voltage drop calc question

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cvirgil467

Senior Member
Location
NewYork
Landscape lightoing is being fed from a three phase feeder distribution. Each of the lights fixtures are alternately circuited from the three phase feeder and the lamps operate at 277v, single phase. At each light pole, one of the phases of the three phase feeder is tapped along with the neutral conductor. In performing the voltage drop calcs, do you perform them on a phase to neutal basis for each of the phase legs or treat it as a three phase system?
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
Re: voltage drop calc question

I would treat it as a three-phase system. I would assume that all of the loads were placed at the end of the run. If that gives an acceptable voltage drop, you are done. If it does not give an acceptable voltage drop, then you can do a lamp-by-lamp calculation, and check the voltage at each point along the run. That is not an easy problem to set up, but it can be done.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
Re: voltage drop calc question

I agree that you should treat it as a three phase system, but compare your resulting voltage drop to the line to neutral voltage (277V).

Steve
 
O

oliver100

Guest
Re: voltage drop calc question

On very long runs (like NYC bridges) lamp-by-lamp calculations is most prudent. Since the system is 3 phase symmetrical and the lights are on 277 volts (phase, neutral), single-phase calculations would be enough.

The current to the first run is FLC the last run gets only the current of a single lamp. Excel would be the best tool for this.
 

cvirgil467

Senior Member
Location
NewYork
Re: voltage drop calc question

You get a big differenc from 3 phase to 1 phase.

Here is the data:

Phase A = 1530VA
Phase B = 990VA
Phase C = 1290VA

Three phase feeder is (4) #10 AWG. Length of run (one way) - 760'-0". Voltage is 480Y/277v

For the calcs i get:

Single Phase = 10.4 volt drop or 3.8%
Three Phase = 7.5 volt drop or 1.5%
 
O

oliver100

Guest
Re: voltage drop calc question

The 3 phase calculations is OK. The differnce between sinle and 3 phase calculatrions is because the neutral is practically underloaded in 3 phase system (0 AMPS if the load is symmetrical). Single phase verification is just in case if the other 2 phases are not available (blown fuses for example).
 
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