Voltage drop and taps at transformer

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hhsting

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Glen bunie, md, us
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Junior plan reviewer
I have situation in which plans call for 480V to 120/208V three phase 75 kva transformer with taps. The transformer secondary feed 120/208V panelboard A. The panelboard A has two branch circuits b1 and b2 . B1 feeds site lights 600 feet away and b2 feeds site lights 400 feet away all with 120V single phase.

Also panelboard A serves small garage loads such as receptacles, heaters.

The branch circuit conductors size for b1 and b2 is #8. I asked for voltage drop calculation and was told that the 75kVA has taps can compensate for voltage drop to branch circuit and their is no need for up sizing the cables.
I always thought it is better to do voltage drop even with taps.


Questions:


1. Can branch circuits 600 feet away be compensated with taps adjustment to not to have voltage drop and cable size remain same?

2. Is their an exception to not having voltage drop from taps in code?

3. Can 250.122(B) apply if voltage is increase due to tap but not cable size?
 
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You must not have much of a lighting load at 600’ if the conductors are #8. IDK what your taps are but if increasing by 5% works, use it. That’s only a 6 volt increase for 120v circuits. Well within tolerance.

Assuming the 8s are specked because of VD, you will need an 8 EG.
 
No doubt LED, an entire Walmart parking lot is now less than a 20 amp load, but that's at 480. 120 volts is unusual for site lighting, unless it's bollards, or short poles. Some strange engineering there.
 
I agree with post #2.
The 2-1/2 or 5% tap, isn't going to make that much difference.
The Code does not address voltage drop except for two or three special conditions so, if you have drawings, follow them.
As started in 15,20 and 30 amp circuits, your equipment ground will be the same size as your phase conductor.
 
electrofelon said:
I guess I don't fully understand your role, but I thought you were a plan reviewer. I don't see what businesses it is of yours to question voltage drop as that is not in the NEC (except for a few special things).
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We have local amendment for site light to follow FPN notes voltage drop NEC
 
hhsting said:
1. Can branch circuits 600 feet away be compensated with taps adjustment to not to have voltage drop and cable size remain same?

2. Is their an exception to not having voltage drop from taps in code?

3. Can 250.122(B) apply if voltage is increase due to tap but not cable size?
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1) I don't see why not, but IMO it is poor engineering.
2) I'm not sure what you are asking.
3) I would think it's apples and oranges, so .. no.

The code recommends a % VD maximum. If you have a 6% voltage drop at 120 volts, then you up the voltage to 126-volts, you still get a voltage drop of close to 6% even though the operating voltage is close to normal. So, technically it's still an excessive voltage drop. :)

Sometimes the PoCo has a +5% voltage, and during this time your voltage after the transformer and prior to your VD will be in excess of +10%.

IMO this is poor engineering. Is there even an engineer involved? Will it work? Most likely just fine.
 
Aside from what was said, what about the rest of the loads in the panels? Your are planing to use taps to get a higher voltage for the lighting circuits, but then feed garage receps and heaters off the same panel that's over nominal? That seems more of a problem?
 
mikeames said:
Aside from what was said, what about the rest of the loads in the panels? Your are planing to use taps to get a higher voltage for the lighting circuits, but then feed garage receps and heaters off the same panel that's over nominal? That seems more of a problem?
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Yes secondary of the transformer would feed panelboard that then feed lights, garage receptacles and heaters. The panelboard itself and receptacles, heaters fed from the panelboard would see increase in voltage along with light circuits.
 
ptonsparky said:
5% is probably still within nominal range unless you already have a primary bumping the high range.
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Ok but where is it in NEC 2014? NEC 2014 does not define this tolerance does it or points back to another code that does? If it does not then install would not be NEC code complaint but industry assumes this to be ok and other documentation maybe mentioning tolerance but AHJ has not adopted other documents or other industry standards
 
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hhsting said:
Ok but where is it in NEC 2014? NEC 2014 does not define this tolerance does it or points back to another code that does? If it does not then install would not be NEC code complaint but industry assumes this to be ok and other documentation maybe mentioning tolerance but AHJ has not adopted other documents or other industry standards
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Sorry to be snotty, but do you know what google is? Often these answers are just a few clicks away. I typed in "utility voltage tolerance" and this is the first thing that comes up. It gives the deviation and the ANSI standard that covers it.


As Tom said, figure =/- 5%, but I have seen high 250's on a 240 service.
 
hhsting said:
Ok but where is it in NEC 2014? NEC 2014 does not define this tolerance does it or points back to another code that does? If it does not then install would not be NEC code complaint but industry assumes this to be ok and other documentation maybe mentioning tolerance but AHJ has not adopted other documents or other industry standards
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As far as code compliance, it would come down to what voltage the equipment is rated for. For example receptacles are often 125v. That takes care of a nominal voltage of 120v that is running a little high. If you are designing in a higher than normal voltage on a system just to have a higher end voltage of two circuits then its poor design as already stated. The percentage of VD is not changing but your final output voltage will on all circuits fed from that panel.
 
mikeames said:
As far as code compliance, it would come down to what voltage the equipment is rated for. For example receptacles are often 125v. That takes care of a nominal voltage of 120v that is running a little high. If you are designing in a higher than normal voltage on a system just to have a higher end voltage of two circuits then its poor design as already stated. The percentage of VD is not changing but your final output voltage will on all circuits fed from that panel.
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But panlelboards are not 120/240V single phase, 277/480v, 208/120V three phase what are their tolerances?
 
You are not going to find that in the Code.
Ask the manufacturer.
spinning wheels again.
 
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