How to figure the load on a flouresent light circuit?

RonRock · Jan 4, 2012

Trying to figure out how to wire the lights I'm planning on for my shop. The ones I'm considering are "8ft 4 Lamp 32 Watt T8 Fluorescent" so do I figure them at 128 watts for each fixture? That gives me 1.066 amps @ 120 volt. So on a 15 amp circuit I could switch 12 fixtures at 12.79 amps, just over the 80% of 12 amps. Am I correct so far?

12 fixtures on a switch seems like too many, so I'll have to come up with a way to divide them up into "lighting zones" That is going to take some thought. They will be wired with conduit and this will be my first conduit project. So I'm sure there is a thing or two I'll need to know before starting.

porcupine73 · Jan 4, 2012

The things are not 100% efficient, so it will take a bit more than the sum of the bulb wattages to operate it, depending on how efficient the ballast is. One of those inexpensive plug in power meters could be a way to check it. The 32W would be a nominal rating, the actual draw might vary by bulb but probably averages to 32W.

I haven't measured the power factor on my cheap fluorescent fixtures. But if it isn't unity power factor, the current draw goes up. I.e. if they are 80% power factor, the current draw goes up by 20%. They might introduce harmonics too.

theoldwizard1 · Jan 4, 2012

Some manufacturers (Square D) make a special breaker for fluorescent "loads". I do not know what the difference is.

Falcon67 · Jan 4, 2012

From reading specs, I found that most regular T8 "32W bulbs" draw 28w and the HO stuff actually gets close to 32W. I'm using 8' four bulb fixtures and just figured 1A per fixture for power.

pattenp · Jan 4, 2012

Guy's you don't use the wattage of the tubes in florescent fixtures to determine the amp draw. You need to get the amp draw straight off the specs of the ballast(s).

I believe you'll find a typical T8 4 tube 32W ballast pulls about .95 line amps.

porcupine73 · Jan 4, 2012

If you specifically wanted to keep the load per circuit down, you could maybe go with say two 15A or 20A circuits. If you want them still controlled by one switch, you could get a double pole switch. I wouldn't use those cheap residential grade switches for much more than a 40W light bulb, those things just do not last. The commercial grade switches seem to be much more durable.

RonRock · Jan 4, 2012

Lots of good info coming in on this.

Thanks.

sberry · Jan 4, 2012

I wouldn't want every light on one switch or one circuit, one of the benefits of running pipe, can run switch's as needed. As for cheap switches and recepts, I have hundreds of them, some have ran thousands of cycles, can't remember the last time I replaced a failed one.

porcupine73 · Jan 4, 2012

It is nice to have more than one lighting circuit, if for no other reason if that circuit trips or has a fault, etc you don't lose all your lights suddenly. Maybe it depends on the particular brand of switch. Or maybe I got a bad batch. I had a dozen or so I bought at a big box store a few years back and within about six months half of them were acting flaky, like flickering the light, audible slight arcing from the switch, stuff like that.

frank1380 · Jan 4, 2012

The power per bulb multiplied by the number of bulbs is a reasonable estimate. I have nine 4-bulb fixtures that draw 8.3A or so.

I void warranties · Jan 5, 2012

pattenp said:
Guy's you don't use the wattage of the tubes in florescent fixtures to determine the amp draw. You need to get the amp draw straight off the specs of the ballast(s).

I believe you'll find a typical T8 4 tube 32W ballast pulls about .95 line amps.

yea! what he said... your using marginal figures, go straight to the ballast.

Aceman · Jan 5, 2012

I always use 1 amp/fixture as a rule of thumb when installing 120v circuits for 4 lamp T8 ballasts.

The real killers are the 6 lamp T5H0 fixtures when you run them at 120v, I think they're around 3 amps apiece!

PatJ800 · Jan 5, 2012

Rule of thumb I have always heard - and always used - is 112w per luminary for 4 tube T-8. Doesn't matter if we used F32T8 or F28T8 we always used 112w for 4 tube. I think in reality it is a little less than that, even with F32T8.

rodm1 · Jan 10, 2012

I would like to add 10 lights in my shop with 10 individual switches. I whose thinking of using a sub panel for lighting. I could get by with 5 is there a cleaner way to do this rather then a sub?

What are high quality 8ft 4 lamp T8 lights with no radio interference or a ballast model number?

Falcon67 · Jan 10, 2012

If you are using 10 switches you'll spend way more on wire than on lights or panels IMHO. Depends on the runs of course. I have two light circuits, 4 220V #12 runs and 5 plug circuits in my plan - I'm approaching 500' used of 12-2 wire and will likely be buying another 250'. It goes quick.

I never had any radio issues with the $40 4 bulb fixtures or with the $18 shop lights. That said - this IS the 21st Century, where's your iPhone/iPad, dock and Pandora?

Charles (in GA) · Jan 10, 2012

rodm1 said:
I would like to add 10 lights in my shop with 10 individual switches. I whose thinking of using a sub panel for lighting. I could get by with 5 is there a cleaner way to do this rather then a sub?

What are high quality 8ft 4 lamp T8 lights with no radio interference or a ballast model number?

A switch for each fixture is "not normal" and I cannot see why you would want to do this. Zoning the garage into two or three areas is one thing, but ten switches is, as Falcon noted, going to cost in wire and switches (and a very large switch box). Not sure why you are suggesting a separate sub panel for lighting, These could all be on one breaker, or better if on two breakers, but you certainly do not need ten breakers.

I have a 60x60 building with a dozen 400 watt MH lights in it. They are on three multi-wire circuits.... three double pole breakers, or a total of 6 slots consumed in a 40 space panel.

Charles

rodm1 · Jan 11, 2012

I think a mini sub by the door for lighting would be much cleaner then 5 to 10 switches that's why I asked. The reason for every light on a switch is at work we must light up half the floor to do a small job in an local area and would like to avoid that.

But thinking about that even lights farther down have a effect on the lighting in the immediate area. I'm in the planing mode at the moment that is why I'm asking.

Charles (in GA) · Jan 11, 2012

rodm1 said:
I think a mini sub by the door for lighting would be much cleaner then 5 to 10 switches that's why I asked. The reason for every light on a switch is at work we must light up half the floor to do a small job in an local area and would like to avoid that.

But thinking about that even lights farther down have a effect on the lighting in the immediate area. I'm in the planing mode at the moment that is why I'm asking.

Breakers do not hold up like switches. Some 15 and 20 amp breakers are actually approved as switching devices for lighting, but they do not last well. They get weak and trip easily, or won't stay set. Just doesn't seem like a good way to go.

Charles

How to figure the load on a flouresent light circuit?

RonRock

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porcupine73

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theoldwizard1

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Falcon67

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pattenp

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porcupine73

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RonRock

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sberry

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porcupine73

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frank1380

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I void warranties

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Aceman

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PatJ800

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rodm1

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Charles (in GA)

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rodm1

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Charles (in GA)

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