Length of circuit to begin applying voltage drop calculations

[Knightryder12]

Exactly. Thank you Larry.

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

I didn't know we could just say "hey this panel will only ever see so many amps", what does the load calculation for the unit panel say? One would think that is what has to be used in the VD calculation right?

 

[winnie]

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

This is the internet. People will read lots of subtext into any question that anyone asks. In fact you have to do so to give reasonable answers. But sometimes we will take things in a direction you don't expect, and sometimes will we focus on a point that you think is incidental to your question.

If you'd said 'I'm on this job, and they've specified these feeders that seem crazy oversized to me.', then you would have gotten a bunch of commiseration, several reports of government jobs where oversized feeders were specified along with their complaints about wasting taxpayer money, and a couple of people trying to explain why the feeders aren't in fact oversized. That is the way the internet works.

IMHO the feeders that you describe are oversize. I did suggest reasons why they might not be (in particular voltage drop on the neutral), but without additional information I tend to agree with you. If what you want is validation of your instinct that the feeders are oversized, then stop reading now, we pretty much agree

At some point someone suggested 'saving your boss money', and several people (including me) chimed in that such 'value engineering' can never be unilateral. If you bid X but deliver Y (less than X) then there are words for that, and they ain't pretty. It doesn't matter if X was excessive. It doesn't matter if _you_ think X is wasteful or immoral or fattening. You bid X, you deliver X, or you renegotiate.

 

[Knightryder12]

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.

The load varies in all facilities. AS far as I know, you have to use the calculated load at the panel or of the load being served to determine actual VD. So what is the calculated load for each tenant panel?

 

[winnie]

I didn't know we could just say "hey this panel will only ever see so many amps", what does the load calculation for the unit panel say? One would think that is what has to be used in the VD calculation right?

If you have experience with energy monitoring in residential units, you will have a pretty strong 'seat of the pants' feel for the energy use of a residential apartment. Article 220 calculations for residential units are very conservative and routinely give numbers quite a bit larger than the real peak consumption.

Things like EV charging are changing this math, but I doubt the OP has EV chargers in the apartments.

My _guess_ is that these panels will _never_ see more than 50A of load, and will only rarely see even 30A of load. But that is a pure _guess_. But I agree, the article 220 calculated load is what should be used for the VD analysis, simply for lack of better information.

 

[Zmax25]

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

This isnt even what the conversation is supposed to be about. The topic is how long of runs do you go before begin to calculate voltage drop into sizing cable. I said 300' and I this is in reference to some 250' long feeders that I will soon be installing. Tortuga mentioned earlier that the IECC limit on VD is 5%. Using this threshold even at the full load of 125A a 2/0 wire would be sufficient for a 250' run. There is no need to size the wire all the way up to 250kcmil when in reality the load on the feeder will never be 125amps, it will rarely sustain 80 amps, if that, for more than 1 hour. Even generously assuming an 80 amp load the VD on a 2/0 cable is still only 3% at 250'.

 

[Zmax25]

The load varies in all facilities. AS far as I know, you have to use the calculated load at the panel or of the load being served to determine actual VD. So what is the calculated load for each tenant panel?

The calculated VA on the 2 bedroom units that would have 250' runs to them is 11132/phase. Given this and allowing for 5%max VD a 2/0 cable should be fine. And we all know the VA shown will hardly ever be hit in these residential settings, which is exactly why table 310.12 exists and why I am saying that in these settings voltage drop is essentially a myth because it has already been factored into our load calculations for dwelling units in most cases.

 

[Zmax25]

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.

which is beside the point of the post. Im asking about voltage drop not about companies bidding process or contractual agreements.

 

[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.

Are you or your company going to take over as EOR since you are possibly going to make all of these VE changes without approval from the engineering firm that designed this project?

 

[Knightryder12]

If you have experience with energy monitoring in residential units, you will have a pretty strong 'seat of the pants' feel for the energy use of a residential apartment. Article 220 calculations for residential units are very conservative and routinely give numbers quite a bit larger than the real peak consumption.

Things like EV charging are changing this math, but I doubt the OP has EV chargers in the apartments.

My _guess_ is that these panels will _never_ see more than 50A of load, and will only rarely see even 30A of load. But that is a pure _guess_. But I agree, the article 220 calculated load is what should be used for the VD analysis, simply for lack of better information.

I agree. In the real world we all know that calculated load is very conservative. But when it comes to permitting and following the code we cannot do this, so calculated load has to be used.

 

[Zmax25]

To everyone who has questions and points about Value engineering and the owners having to share any savings with the builders, here is the case just fyi.
This entire project is value engineered, everything is done as cheap as possible and yet for some reason we are oversizing conductors by 3 to 4x? To the point where I now have to downsize my conductor at the termination points to land them creating even more labor and more material cost. The entire thing is stupid and the opposite of any type of value engineering. I love my job as an electrician, but one of the things i hate the most about the construction industry is how wasteful it has become, so when I see something like this, I am more than willing to call an engineer out on it and get it changed to what it should be.

 

[hmspe]

5% at the panel is not correct. See Informational Note 4 at 210.19(A) in the 2017 or IN 3 at 219.19(A) in the 2020. In the 2020 International Residential Code see C405.10.

 

[Joethemechanic]

Being a "single phase" from a wye, the neutral can end up adding to your VD somewhat, but even then for residential, or your typical offices (Not including HVAC or Kitchen) my habit of amp clamping tells me that the loads are typically like 1/5 of what the installation are designed for. Especially with all the modern LED lighting. And even when they hit that 1/5 mark it's only because someone is running the microwave and the toaster oven in the kitchenette.

But if the engineer came up with a spec, and the customer agreed to it...........................

 

[Knightryder12]

The calculated VA on the 2 bedroom units that would have 250' runs to them is 11132/phase. Given this and allowing for 5%max VD a 2/0 cable should be fine. And we all know the VA shown will hardly ever be hit in these residential settings, which is exactly why table 310.12 exists and why I am saying that in these settings voltage drop is essentially a myth because it has already been factored into our load calculations for dwelling units in most cases.

Its not 5% VD its 3% thats allowed on feeders and 2% on branch (or 5% total) meaning to the farthest device. But you cannot use 5% just for the feeder.

 

[jim dungar]

which is beside the point of the post. Im asking about voltage drop not about companies bidding process or contractual agreements.

I said you need to ask the sizing question of the engineer for this project, not a bunch of people on this forum. You stated you have already submitted an RFI.

After some 52 posts, I don't think there is a single one that said the cables were not oversized. But, the VD calculations are a design choice and not addressed by the NEC.

The contract point was introduced by you, in post #14, when you said you would keep all of the money from any savings.

 

[readydave8]

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

I think that mentioning rfi to engineer in original post is making it harder for you to get the answer you want

Yeah man, sounds like those wires are way oversized, you got that right, good catch
️

 

[kingpb]

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 properly you aren't going to "save" him money. What you are doing is trying to make money by getting away with installing something less than what was on the plans, unless you get approval to install something less, in which case the Owner will certainly want money back. So, not saving anything. If you install without approval, you risk having to rip and replace, have warranty issue, increased liability on design, withheld payment, potential license/disciplinary action, and ultimately could be criminal fraud.

Again, the installation things, such as extra labor to pull, or pins to fit breakers should have already been included if it was bid properly.