How many lights on one circuit?

[NOMAD]

I'd like to get some cheap lighting for starters in my garage. I saw some flourescent 4' lights at home depot, smple, industrial type for maybe $9-$12 each. I think they were single bulb but could have been double bulbs.

I already have wiring on a switch to one set of non-working lights so I thought I could run several of the lights off this one line of romex. I don't mind all the lights turning on with one switch as i have a single bulb for basic use as well.

How many of these should I run? I have yet to check the wire guage and the rating of the lamps.

I was thinking 3 or even 4 would be fine for my 20x24 garage.

Can these all be wired off of one switch? I have a DIY book that I'll follow for the wiring. But I'm a beginner (if you couldn't tell).

 

[markb1]

Look on the ballast(transformer) inside the fixture. It will list amps.

14 ga. wire is typical for use as lighting circuit, good for 15amps.

I'd go 80% max or 12 amps.

 

[brownbagg]

1 watt = 1 volt X 1 amp.

so using a 20 amp wire 12 gauge. X watts= 110 volts * 20 amps = 2200 amp
but you must prorate 80% to keep heat of wire down. 2200 * 80% = 1760

you going to have 40 watt bulbs, each bulb will be 40 watts, 2 in a light that will be 80 watts, 4 in a light that will be 160 watts

so 1760/40= 44 bulb, you can legal ave 44 bulbs on a 20 amp circuit but there will be some start up surge so you need to allow for that too.

I have nine two bulb unit and 2 4 bulb unit, that 26 bulbs with no problem. I could proberly get close to 36 before having problems.

That mainly the math of it, you got to allow for surges, room temp, conduit temp. I would stay close to 1400 watts per circuit.

 

[Bib Overalls]

I don't care for the cheap "shop lights". I used them in my shop and over half of them died within a few months. You are better off using the units that cost about $20 and have a real ballast in them. As my shop lights failed I swapped out for the higher grade 4' units and, in a couple of cases, for the 8' units. Wish I had poinied up the additional bucks in the first place.

 

[V-10 Killer]

I agree. I bought some cheapie 4 footers a long time ago, and they were starting to fail within a month. Get some energy star rated ones with the T8 bulbs if you want 4 footers. I have some 8' T12 fixtures that work great and will light at colder temps than I want to be in the garage for.

 

[Aceman]

Brownbagg, not everything you posted is entirely correct.

1. Calculations are made at standard voltage ratings. 120, 208, 240, etc.
Lighting is considered a continuous load so branch circuits are loaded to 80%.
120v x 20 amp x 80%=1800 watts
120v x 15 amp x 80%=1440 watts

2. You can't add bulb wattage ratings when using a fixture with a ballast. You MUST read the ballast to find out how much it draws.

 

[Charles (in GA)]

2. You can't add bulb wattage ratings when using a fixture with a ballast. You MUST read the ballast to find out how much it draws.

That ole power factor rears its ugly head again. All lighting that is ballasted, Flourescent, Metal Halide, Sodium, Mercury Vapor, and induction electric motors, all are affected by power factor to some extent. A 400 watt metal halide for instance draws 4 amps (on 120v) or about 480 watts.

Charles

 

[brownbagg]

I went out and check the ballast, my ballast uses 0.75 amps at 40 watt

 

[scott657]

Go with electronic ballasts and T8 lamps whether 4 or 8 foot. I have retro fitted 100s maybe 1000s of these. better lighting, you can remove 1 lamp and still get good lighting no hum etc etc. The ballast cost about the same as the magnetic ballasts the tubes are a little more expensive the energy savings are awesome

 

[NOMAD]

Thanks I'll do a little math and go shop around.
I'm still building all my rolling tables and such so it'll be a while before I can share my little garage with folks.
It's still useable though. Today I swapped out the suspension on a friend's Classic Mini. Finished while it was still light so no lighting issues yet.

 

[RebelAngelRuth]

This post is fairly old...and wasn't exactly what I was searching for...however, I decided to add my findings for anyone who may be interested!
I was searching for, "how many light fixtures can one circuit hold?", as I wanted to add lights in my bathroom.
My findings...
Rule of thumb....
If running off an existing circuit, make sure there are not more than 15 loads on the circuit (each light or outlet and switch being one load).

 

[redsky49]

That may be a dangerous "rule of thumb". What is your source?

 

[R6 Racer]

code is 11 up here.

 

[Fudge0514]

FWIW Home Depot sells at least three of the "cheapy" dual 4' T8 shop lights. The 'low' quality one is $19, then you have $29 then $39. The $29 is diamon plate aluminum and is somewhat reflective. The $39 has a mirrored finish so it reflects most light, I'd go with that, since you'll probably have much more light output via reflection than the $19 ones.

Also, get the 6500k (daylight) tubes, they're awesome! A two-pack is just under $7 at Home Depot right now. The soft white and bright white just aren't as bright. You'll thank me when you install these.



Edit - I meant the 6500k tubes, that's what I got.

 

[gary300]

That may be a dangerous "rule of thumb". What is your source?

I wouldn't even think about 15 loads, unless I knew how much power each of those loads would take. If you had a compressor running on a 15A breaker and then turned on an electric heater on the same breaker it would probably trip that breaker, and you only have TWO loads. You need to know either the wattage, or the current draw for each load on the circuit and then derate it 20 percent to allow for power factor and voltage drop in your wiring.

120 V x 15 A = 1800 W x .8 = 1440 W safe load on a 15 A breaker

120 V x 20 A = 2400 W x .8 = 1920 W safe load on a 20 A breaker