Length of circuit to begin applying voltage drop calculations

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Zmax25

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Colorado
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Electrician
Im wiring an apartment building and all units are 125a feeds. Based off 310.12 I should be able to run 1/0 for this. My feeder schedule shows the apartments furthest from the meter stack being ran in 4/0 and a few are even sized at 250. I wrote an RFI to the engineer asking if I could just run 1/0 and maybe bump it up to 2/0 if its over X amount of feet, say 200. I typically dont begin to account for voltage drop until at least 300' so I thought upsizing once should be plenty here, if needed at all. none of these runs will be more than 250'. 310.12(C) says that in no case will a feeder be required to be larger than specified in 310.12(A) or (B) so really I think I should be able to use 1/0 for every feed, even the 250' ones.
All in all, unless the load being served is critical, or very very long like parking lot lights, I feel like voltage drop is mostly a myth that people think they need to account for but 95% of the time they dont. No where in the code makes us account for voltage drop, so what is your go to standard regarding the subject?
 

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A quick calculation tells me that at 200 feet, a 1/0 feeder to a 125 amp panel yields a voltage drop (to the panel) of 5.38%. VD along branch circuits could add another percent or two. That is not good enough. VD is not a myth. Equipment does not operate as well as it should when the applied voltage is low. It is incumbent upon designers and installers to provide our clients with an adequate voltage supply.

Yes, the NEC includes no required maximum amount of voltage drop for the majority of the equipment it covers. It does offer what is essentially a suggestion. But that does not leave us free to ignore VD.
 
charlie b said:
A quick calculation tells me that at 200 feet, a 1/0 feeder to a 125 amp panel yields a voltage drop (to the panel) of 5.38%. VD along branch circuits could add another percent or two. That is not good enough. VD is not a myth. Equipment does not operate as well as it should when the applied voltage is low. It is incumbent upon designers and installers to provide our clients with an adequate voltage supply.

Yes, the NEC includes no required maximum amount of voltage drop for the majority of the equipment it covers. It does offer what is essentially a suggestion. But that does not leave us free to ignore VD.
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Especially if it is a 208/120 wye system. Though designed for 208/240 systems, tenants will probably not like the extended times to dry clothes, or heat water. Or dimming lights.
 
Interesting question. As an engineer, I would say run what the engineer said. How was it bid? If you run the larger sizes will it cost you anything, or are you simply trying to value engineer the job for the owner? Do you care for some reason that they are potentially larger than you think are needed? It's nice you don't want to be wasteful, but in the end, if it was bid that way, it won't cost you anything. If you have the engineer do the calcs, and he changes to smaller, will you get to put the money in your pocket?
 
hillbilly1 said:
Especially if it is a 208/120 wye system. Though designed for 208/240 systems, tenants will probably not like the extended times to dry clothes, or heat water. Or dimming lights.
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A 208Y/120V is a three phase system. i think you may have meant a 120/208V system which is single phase derived from a three phase system.
 
On the one hand, NEC calculations for residential use are notoriously conservative. A 125A panel going into an apartment will rarely see more 30A of load, but will sometimes see pretty high peaks, especially if there are any electric heating appliances.

On the other hand, when you use 120/208V for lots of 120V loads, the voltage drop seen by the 120V component of the loading is about twice the voltage drop you'd expect in a similar 120/240V system.

-Jonathan
 
I do a lot of wiring designs inside of large grocery stores, where we see 300' runs all the time either single phase or three phase. We have never had an issue and have seen that the VD calculations are pretty conservative. They seem to be "rounded up" if you know what I mean. For example the lighting in the back of the store close to the panel look exactly like the lighting way off on the other side of the building. No dimming or fading at around 260' run of #12 lighting circuit.
 
Exactly. Thank you Larry.

This panel might just see 40-60A at any given time. VD is a very widely misunderstood phenomena.
 
charlie b said:
A quick calculation tells me that at 200 feet, a 1/0 feeder to a 125 amp panel yields a voltage drop (to the panel) of 5.38%. VD along branch circuits could add another percent or two. That is not good enough. VD is not a myth. Equipment does not operate as well as it should when the applied voltage is low. It is incumbent upon designers and installers to provide our clients with an adequate voltage supply.

Yes, the NEC includes no required maximum amount of voltage drop for the majority of the equipment it covers. It does offer what is essentially a suggestion. But that does not leave us free to ignore VD.
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Im saying its a myth because it does not have nearly the effect in the real world as people make it out to. I understand it does actually occur but its not as simple as saying a 125amp panel will have S amount of VD at Y feet. It depends on the load applied, and the code understands that the load applied to dwelling unit services is typically far less than it is actually rated for, and when it is fully loaded its only for a short time. Its not like some factory that is constantly running at 80% of its load capacity. Ive installed all types of systems and seen VD first hand in the real world, not just a calculation on a piece of paper. You cant honestly tell me that you would be upsizing your feeders by 4 wire sizes just because the feed is 250' from the meter, thats insane.
 
LarryFine said:
Remember that voltage drop is dependent on actual load current. A125a panel is not a load.
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This is my exact point, thanks Larry. People type in voltage drop calculator into google and the southwire VD calc is always the first one to show up. They go into it and naturally choose the option that says "minimum wire size". They type in their parameters, one of which is amperage. Well, its fed from a 125Amp breaker so I put in 125 right? WRONG!!! That is the actual load applied to the circuit, not the OCPD size, which in a residence is very rarely even 75% of that, and especially not getting near "continuous load" status. this is the whole point of table 310.12
 
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