I have a three phase service with 208/120 volts we installed 4 foot HO fixtures with electronic ballast in parking garages. I put an amp probe on the branch ckt wire on load side of breaker. leg A-B-N and I had a reading of 16 amps on A 14 amps on B and 23 amps on the N. Is the N high do to reading through the ballast or because my C phase wasn't wired in yet so it was not cancelling out, all phase had same amount of figures at 2.5 amps per
Light fixtures
- Thread starter Big Guns
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- New Jersey
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- Journeyman Electrician
Sounds like a non-linear load problem causing the high amperage on the neutral. 16 amps on a 20 amp circuit is borderline high for a continuous load.
Did you use this formula in you calculations? If the load Pf is not unity, you will get an error.
I was skeptical about the harmonics. But if we assume a 20% THD ballast, that would be about 3.2 amps of harmonics per phase. So 15 amps + 3.2 + 3.2 would be about 21.4 amps. That's still a little less than the 23 amps measured, but not too far off.
Still, most ballasts are 10% THD.
Guns: Can you be more specific about what type of ballasts they have?
I didn't directly use that forumla. I kind of did a graphical vector addition. But I think it probably gives the same result as that forumla.
I was assuming that the PF would be off by the same amount on both phases. So I think that formula should still be pretty close to the real result.
Steve
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