Number of circuits

[bigpapa400lb]

How many 120v, 2 wire, 20 ampere branch circuits are required to supply 52 parking lot lights which are rated 500 VA each?

I get 17 or 16.9 answer key says 18 circuits?

[augie47]

I keep getting 14, so I can't testify.

[boku0003]

Here's what I tried. 52 x 500VA = 26kVA. Then take 26kVA / (1500VA/circuit) = 17.33 round up to 18. Not sure why you would use 1500 VA instead of 120x20=2400VA per circuit but it yields the answer they were looking for.

One quick question I have is when calculating number of circuits, do you use 1.25 for continuous loads? Or is that only for sizing the conductors and OCPD?

[roger3829]

Originally Posted by augie47

I keep getting 14, so I can't testify.

That's what I got too.

[sparks2000]

why are you using 1500VA???

[roger3829]

52 fixtues times 500va = 26000va

26000va/120v = 216.67a

20a @80% = 16a/circuit

216.67a/16 = 13.54 circuits or 14

[raider1]

Originally Posted by augie47

I keep getting 14, so I can't testify.

That is what I get as well.

Chris

[bigpapa400lb]

Originally Posted by boku0003

Here's what I tried. 52 x 500VA = 26kVA. Then take 26kVA / (1500VA/circuit) = 17.33 round up to 18. Not sure why you would use 1500 VA instead of 120x20=2400VA per circuit but it yields the answer they were looking for.

One quick question I have is when calculating number of circuits, do you use 1.25 for continuous loads? Or is that only for sizing the conductors and OCPD?

I used 125% and the answer key does refer to 210.19(A)

[boku0003]

14A is what my answer would be based on 52 x 500VA x 1.25 = 32.5kVA then divide by 120x20A to get 13.54A, round up to 14 circuits.

I was just throwing 1500VA out there as it is what you use for other circuits (small appliance and bathroom...) and it gets an answer of 18 amps. If I was taking an exam and 14 A was not one of the answer selections, I'd start trying other options.

[Dennis Alwon]

I disagree with the other that 14 circuits are needed. We have to assume that these lights will burn continuously or for 3 hours or more. So at 2400 watts for a 20 amp circuit we must take 80% and we get 1920 watts per 20 amp circuit. Now with 1920 as the max for the circuit we can only get 3 lights (1500 watts) per 20 amp circuit. We have 52 lights so 52/3= 17.3 or 18 circuits. Answer key is correct, IMO

This is not a simple problem as total wattage being used since the circuits must be limited to 1500 watts simply because adding the 4th light would be at 2000 watts which would be over the 80%