Bad Phase on High Mast Lighting Pole

[dcooph]

Two parter...

I'm putting together a proposal to re-feed a high mast light pole - according to the electrician on-site, there is a bad phase leg (with the actual problem lost underground somewhere upstream) and naturally, lamps are not operating.

The pole currently consists of 8-1000W hps lamps and is fed via a 40A-3P circuit breaker at 480V. The electrician has recorded current draws from 26 & 29A in the good phases.

First question is, why are those phases drawing so much? By my calculation, the total power feed (including some ballast lost) is about 9600W (or 11.6A per line). I realize that the lamps are connected single phase (480), but wouldn't that affect the phase currents and not the line currents?? These numbers seem too large - even with considerable voltage drop over the 1500' run.

The second question (which ties into the first) is: would the bad phase leg have a hand in this? And if so, that still doesn't explain why the original OCP was sized at 40A??

Thanks...

 

[hmspe]

For a 3 phase system I would expect to have about 1/3 of the lights operating. I agree that the currents reported are too high for the stated load. The length of the run won't materially affect the current drawn.

Is this a single pull, or are there pull boxes? I'd be looking for bad connections in handholes or in other poles. The possible explainations for using a 40A breaker I see are (1) having more than one pole on the circuit, or (2) an issue with starting current.

I think this one needs a bit more investigation. It's not impossible for just one of three conductors to open somewhere in the middle of a closed conduit run, but it's by no means common unless the conduit has been damaged, and if there's damaged conduit this won't be a simple re-pull.

 

[bob]

Two parter...


The pole currently consists of 8-1000W hps lamps and is fed via a 40A-3P circuit breaker at 480V. The electrician has recorded current draws from 26 & 29A in the good phases.

First question is, why are those phases drawing so much? By my calculation, the total power feed (including some ballast lost) is about 9600W (or 11.6A per line). I realize that the lamps are connected single phase (480), but wouldn't that affect the phase currents and not the line currents?? These numbers seem too large - even with considerable voltage drop over the 1500' run.

The second question (which ties into the first) is: would the bad phase leg have a hand in this? And if so, that still doesn't explain why the original OCP was sized at 40A??

Thanks...

First: With one phase out of service you now have single phase service and the 1.73 factor may not be used. What you now have is 2 of the 3200 watt lamps in series (total 6400 w) which is in parallel with the third 3200 watt load. I am assuming the lamps are connected delta.
Load now = 3.3 amps + 6.7 amps total = 10 amps.

Second: Yes. Dont know about the 40 amp breaker.
Edit: Corrected math.
Thanks Larry

 

[LarryFine]

Load now = 6400/480 = 13.3 amps + 3200/480 = 6.7 amps total = 21 amps.

Math wrong.

Yes, at 480 volts, the 3200 watt load is 6.7 amps, but with two in series, you'd end up with half the current, or 3.35 amps. Now parallel that with the still-full-voltage 6.7 amps for a total of 10.05 amps on the two intact lines.

All of this assumes (careful!) only three loads per circuit, evenly distributed among the three lines Delta fashion (as it would be with no neutral). With eight luminaires, there's no way to have a fully-balanced load.

"Must re-evaluate! Re-evaluate!" ~ Nomad from Star Trek

 

[dcooph]

Thanks for the help fellas...

Even if the load is imbalanced, on paper I don't seem to be getting anywhere near the 25+ amps measured in the field - something must have been wrong with the actual measurement. Either that, or my assumed data is wrong.

I DID receive the following correction - only 30A breakers are being used. This at least solves the over-overcurrent protection mystery. But I still don't understand why these lines are drawing so much. No such thing as a 1500W HPS lamp, is there?! Or perhaps the ballasts are just that inefficient?

Thanks again...

 

[hockeyoligist2]

For a 3 phase system I would expect to have about 1/3 of the lights operating. I agree that the currents reported are too high for the stated load. The length of the run won't materially affect the current drawn.

Is this a single pull, or are there pull boxes? I'd be looking for bad connections in handholes or in other poles. The possible explainations for using a 40A breaker I see are (1) having more than one pole on the circuit, or (2) an issue with starting current.

I think this one needs a bit more investigation. It's not impossible for just one of three conductors to open somewhere in the middle of a closed conduit run, but it's by no means common unless the conduit has been damaged, and if there's damaged conduit this won't be a simple re-pull.

And don't forget about Fire ants! We lose a lot of lights due to them, they seem to really like the pole lights and build nests in any box along the way! The wire nuts will be full of fried ants but there are still plenty around to sting (or bite?) But if you are up north they may not be there "yet".