i have a few questions that i’m having trouble getting straight answers to, so before i started relying on the kindness of strangers in orange aprons i thought i’d ask you guys:
how many 2 tube, 8′ fluorescent fixtures can one safely put on a 15A, 14/2 wired circuit? does the type of ballast (reg or electronic) make a difference? btw- i have space in the panel, just need to know how many circuits to dedicate to overhead lighting.
advantages/disadvantages of regular garden variety ballast and the electronic type?
pros/cons of using 4 tube, 4′ fixtures in a couple places where it might be preferable to have more light over a smaller area, instead of the twin 8s?
thanks!
m
Replies
Mitch,
If you look on the ballast there should be an amperage rating. If you add up the total amps it should not exceed 80% of the maximum circuit (15amps x .8 = 12amps). Some people just add up the wattage of the tubes but with the variety of ballasts out there - electronic, magnetic, high output, etc. it is better to take the info off of the ballast.
I prefer only using 4ft units. I think that there are a large varity of quality bulbs available for them then the 8ft. Specially from the box stores and hardware stores.
I have 8ft and had to go to a couple of electrical supply stores until I got some high CRI bulbs.
But I can find them in 4ft even at wally world.
If you figure 80 watts (or 40 for 4ft) per bulb and then add about 10% you will be safe.
to save me the trouble of looking this up (my electrical experience does not extend far beyond black to black, white to white, bare to bare and/or fixture/box) would you mind translating watts to amps? although i will probably take chris' advice and check the ballasts i end up getting. loadwise (assuming wattage is 1:2), are four 4 footers a straight swap with two 8s, or are there other factors?
any comments on different types of ballast? this will be a heated space in winter so lo-temp start-up and buzzing shouldn't be a problem.
thanks again-
m
Watts is voltage times "current"For 80 watts that is 0.67 amps. Or after adding 10% and rounding off 0.75 amps.I used "amps" because only the part of the current that is in phase counts. That does not really matter in a typical shop, but does in a large office building.But that is why I through in the extra 10% and also most bulbs are now 60-75 watts, but I used 80.Yes 40ft/40 watt fixtures can be sustituted 2 for 1.The new "standard" fixture uses T8 (1" dia) bulbs while the older ones are T12 (1.5"). All of the T8's use electronic ballast. They are more efficent (in both the bulb and ballast). And the elctronic are less prone to noise and the strobing which bothers some people.But the box stores don't stock the full compliment of T8 bulbs yet so you might need to search for them.
If you divide watts by 100 you end up with a pretty close answer, with roughly the 20% buffer built in. (To be more exact, with no buffer, divide by 120.) But for fluorescents you need to add another 10-20% for the ballast and power factor.So a real quick thumb suck would be that each 80W bulb is about 1 amp of circuit rating, which is to say you should be able to put close to 20 80W bulbs on a 20A circuit, while staying within the 80% of circuit rating restriction. (But double check what it says for amp rating on the ballast, since power factor is an issue.)(I personally don't care for 8' bulbs because they're so daam awkward. Even 4-footers are a PITA in many cases.)
Hi Mitch,
Here is a place that should help you decide about ballasts, bulbs, and fixtures.
http://www.ldpi-inc.com/technical/
Darned, can't seem to hot link it today, anyway this manufacture makes some good products and has good information regarding to fluorescent lighting.
Regards,
Virginbuild
so, at .67A per 80 watt tube, and using 80% of the 15A circuit, or 12A, that works out to roughly 18, 80w, 8' tubes (or 9 twin fixtures, or 9, 4' quads) on only one circuit? and even more with 60-75w tubes. wow- that kinda seems like alot. is there any start-up load to figure in, like with some motors, or if there is, is it already factored into the 80%?
m
Well you are pushing it a bit. That is why I suggested a 10% over and rounded it off to 0.75 amps. That gives you 16 8 ft units.Because the ballast have losses in them and they have power factor issues.And breakers have an inverse time delay that allows they to supply a relatively large amount of current for a short time period. Starting is not an issue.
as it happens, i don't actually need that many fixtures, more like 6 or 7 total. so unless i wanted (if i needed the panel space) to combine all the overhead lighting in both shop spaces i'll be fine. maybe i'll split the lighting in the big room into two circuits anyway just so i can add units with impunity if i find i need more light some place.
thanks again-
m
thanks for the link- most of it's too technical for me but some of the simpler stuff was helpful.
m
The cheap "garden variety ballast" 8 foot fixtures will buzz and interfere with AM radio.
The electronic ballasts won't.
You can use 8 foot fixtures with 4 foot ganged tubes....little or no 60 cycle...er....hertz hum.
Another vote for the 4' bulbs here. They're a lot easier to deal with, changing 8' bulbs (or trying to get them home from the store) is a pain. The T8 bulbs and electronic ballasts are the way to go as well. If you check around you should be able to find bulbs with good color that won't drive you nuts working under them all day, although you'll probably have to go to a lighting store instead of the big box for them.
The local Home Desperate stocks 8' T8 bulbs in C50 and 2 or 3 other types. Not cheap - $9 each for the C50's - but well worth it for the quality of light.
mitch,
There are many different types of 8' lamps, and the wattage and type of lamp (and type of ballast) determines how much current the fixture will draw.
A two-lamp 8' fixture running at 120V can draw anywhere from 0.92 amp to 3.7 amps. If the fixtures are new standard grade shop lights, they probably have F96T8 lamps (at 59 watts each) and an electonic ballast. That translates to a system input of about 0.92 amps @120V.
Your fixtures could have different lamps and ballasts. Lamps and ballasts made to start in cold temps draw more power. Very high output lamps/ballasts draw a lot more power. Look at the ballast specs of your fixtures for amps or for "system watts" and figure for a residential occupancy, a 15 amp circuit will supply 1800 watts maximum and a 20 amp circuit, 2400 watts max.
Although not required for residential, it's a good idea to keep the load below 80 percent of the max circuit rating, as required in commercial occupancies.
I agree with the other posters who suggested using 4' lamps. Not that the 8-footers are impossible to manage, but it gets easier with experience. Once you've had to re-lamp the 8' fixtures, you'll wish you'd installed the smaller ones. And transporting and storing the 8-foot lamps is a pain compared to the 4-footers.
Cliff
while we're on the subject- anybody know anything about these:
http://www.griotsgarage.com/catalog.jsp?L1=L1_3000&L2=L2_3070&SKU=13014
this company caters to the upscale car owner and their prices usually reflect that fact. any idea where i could find this sort of fixture cheaper than $300?
thanks,
m