Length of circuit to begin applying voltage drop calculations

[tortuga]

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.

Well said and I'd just add that since the NFPA decided the regulation of voltage drop was not in the scope of the NEC it efforts to regulate it moved to the International Energy Conservation Code (IECC), so its in a adopted code just not the NEC.
And since engineering contracts between a PE and a client typically say the drawings shall "meet all local codes" it could be an 'error or omission' of a PE to not include voltage drop in stamped plans (in CO) where wire sizes are given, as voltage drop is regulated by the state of Colorado IECC section C405.10 in the 2021, which says the total voltage drop across feeders plus branch circuits shall not exceed 5 percent.
While it may not be enforced by the electrical AHJ, it could be enforced down the road by a litigious client as a construction defect ('error or omission' claim) not just the AHJ.
Colorado training materials show that exact commercial requirement but I am not clear on resi. However for residential it depends, Low-rise multifamily, 3 stories or less above grade, is treated under the IECC residential provisions and are likely exempt. DOE's Colorado/IECC materials state that the IECC residential provisions are limited to multifamily buildings that are three stories or less above grade but I'd suggest the OP do submit a RFI and referece IECC section C405.10 or whatever it is at present.
And as Larry correctly pointed out the calculation is based on NEC calculated load of the feeder not panel size.

 

[hillbilly1]

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.

????? Yes, the loads and panels may be single phase, but it is still a 208/120 wye system supplying it. You will still have 208 at the panel, not 240.

 

[Zmax25]

????? Yes, the loads and panels may be single phase, but it is still a 208/120 wye system supplying it. You will still have 208 at the panel, not 240.

its the way its listed that matters. 208Y/120V indicates a 3 phase system where as 120/208V indicates single phase pulled off a 208V 3 phase system.

 

[jim dungar]

????? Yes, the loads and panels may be single phase, but it is still a 208/120 wye system supplying it. You will still have 208 at the panel, not 240.

But the ANSI methodology is to identify single phase by showing the L-N Low Voltage followed by a slash and then the L-L High Voltage, thus 120/208. Three phase is identified by the L-L High Voltage followed by a Y if the source is a wye, then by a slash and then finally by the L-N Low Voltage (if applicable) For some reason US power companies tend not to follow this methodology.

 

[Zmax25]

Well said and I'd just add that since the NFPA decided the regulation of voltage drop was not in the scope of the NEC it efforts to regulate it moved to the International Energy Conservation Code (IECC), so its in a adopted code just not the NEC.
And since engineering contracts between a PE and a client typically say the drawings shall "meet all local codes" it could be an 'error or omission' of a PE to not include voltage drop in stamped plans (in CO) where wire sizes are given, as voltage drop is regulated by the state of Colorado IECC section C405.10 in the 2021, which says the total voltage drop across feeders plus branch circuits shall not exceed 5 percent.
While it may not be enforced by the electrical AHJ, it could be enforced down the road by a litigious client as a construction defect ('error or omission' claim) not just the AHJ.
Colorado training materials show that exact commercial requirement but I am not clear on resi. However for residential it depends, Low-rise multifamily, 3 stories or less above grade, is treated under the IECC residential provisions and are likely exempt. DOE's Colorado/IECC materials state that the IECC residential provisions are limited to multifamily buildings that are three stories or less above grade but I'd suggest the OP do submit a RFI and referece IECC section C405.10 or whatever it is at present.
And as Larry correctly pointed out the calculation is based on NEC calculated load of the feeder not panel size.

Thanks for this comment Tortuga, I was unaware of the 5% requirement coming from another code. My question now would be how can you accurately calculate voltage drop on a residence where the load applied is constantly changing? To me, you cant, this is why we perform load calculations and size things the way we do. Commercial and industrial sizing is very different because in those applications the load being served is usually explicitly stated and you know how much certain things will be running. That is never the case in dwelling units, which is why we already oversize them the way we do. Lets be honest here, these apartments will rarely see more than 60-80 amps of load across both phases. The way I see it, in most cases for "Dwelling Units" the voltage drop has already been accounted for with the ammount of VA we apply during our calculations.

 

[tortuga]

Thanks for this comment Tortuga. My question now would be how can you accurately calculate voltage drop on a residence where the load applied is constantly changing? To me, you cant, this is why we perform load calculations and size things the way we do. Commercial and industrial sizing is very different because in those applications the load being served is usually explicitly stated and you know how much certain things will be running. That is never the case in dwelling units, which is why we already oversize them the way we do. Lets be honest here, these apartments will rarely see more than 60-80 amps of load across both phases. The way I see it, in most cases for "Dwelling Units" the voltage drop has already been accounted for with the ammount of VA we apply during our calculations.

I don't disagree the load will be very low, and if the building is under 3 stories I don't think the energy code applies, but if the energy code applies the voltage drop calculation is based on the article 220 calculated load for the dwelling unit (I'd use 220.82) and the design voltage is based on 220.5(A). Just be aware it is in the code (may not apply to you) but CYA

 

[winnie]

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.

I'm going to throw a couple of wrenches (some helpful) at the above:
1) By heat pumps, my guess is that they are minisplit systems. If so, there is a good chance that they are inverter fed systems. The running load will probably be less than the MCA most of the time, except for some rare worst case situation. This is good for your installation.
2) If by chance these are _not_ inverter fed systems, then the transient compressor starting current will be considerably larger than both MCA and Max OCPD
3) Since these units are 120V, their impact on voltage drop will be as 120V loads. You don't have the benefit of current canceling on the neutral. So your worst case scenario becomes 30A at 120V when you do your voltage drop calculation.

