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

 

[herding_cats]

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.

 

[LarryFine]

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%.

Remember that voltage drop is dependent on actual load current. A125a panel is not a load.

 

[herding_cats]

Exactly. Thank you Larry.

This panel might just see 40-60A at any given time. VD is a very widely misunderstood phenomena.

 

[Sberry]

That one seems hard to get sometimes doesnt it?

 

[Zmax25]

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.

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.

 

[Zmax25]

Remember that voltage drop is dependent on actual load current. A125a panel is not a load.

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

 

[herding_cats]

Correct. This is why my lighting circuits have no issues. We have them fed from 20A breakers, but the actual load is like 4-5A for LED lighting strips. That's my point on people looking into VD and getting it wrong. I was on a job at a rural farming operation and the guy there was pulling #8 wire for outdoor outlets 120vac. Distance is about 100'. Totally insane. He couldn't understand what I was trying t explain to him about the unnecessary wire size.

"But it's on a 20A breaker". That does not matter.

 

[Zmax25]

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?

It was bid for the size of wire on the plan, but we are a fairly small company and if I can save my boss what would be a ton of money on this wire order to feed 252 apartment units why wouldnt I do that? What calcs do you propose were done by the engineer to come up with the sizes listed on the feeder schedule I attached to the post? Because I have done the calcs myself and it looks to me like they are sizing this wire based off of the OCPD and not the actual load applied. This is not only wasteful, its stupid. Its way harder for my guys to pull a 250kcmilSER 250' than it is to pull a 1/0 or 2/0 the same distance. On top of that when the wire is increased over 2/0 now they dont fit in the breakers and panel lugs so now I have additional cost and labor to install reducing pins or tap it down to the normal size at the connection points.

 

[winnie]

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.

But is this 120V or 277V lighting? And just what sort of ballast?

I'm not disagreeing that voltage drop is often misunderstood, or often worried about when it isn't an issue.

But 277V lighting with ballasts that compensate for voltage changes is rather different than 120V lighting running incandescent bulbs.

When you are looking at voltage drop you need to know:
1) The actual load being served.
2) The actual sensitivity of the load to changes in supply voltage, both in steady state and during startup transients.
3) The actual sensitivity of other loads sharing the same circuit.

As has been said 3x above: a 125A residential panel is not a 125A load. But an example of something to consider: if you have a large motor load such as a heat pump, you might see large transient voltage drop every time the compressor kicks in.

 

[herding_cats]

Both 120 and 277. All modern LED. I understand what you are saying.

One thing to watch out for in an industrial environment is a severe voltage drop could equate into really high demand charges. All it takes is 1 blip of high wattage and its on your bill for 12 months as "actual demand".

 

[winnie]

It was bid for the size of wire on the plan, but we are a fairly small company and if I can save my boss what would be a ton of money on this wire order to feed 252 apartment units why wouldnt I do that? What calcs do you propose were done by the engineer to come up with the sizes listed on the feeder schedule I attached to the post? Because I have done the calcs myself and it looks to me like they are sizing this wire based off of the OCPD and not the actual load applied. This is not only wasteful, its stupid. Its way harder for my guys to pull a 250kcmilSER 250' than it is to pull a 1/0 or 2/0 the same distance. On top of that when the wire is increased over 2/0 now they dont fit in the breakers and panel lugs so now I have additional cost and labor to install reducing pins or tap it down to the normal size at the connection points.

If it was bid to the plan, and the plan is nutso oversized, then any deviations will require a negotiation between your company and the customer. Your company can't simply pocket the savings. You have to deliver what was bid or get the customer to agree to the change, and the customer will want some of the savings.

-Jonathan

 

[Zmax25]

But is this 120V or 277V lighting? And just what sort of ballast?

I'm not disagreeing that voltage drop is often misunderstood, or often worried about when it isn't an issue.

But 277V lighting with ballasts that compensate for voltage changes is rather different than 120V lighting running incandescent bulbs.

When you are looking at voltage drop you need to know:
1) The actual load being served.
2) The actual sensitivity of the load to changes in supply voltage, both in steady state and during startup transients.
3) The actual sensitivity of other loads sharing the same circuit.

As has been said 3x above: a 125A residential panel is not a 125A load. But an example of something to consider: if you have a large motor load such as a heat pump, you might see large transient voltage drop every time the compressor kicks in.

Lets use real world examples here. These apartments have one heat pump per bed room plus another for the living room, so a 3 bedroom has 4 and a 2 bed has 3. Nice for me the 3 bedrooms are right next to the meter stack so those shouldnt be an issue. So the furthest 2 bedroom is 250' away. The heat pumps are 120V ( dont ask me why), the fans are listed to draw 0.4 amps and the compressors are 12.3 amps for a total of 12.7 amps. The MCA is listed as 17A and the Max OCPD is 25 amps (although 20 should be fine). That means even if all 3 of these kicked on at the same time you would see at most 15 amps on one phase and 30 amps on another phase and that would be for a very breif moment before it would go down to nominal ranges or less - Ive seen these same heat pumps typically draw 6-9 amps once running. All that is to say that even if all the heat pumps kick on at the same time it will hardly put a dent in the load capacity.

 

[Zmax25]

If it was bid to the plan, and the plan is nutso oversized, then any deviations will require a negotiation between your company and the customer. Your company can't simply pocket the savings. You have to deliver what was bid or get the customer to agree to the change, and the customer will want some of the savings.

-Jonathan

Not in this case. The price negotiated is the price we get paid, regardless of if this wire size were to change or not.

 

[jim dungar]

Not in this case. The price negotiated is the price we get paid, regardless of if this wire size were to change or not.

Really?

I dont think I have ever seen a contract where the contractor can supply whatever they want, regardless what was bid per specifications, and pocket the entire savings.