Voltage Drop

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NEC User

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My understanding is:
NEC recommends having a maximum of 3% voltage drop from the last circuit breaker to the last load to provide a reasonable efficiency of operation.
NEC recommends having a maximum of 5% voltage drop from the feeder to the last load to provide a reasonable efficiency of operation.

My loads are all lighting and I'm restricted to using conduit within a concrete parapet which is filled with existing wiring. Due to the other circuits I can't increase the wire size. My intention is to reuse the existing wiring.

My wiring diagram consists of an existing 120/208V service from the utility to an existing loac center which is a 120/240V panel which uses single phase breakers to feed all 120V lighting loads. The utility service is located within 5 feet of the 120/240V panel and the voltage drop is less than 1%.

My voltage drop calculations from the panel to the last light come to 4% for two of the breakers.

My Questions are:
Are my ok even though I get a 4% voltage drop on the branch circuit since the total voltage drop is less than 5%?
What is what is a reasonable efficiency of operation; will the equipment still work just cost more money in utility bills? What happens if these recommendations are exceeded? Is there's some room for error where the equipment will still work?
 

augie47

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Location
Tennessee
Occupation
State Electrical Inspector (Retired)
My understanding is:



My Questions are:
Are my ok even though I get a 4% voltage drop on the branch circuit since the total voltage drop is less than 5%?
What is what is a reasonable efficiency of operation; will the equipment still work just cost more money in utility bills? What

happens if these recommendations are exceeded? Is there's some room for error where the equipment will still work?
You should be in good shape, IMO. There are numerous systems that operate well with voltage drop far in excess of what you have.
Incandescent lighting is least effected and you need to look at the specifics of the type lighting you are installing. Most HID systems have voltage tolerances well beyond the NEC "recommended" values.
 

NEC User

Senior Member
Thanks,
do you know what the consequences of exceeding the NEC recommendations are? Like whats the worst thing that can happen. Also, we double checked our calcs and it is actually 4.8% voltage drop in the branch circuit alone. What is the largest voltage drop you encountered without encountering any problems?
 

augie47

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Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
I used to have an operating voltage sheet for HID ballasts but I can't locate it and Goggle let me down. I'm sure someone has the specs and can post. As I recall the ballasts operate at a +/- 10%.
I can't say that a 4.8% drop won't have some effect and a lighting guru may chime in, but in installations I have seen it would be no cause for concern. It might pay to check your actual supply voltage. 120v nominal can vary greatly in reality....as a rule I see it higher.
 

NEC User

Senior Member
Thanks again. I've been calling the manufacturer of the light fixtures. They are recommending to stay within 10% but the fixtures can actually handle quite a bit more voltage drop.
 

augie47

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Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
That coincides with my field experience but I don't want to discredit some of the sharp fellows on the Forum that might give you more detail on the effects of a 5% drop.
 
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