 

[winnie]

????? Yes, the loads and panels may be single phase, but it is still a 208/120 wye system supplying it. You will still have 208 at the panel, not 240.

This is just a standard naming convention; you have the physics right.

If you have a wye system with 208V L-L and 120V L-N, you call it 208/120V when it is three phases supplied to a panel, and you call it 120/208V when it supplies a single phase. Pot-ay-toe pot-ah-toe I've proposed calling 120/208V supplied to a panel 'psingle phase' but that never took off.

 

[tortuga]

This is just a standard naming convention; you have the physics right.

If you have a wye system with 208V L-L and 120V L-N, you call it 208/120V when it is three phases supplied to a panel, and you call it 120/208V when it supplies a single phase. Pot-ay-toe pot-ah-toe I've proposed calling 120/208V supplied to a panel 'psingle phase' but that never took off.

The NEC trips all over itself trying to describe these systems, I call them open-wye, avoiding wording that implies neutral current canceling.

 

[readydave8]

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

What does the engineer gain by approving reduced wire size? If he's working for the customer, it would be stupid for him to do so.

 

[JoeStillman]

... 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

We do the VD calcs using the 220 Part IV calculated load. You could make a good case for sizing the wires based on THAT. Did the engineer include the calculated loads on the drawings? Not just the total loads on a panel schedule. He should have.

 

[roger]

The customer paid for the design and if it were me I would expect to get what I paid for. If there is going to be a form of VE the customer is due a refund.

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?

In reality your boss should have marked up the material and would be making money on it.

 

[kwired]

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.

Actual load is the first concern, a second concern might be voltage drop when say an AC compressor is starting even though it is very short term it can be noticeable by light dimming during that time. Many cheap LED's will have some dimming even when the voltage drop is practically negligible though.

Your boss should have looked at the specifications when he bid the job? If it called for these larger conductors then he should have included them in the bid. If he intentionally did not include it to lower his bid price he likely would have already told you we are not running that size.

Designers often do over specify things, but they may have their reasons, none of which are code driven but are simply design decisions.

 

[Zmax25]

Actual load is the first concern, a second concern might be voltage drop when say an AC compressor is starting even though it is very short term it can be noticeable by light dimming during that time. Many cheap LED's will have some dimming even when the voltage drop is practically negligible though.

Your boss should have looked at the specifications when he bid the job? If it called for these larger conductors then he should have included them in the bid. If he intentionally did not include it to lower his bid price he likely would have already told you we are not running that size.

Designers often do over specify things, but they may have their reasons, none of which are code driven but are simply design decisions.

Im well aware that actual load is the first concern.
I have more than twice as much experience as my "boss", the owner of the company, why shouldn't I help him out if he misses something?

 

[kwired]

Im well aware that actual load is the first concern.
I have more than twice as much experience as my "boss", the owner of the company, why shouldn't I help him out if he misses something?

If his bid was for what was specified then what was specified should be installed. If you feel something is in error, you bring it up, and possibly make price adjustments accordingly if the actual installation ends up being different from original plans. He shouldn't bid different from the specifications without noting said differences in the bid somehow.

If what is on plans is overkill, you also need designer and/or owner approval if you install to lesser specifications, and likely give the owner a break on price as well.

Would you downgrade without saying anything to designer/owner the switches and receptacles from spec grade if that were called out on plans to the cheapest lowest grade devices you can find from Amazon so your company can make some more off the job?

 

[winnie]

Im well aware that actual load is the first concern.
I have more than twice as much experience as my "boss", the owner of the company, why shouldn't I help him out if he misses something?

You absolutely should help your boss out. I think we all agree on the physics of the situation and the fact that these feeder conductors are probably oversized.

But your boss might be constrained by the contract with the customer to use the oversized feeders. Or they may need to negotiate with the customer to reduce the size of the feeders. Or your boss may have recognized that the feeders are oversized, and simply included this in the bid with suitable padding; maybe your boss wants to sell a 'premium' (meaning oversized) install and take the profit on that 'premium' design.

 

[Zmax25]

What does the engineer gain by approving reduced wire size? If he's working for the customer, it would be stupid for him to do so.

Im sorry I forgot that we cant hurt the engineers feelings by telling them they are wrong huh? If you've ever done a load calc and actually understand how voltage drop works there is no way that you can justify pulling 250kcmil or even 4/0 for what is typically a 1/0 feed with a highly variable but generally low load.

 

[Zmax25]

If his bid was for what was specified then what was specified should be installed. If you feel something is in error, you bring it up, and possibly make price adjustments accordingly if the actual installation ends up being different from original plans. He shouldn't bid different from the specifications without noting said differences in the bid somehow.

If what is on plans is overkill, you also need designer and/or owner approval if you install to lesser specifications, and likely give the owner a break on price as well.

Would you downgrade without saying anything to designer/owner the switches and receptacles from spec grade if that were called out on plans to the cheapest lowest grade devices you can find from Amazon so your company can make some more off the job?

all of that is beside the point of the actual topic. Im asking about peoples opinions on voltage drop not how companies bid jobs.

 

[jim dungar]

Im sorry I forgot that we cant hurt the engineers feelings by telling them they are wrong huh?

Who said you shouldn't question the engineer's design? I have always considered this as a standard practice on most projects.

Most comments have been saying the owner probably expects a cut of any Value Engineering you get permission to implement.

 

[Knightryder12]

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.

How would you be saving your boss money? Are you not going to offer a refund to the owner for the smaller wire?