How Many T-12s on a 15 Amp Circuit?

[Jim Eller]

The question is, how many T-12 fluorescent lamps can you run on a 15 amp circuit?

The units have four lamps each with a ballast for each pair of lamps. The ballast read ".80 amps" which would be 1.6 amps per unit. So 15 divided by 1.6 is 9.375.

Is that correct? Nine four lamp units on a 15 amp circuit or am I missing something here?(It's happened before!)

Jim

[Chris Friesen]

I believe you're not supposed to have a continuous load of more than 80% of the breaker, in which case you'd be limited to 12A, giving a max of 7 quad-bulb fixtures.

Edit:
I was curious so I looked it up. Section 210.19, A, 1, says that for continuous loads (3 hrs or more in duration) the minimum conductor size must have an allowable ampacity of not less than 125% of the load, unless the assembly (including the overcurrent devices) is listed for operation at 100% of its rating. Section 210.20, A, says that the rating of the overcurrent device must be not less than 125% of the continuous load, unless it's listed for operation at 100% of its rating.

[Chris Padilla]

This sounds a little low to my ears but maybe it is correct. I haven't looked at a ballast recently so I'm wondering what "kind of" 0.8 A that is? Is it full load? Continuous? Does it say anything else on there?

Can you plug your fixture in and get a reading with a current meter?

However, in the end, 7 or 9 fixtures...this is a lot of light!! That is 28 - 36 bulbs!

I have 24 T8s in my 20x20 shop and it is like being on the surface of the sun in terms of light!

If you really need/want that much light, I wonder if the T8s are more efficient? I believe they are but I don't have any numbers.

[Jason Beam]

I wondered the same, Chris...

I have all T8s - daylight - i think i only have 20 bulbs, though. I'm just off the surface of the sun, i guess :P

Most of the T12s I used to have ran at about 34 watts, if I remember correctly. 120v * .8 = 96 - that's 48w per bulb which to me actually seems a little high - but then, the ballasts likely take some wattage, too. I guess ya go by whatever the rating is on the ballast.

I do know that my T8 fixtures are so much better than any T12 lights I've ever used. They never make a sound, don't flicker in the slightest, start at nearly 100% brightness no matter how cold my shop gets and so far I've not lost a single bulb in over 2 years. That's with a move in there, too. I am a HUGE fan of the T8s. They seem to be superior in nearly every way.

I'm not sure about that 80% loading in the case of lights, either. Then again, maybe there's some startup current with them, too? I was more concerned about separating the lighting over different circuits in case of popping a breaker than I was about loading of the circuits.

I'm a firm believer of splitting my lights over at least two circuits no matter what. I would love to have so much space that two circuits might not be able to handle so many lights! :P

[Chris Padilla]

Yep, fully agree about the virtues of T8s over T12s. Zero sound, zero flicker, and instant on regardless of temperature. I finally had 1 T8 of my total of 28 T8s that I've installed around my home/shop die. It took 3+ years and in all honesty, I do not notice or miss the bulb being out. It is still connected and just sitting there ready to be changed out!

[Jim Eller]

The ballast reads, ".08 amps (For standard bulbs)"

I would have put in T-8's but the fact that I have 14 of the T-12 quad units doesn't make economic sense at this time.

I have pull switches on all of them so I can control the light as needed.

BTW - I did wire the circuit 20 amp with 12-2.

Thanks,
Jim

[Jason Beam]

I wondered about that - If you've already got the fixtures, by all means! It sounds like you've also already got the one circuit ran so you're kinda locked in, there, too, huh?

Two separate circuits would be my preference and since it looks like you're bumping into the limits of even a 20a circuit, is there a chance you could run a 2nd one and split the load in half? This way if for some silly reason one of them pops, you'll still have half your lights on and won't have to fumble around in the dark trying to fix the problem.

14 quad fixtures, huh? Holy ... 56 bulbs oughta light yer way!

[Jim Eller]

Jason,


They will not all be on the same circuit. I was just wanting to know the max for a 15/20 amp circuit.

I have a couple along a wall that are just on a pull cord also and not on a wall switch.

At 65 it seems I never have too much light. If it becomes the case, I will start pulling cords and turnin' 'em off.

Thanks,
Jim

[Chris Padilla]

The ballast reads, ".08 amps (For standard bulbs)"

Well, Jim, now we have an order of magnitude difference. Is it 0.08 A or 0.8 A? I'll put money on 0.8 A.

[Tom Veatch]

... I was just wanting to know the max for a 15/20 amp circuit....

Can't tell you the "max", Jim, but just for a comparison point, my shop has two 15 amp lighting circuits. One has 6 HO fixtures with 2 80w bulbs for a total of 960 watts. The other has 2 HO fixtures with 2 80w bulbs and 6 fixtures with 4 40w bulbs each for a total of 1280 watts. So my total wattage, 2240, should be very close to yours. I imagine the 2nd circuit with 1280 watts is getting pretty close to the practical limit. They were configured that way by the electrician that wired my shop.

I've not measured the amperage draw on either of the circuits. Next time I have the cover off the panel, I probably will because I'm thinking about adding a little more light in one area of the shop. My own rule of thumb based on nothing but a WAG is to keep the calculated amperage (watts/voltage) below the 10-12 amps range on a 15 amp circuit (13-16 for a 20 amp circuit).

EDIT:

OOPS! Above was from memory and I forgot about the second half of the building. It has a 20 amp lighting circuit with 8 HO fixtures with 2 8' bulbs each. And, I just checked, and those bulbs are 110w instead of 80w. So the corrected figures are (all T12 sizes):

1 15 amp circuit with 6 x 2 x 110w = 1320w
1 15 amp circuit with 2 x 2 x 110w + 6 x 4 x 40w = 1400w
1 20 amp circuit with 8 x 2 x 110w = 1760w

From that, I'd say the electrician figured that 1400w was OK on a 15amp circuit but went to a 20amp circuit for the 1760w load. I'd interpret that as (calculated amperages) saying that 1400w (11.7 amps) was OK on a 15 amp circuit but he needed to go to 20 amps for 1760w (14.7 amps). And that sorta matches my "rule of thumb".

[Jim Eller]

Chris,

My mistake. It is what I said in my original post - ".80 amps". That's the way it's written on the ballast.

Please excuse me............I'm old and .


Tom,

Thanks for the info.


Jim

[Michael Wetzel]

Amps = watts divided by volts


30*40w=1200/120=10

[Rod Sheridan]

Amps = watts divided by volts


30*40w=1200/120=10

Actually Michael, that's only true for DC, for AC you need the power factor if you want to calculate the current of a device given voltage and power.

The OP had a ballast that has a current rating of .8 Amperes, the maximum continuous load of the circuit is 80% of 15 Amperes, which is 12 Amperes.

12 Amperes divided by .8 Amperes is 15, so 15 ballasts can be supplied by a 15 Ampere circuit.

Since the OP has 2 ballasts per fixture, he can install 7 fixtures on a 15 Ampere circuit.

Regards, Rod.