Current used by florescent lighting at start-up?
March 11, 2010 7:01 AM   Subscribe

I want to add some more florescent shop lights in my basement but I have a question about how many I can safely add.

My basement lights are all on their own 15A circuit. The fixtures are all 2x32W 4-foot "premium" shop lights for a total of 64W per fixture. If I stick to 80% of the breaker load (code, shmode, this is for my own piece of mind) then I can have 1440 total watts (15a x 120vac) on the breaker for a total of 22.5 of these light fixtures. Awesome. Way more than I’ll ever need.

My question is about the extra current used by the ballast (or starter?) when the lights come on. Is there a surge used at start-up? If so, is it significant? I don’t want to tear open one of my fixtures if I can help it. Is there a rule-of-thumb or standard I can use to calculate the current load at start-up? Something like “a typical starter on a 2x32w fixture would need X number of amps at start-up.”

Googling around confirmed that this is something I probably need to think about but I didn’t find any clear answers. I looked at some on-line specs for similar fixtures, and drilled further down to ballast model numbers, but none of them gave the information I’m looking for.
posted by bondcliff to Home & Garden (7 answers total) 1 user marked this as a favorite
Are you putting them all on one light switch, or in banks?

If you have 4 banks, then you need to worry about the draw of all 4 banks for running, but only 1/2 or 1/4 for the starters...
posted by sodium lights the horizon at 7:17 AM on March 11, 2010

Response by poster: Are you putting them all on one light switch, or in banks?

That's a good question. They'll be on two banks, but the switches are in the same location so I tend to flick both switches on at once. I should probably count on them all starting up at the same time.
posted by bondcliff at 7:30 AM on March 11, 2010

I can't give you hard data, sorry. But the ballasts draw significant current until the lights come on. So if you want to run lights using an appreciable fraction of the breaker, you'll need to put them on a bank of switches and turn them on one at a time s-l-o-w-l-y. Real ballpark safety estimate would be four banks of four 80w fixtures each if they're all on one breaker. I wouldn't install more than that.

HOWEVER, I suggest looking at more modern T5 "pencil" bulb fixtures as opposed to the traditional T12 fatty "shop lite" fixtures, which cost more up-front but put out more light per watt and run cooler too. So you'll need fewer of these fixtures for the same light.

FWIW, in my workshop I mounted cheap twin Edison socket ceiling fiixtures in the corners and loaded them up with 42 watt spiral fluorescents in daylight and warm white mixes. No harsh shadows, good colour rendition, fairly easy to wire. Not really pretty to look at, though.
posted by seanmpuckett at 7:38 AM on March 11, 2010

A shop light plugs in and therefor should have a rating plate telling you what the amperage draw is. If not I wouldn't worry about the current spike1.

Modern electronic ballasts don't draw any where the power that old magnetic ballasts did. As long as you've got T8 bulbs (which for efficiency reasons is what you should be buying) you have electronic ballasts. Bonus: electronic ballasts don't hum anywhere even close to as loud as magnetic ballasts.

1Modern electrical codes don't worry about surge values much because the wire will handle these spikes without problem. This what allows you to buy and use, for example, a 2 horsepower air compressor (total running draw ~1500 watts and a locked rotor amperage draw around twice that) on a regular residential plug. Such a large motor will easily momentarily dim the lights on any residential circuit it is plugged into without tripping a breaker or creating a fire hazard.
posted by Mitheral at 8:27 AM on March 11, 2010 [1 favorite]

Response by poster: A shop light plugs in and therefor should have a rating plate telling you what the amperage draw is.

These will be hardwired. It's probably just a matter of going to Home Depot and reading the package, possibly opening one up and checking the ballast, but I was hoping I could start planning it before I can get there.
posted by bondcliff at 8:46 AM on March 11, 2010

You can get delay fuses (maybe breakers too, unsure) for loads that draw a high start-up current and then settle back down. Typically they're used for AC units (like a window unit) but you could use one of those for this as well. You wouldn't blow out on start up but a real short would still blow the fuse.
posted by GuyZero at 11:01 AM on March 11, 2010

Here is a time delay plot for a typical residential circuit breaker. It shows that the circuit breaker will delay up to 1 second when at 6 times the rated current. So this means you would have no problem pulling 90 amps for 1 second. It also shows that you can pull up to twice rated current for 15 seconds. It takes a long time for the wires to heat up to dangerous temperatures even at these current levels. These time and current ratings are conservatively designed for safe use, so you should have no problem with start up current on your lights. The
posted by JackFlash at 5:22 PM on March 11, 2010

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