Help calculating the number of 20 lighting circuits required.

[julian padilla]

Our shopping center parking lot has fifty-two (52) 120 Volt, 500 VA luminaires. The minimum number of 2-wire, 20 ampere circuits required to supply this lighting load is:
1. 11 circuits
2. 13 circuits
3. 14 circuits
4. 18 circuits
What is the calculation/math for this problem? :?
Julian

 

[david luchini]

52 fixtures / 3 fixtures per circuit = > 18 circuits.

 

[texie]

52 fixtures / 3 fixtures per circuit = > 18 circuits.

You took all the fun out of it. We wanted to see the OP's take first. I think many, at first glance, would say 14.

 

[infinity]

Is this a homework question? Here's a hint you need to consider these lights to be a continuous load.

 

[Johnhall30]

Max continuous load you can put on a 20A circuit is 16A (80% x 20 = 16). 16A *120V = 1920VA, which is max continuous load allowed on a 20A circuit.

3 lights is 1500VA, 4 lights is 2000VA. 4 lights is too much, so you can only place 3 lights per circuit.

 

[julian padilla]

Is there a NEC 2017 code reference that covers rule that 3 luminaires shall be calculated as a continuous load (125%)?
Please advise.

 

[Dennis Alwon]

Max continuous load you can put on a 20A circuit is 16A (80% x 20 = 16). 16A *120V = 1920VA, which is max continuous load allowed on a 20A circuit.

3 lights is 1500VA, 4 lights is 2000VA. 4 lights is too much, so you can only place 3 lights per circuit.

If your post was directed to Texie, then I believe he knew that that is why he said that most would get 14 circuits. Easy error to make.

 

[julian padilla]

Is this a homework question? Here's a hint you need to consider these lights to be a continuous load.

I'm looking everywhere in the NEC, but haven't been able to find a reference that states that luminaires are considered a continuous load calculated a 125%.
Does anyone know where I can find this in the code? :?

 

[petersonra]

I'm looking everywhere in the NEC, but haven't been able to find a reference that states that luminaires are considered a continuous load calculated a 125%.
Does anyone know where I can find this in the code? :?

Continuous Load. A load where the maximum current is
expected to continue for 3 hours or more.

Most lights will be on for at least 3 hours at least some of the time. I suppose the OP could put his parking lot lights on a timer so they could only stay on for 179 minutes at a time.

 

[Adamjamma]

Is there a NEC 2017 code reference that covers rule that 3 luminaires shall be calculated as a continuous load (125%)?
Please advise.

most commercial lighting exceeds the number of hours rules so is considered continuous, especially in winter months.

 

[julian padilla]

Awesome guys. Thanks for the help!
NEC 2017 - 210.11(A), and 210.19(A)(1). 52 luminaires / 3 luminaires per 20A circuit = 18 circuits. See steps below.
Step_1: Understand the rule that lighting/luminaires are considered a continuous load calculated a 125% unless otherwise noted in the question as non-continuous.
Step_2: Calculate the maximum ampacity on a 20 A circuit; 20 amperes / 125% = 16 amperes.
Step_3: Calculate the maximum continuous load on the 20 A circuit; 16 amperes x 120 volts = 1,920 VA.
Step_4: Calculate the maximum luminaires per circuit; 1,920 VA/Circuit / (500 VA/luminaire) = 3.84 luminaires, so round down to a maximum 3 luminaires/circuit.
Step_5: Calculate; Number of Circuits = 52 luminaires / 3 luminaires/circuit = 17.33, so round up to 18 circuits required.
Julian