Length of circuit to begin applying voltage drop calculations

[Zmax25]

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?

 

[charlie b]

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.

 

[hillbilly1]

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.

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.

 

[kingpb]

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?

 

[kingpb]

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.

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.

 

[winnie]

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