Lighting Circuit Size

Status
Not open for further replies.
P

philly

Senior Member
I'm trying to calculate how many lights I can put on a particular lighting circuit.

The lights that I have are multitapped 120/208/240/277. We are going to supply them with 208V. The lights have an ouput rating of 100W and an input rating of 129W. The start amps and operating amps are both listed a 0.7A draw. Based upon this, if I have a 20A circuit and run #12 the I can put aprox 22 lights on this circuit. Here are my calcs:

Continuous duty requires that my avaliable OCPD is rated 125% of continuous load.

20A * .80 = 16A connected load

16A / 0.7A (per light) = 22.8 lights.

So I can put 22 lights on this circuit with an OCPD of 20A anc #12 wire.

Does this seem right? I cant find anything in NEC that says other.
 
Looks good to me. But you won't find anything in the NEC. It is not a design manual, and doesn't help with this type of problem.
 
philly said:
I'm trying to calculate how many lights I can put on a particular lighting circuit.

The lights that I have are multitapped 120/208/240/277. We are going to supply them with 208V. The lights have an ouput rating of 100W and an input rating of 129W. The start amps and operating amps are both listed a 0.7A draw. Based upon this, if I have a 20A circuit and run #12 the I can put aprox 22 lights on this circuit. Here are my calcs:

Continuous duty requires that my avaliable OCPD is rated 125% of continuous load.

20A * .80 = 16A connected load

16A / 0.7A (per light) = 22.8 lights.

So I can put 22 lights on this circuit with an OCPD of 20A anc #12 wire.

Does this seem right? I cant find anything in NEC that says other.
Click to expand...

It looks good and meets the NEC requirements.:)

When you max out your circuits, you can paint yourself into a corner. If this is a warehouse or similar design you may be OK, but if it is a freeflowing structure with many levels/platforms and multiple various size equipment it is good to use about ~70% of the circuit capacity so if you need to add lights here and there you have the capacity and you don't have to run a new circuit.
 
philly said:
I'm trying to calculate how many lights I can put on a particular lighting circuit.

The lights that I have are multitapped 120/208/240/277. We are going to supply them with 208V. The lights have an ouput rating of 100W and an input rating of 129W. The start amps and operating amps are both listed a 0.7A draw. Based upon this, if I have a 20A circuit and run #12 the I can put aprox 22 lights on this circuit. Here are my calcs:

Continuous duty requires that my avaliable OCPD is rated 125% of continuous load.

20A * .80 = 16A connected load

16A / 0.7A (per light) = 22.8 lights.


So I can put 22 lights on this circuit with an OCPD of 20A anc #12 wire.

Does this seem right? I cant find anything in NEC that says other.
Click to expand...


The NEC will allow it.

Personally I would be shooting for 10 to 12 amps of calculated load on each circuit. Nice to have some room to add a few fixtures if the need arises.
 
Thanks for the responses. It makes sense to load the circut to about 70% to leave room for future lights as others have said.

I was wondering how the light itself if protected from and over current. Are the lights rated for 20A or are most of the ballasts internally fused/protected.
 
I have about 24 lights I was going to try to connect. I was thinking about breaking these up on two circuits fed from two 20A double pole breakers a 208V.

Someone mentioned to me the idea of instead using a 3-pole breaker and having the circuits divided up in the field among the three phases. In other words run the three legs off of the 20A three pole breaker and then divide the lights up between the three phases, A-B, B-C, and C-A. This would have all 25 lights on the one circuit and about 8 lights betwen each of the legs. Is this a good design practice?

My other option is putting 12 lights on two double pole 20A circuits. Would there be a concern with an unbalance issue in the panel with this arrangement?
 
philly said:
Someone mentioned to me the idea of instead using a 3-pole breaker and having the circuits divided up in the field among the three phases. In other words run the three legs off of the 20A three pole breaker and then divide the lights up between the three phases, A-B, B-C, and C-A. This would have all 25 lights on the one circuit and about 8 lights betwen each of the legs. Is this a good design practice?QUOTE]

If these are HID Luminaires using clear lamps, and if there is rotating machinery operating in the space, it would be best to use all three phases to minimize the stroboscopic effect.
But I would not use a 3-pole breaker - if you need to work on a luminaire, switch, etc, you want to be able to isolate that circuit and leave the remaining lights on.
db
Click to expand...
 
barclayd said:
If these are HID Luminaires using clear lamps, and if there is rotating machinery operating in the space, it would be best to use all three phases to minimize the stroboscopic effect.
But I would not use a 3-pole breaker - if you need to work on a luminaire, switch, etc, you want to be able to isolate that circuit and leave the remaining lights on.
db
Click to expand...

If I use (2) two pole breakers and have two circuits connected on A-B and B-C. I could only connect lights across these two phase combinatnions correct? I could not connect lights between A-C because that would be connecting a light connected on two different circuits. Is this right?
 
philly said:
I have about 24 lights I was going to try to connect. I was thinking about breaking these up on two circuits fed from two 20A double pole breakers a 208V.

Someone mentioned to me the idea of instead using a 3-pole breaker and having the circuits divided up in the field among the three phases. In other words run the three legs off of the 20A three pole breaker and then divide the lights up between the three phases, A-B, B-C, and C-A. This would have all 25 lights on the one circuit and about 8 lights betwen each of the legs. Is this a good design practice?

My other option is putting 12 lights on two double pole 20A circuits. Would there be a concern with an unbalance issue in the panel with this arrangement?
Click to expand...

If this is residential, it is common to use a single phase supply rather than 3-phase.
 
Status
Not open for further replies.
Top