How many lights can be run from one breaker I would like to add two two tube fluorescent lights to a string of lights I have already.

  • 1
    What size breaker? – Tester101 Oct 13 '14 at 10:12
  • 4
    Use LED lights. They only use 14 watts versus 100 watts for conventional incadescent ones - U can have 8 times more light bulbs. – Dannyboy Feb 15 '16 at 15:21
  • Do not change out a 15 amp breaker by putting in a 20amp breaker, the wires are not able to handle the possible load. Instead of tripping the 15amp the 20 amp will not flip, but you have a fire hazard. I am not by all means an electrician, just have same questions. – Jon Dec 16 '18 at 2:48

As the other two posters have indicated, it depends.

For 20 AMP you have 20A*120V or 2400 watts and 15 AMP you have 15A*120V or 1800 watts. Good Electricians will only plan for 80% load so you have 1920 and 1440 watts respectively at your disposal per circuit.

Lets use 1440 as a conservative estimate.

Next, find out what's on the circuit. To do this, leave the lights on that you want to connect to. Open your electrical panel and flip off the breakers one by one until the lights turn off. Now turn off all the breakers except for the one the light are on and see how many lights and outlets are powered by the circuit. It's then just a matter of adding up what is on the circuit. Let's say you have 8 50 watt lights on the circuit, that roughly 400 Watts of draw (roughly). That gives you and extra 1000 watts, more than enough to add a few flourescents. Adding up the lights are easy. But the plugs will be a little more difficult, because there is a difference between having your entertainment center (TV, stereo, etc) on the circuit versus an alarm clock radio and just a lamp. For the current ligths and appliances that are connected, you can plan whether adding another light is feasible. Some circuits are lightly loaded, others not so much, so your mileage will vary.

  • 1
    More like misinformed electricians will use 80%. A 20A breaker on a #12 (minus voltage drop) is good for 20amps. 80% rule is only for continuous (3 hour or more) loads. – Kris Jun 3 '17 at 17:27
  • To tell the truth electricians only count loads on residential if the local AHJ requires it. The NEC only states the maximum for commercial & industrial, but the 80% rule is a good guide using the max wattage for each fixture & 180va per yoke on outlets. – Ed Beal Mar 13 '18 at 23:15

That depends on how many Volt * Amperes (VA) the lights already on the circuit draw, and the VA of the additional light, and the size of the breaker.

There is no "one size fits all" answer, such as "7"

If the circuit is a 120V circuit, that's the Volts part of Volts * Amperes. Likewise for 240 or 277 volt circuits. The Amperes is found by looking at the information on the fixture or the ballast which will either list Amps or (possibly) VA or both.

Lighting loads are typically continuous (may be on for more then 3 hours, per electrical code that's considered a continuous load), so a 15 amp 120V breaker can have at most 12 amps, or 1440 VA (VA might seem like it is the same as Watts, but it's not, except for incandescent bulbs.)

A 15 amp 240V circuit is the same 12 amps, but 2880 VA since the voltage is doubled.

  • Funny E×I= P how is that different? Than V×A they are the same. – Ed Beal Mar 13 '18 at 23:18

It's usually safe to add a few more lights to an existing circuit. But that said, you should take a closer look at the circuit.

Is it only used for lighting?

If there are receptacles on the same circuit, it will be difficult determine if you will be always be fine: how can you guarantee that someone won't plug a high wattage space heater in to an outlet on the same circuit?

How many lights and what kind of wattage or current for each one?

You could approximate a 100 Watt 120 Volt incandescent light bulb as using about 1 Amp of power (it uses a bit less than 1 Amp after the initial surge of current when you just flip the switch and the bulb filament is cold and has less resistance temporarily allowing greater current to flow through) A 10 Watt energy saver bulb could then be approximated as 0.1 Amp.

What is the capacity of the circuit? 15 Amps? 20 Amps?

If when you add up all of the currents that the lights would use on that circuit if each light was turned on and it is well under the circuit capacity, then you should be fine. If you get close to the limit or exceed it, then you could add another circuit to meet the possible load of having all of the lights on.


The breaker size is determined by the wire size. A 12 Gauge wire should have a 20 amp breaker and a 14 Gauge wire should have a 15 amp breaker. If you know your total wattage of your circuit you can divide it by your circuit voltage which will give you the total amps of the circuit. So if you have 120 volt circuit that uses 1500 watts your current load is 12.5 amps. If you base your loading at 80% of max load a 15 amp circuit should be 12 amps. So in this example you should use a 12 Gauge wire which means you can us a 20 amp circuit.


I think the basic has been said you just sum up the total watt of your lightings. you will see this written on the bulb for instance you having say 50watt bulb each and you had 10 already maths gives 500watt. to calculate safer breaker. if u are in UK or country that uses 240v. simply multiply 240v*20A*80% gives 3840VA all together. this shows that you can still add as much as 3340watt additional bulbs


Non on the above is entirely true. There is also Residual current leakage to consider. LED fittings generate .5mA of residual current. If you have an 30mA RDC (Residual current device) on that Circuit breaker then you should't really be exceeding more than 20 Ballasts / circuit. Most fittings have their own ballast so 20 fittings +/- a few is safest.

protected by Community Dec 16 '18 at 16:06

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