selection for quality fixtures. The ones at the big box stores are really cheap in both price and quality, IMO.

Good luck!

Nick

[Mike Henderson]

Let me see if I understand the question. You can figure out the current draw of the bulbs because you have the wattage of each bulb. It sounds like you're asking how much additional power is consumed by the ballast so you can add that power into your calculations. Is that correct?

If so, the ballast will consume a very small amount of power, probably small enough that you can ignore it.

Mike

[Old time inductive ballast could cause a significant change in the power factor, which means that you can have more current than you'd calculate just from the wattage of the lamps. But if you put in a safety factor in your calculations -such as don't load beyond 80% of the rating of the circuit breaker - you'll probably be okay.]

[Buck Williams]

I did a quick Google search, the operative term is "system watts" for a 4 lamp 34 watt fixture. This site offers a handy chart.
4 - 34 watt lamps with a magnetic ballast = 156watts, 119 watts wit an electronic ballast.

http://www.harrislights.com/media/55...20wattages.pdf

[Kirk Poore]

How many lights per circuit? Half of them. That way, if a breaker pops, you still have enough light to go figure out the problem.

Really. No, really.

Kirk

[Dave Wagner]

Depending on the current draw and using the 80% rule. Lets use the above example) POWER = VOLTS * AMPS
So,
156 watts / 120 Volts = 1.3amps Each. So, if you have a 15amp circuit (most lighting) x 80% = 12 amps.
If you use 12 amps / 1.3 amps (Each) = 9 lights approx.

[ray hampton]

is ballasts similar to transformers, will transformers deliver more than the output volts/amps that it are design without burning out ?
the ballast rating should be printed on the ballast case

[Bob Cooper]

Sorry it's taken me so long to respond. The gist of my question was how much do i need to consider the power consumption of the ballast itself. I drew everything out figuring 80% of max amperage draw due to the continuous nature but later started wondering if i needed to alot for the ballast itself when doing that calculation. I'm on the road and not able to stop by a Lowes or equivelant to see what the actual fixture says.

When I did the calculation I used 4*35W for the total consumption. The Lithonia site tells me the ballast (electronic instant start) but doesn't really mention power consumption for the entire unit. If it consumes little then I think all is well.

[Rod Sheridan]

The ballast will have a specification that lists the current with the correct lamps. You don't use power consumption because you don't know the power factor ( we're solving for current).

You can load your circuit to 80% of rating so for a 15 ampere circuit you can have 12 amperes of continuous load, simply divide 12 by the current consumption of your chosen ballast to find out how ballasts you can run from the 15 ampere circuit.

If you have a 20 ampere circuit, obviously use 16 amperes for the continuous load...................Regards, Rod.

[Jason Roehl]

I wouldn't put more than 10 fixtures on a circuit, so it would be a moot point. 4 x 34W = 136W per fixture, 10 of those would be less than 1400W, well within the safe current limits of a 20A circuit.