Calculating house panel voltage drop from utility transformer or meter?

[TheWantonElectrician]

I have three questions.

I have a customer building a house approximately 250ft from the existing standalone meter pedestal. The meter pedestal is roughly 250ft from the utility transformer.

I'm trying to determine wire size for the house feeder from that meter.

When doing the calculations:
1) Should I use 250ft or 500ft in my calculations. If it's 500ft, then I will probably try to get the utility to set a transformer closer to the house and run a primary to it and then I'll set the meter on or near the house.
2) should I use 120V or 240v?
3) should I use the panel OC rating (200A), 166A (using 83% rule), or try to calculate the actual house load (presumably <166A)?

 

[augie47]

Your calculation should be based on the actual house load (taking into account realistic future loads).
It's probable that you will have a different wire size on the Line & Load side of the meter so I would do two calculations
240v

 

[wwhitney]

1) Should I use 250ft or 500ft in my calculations. If it's 500ft, then I will probably try to get the utility to set a transformer closer to the house and run a primary to it and then I'll set the meter on or near the house.

That's partially a regulatory question. If you consider the utility transformer fixed, then you should use 500 ft. But if the utility owns the 250 ft from the transformer to the meter, then typically the utility is regulated to provide a minimum voltage at the meter or thereabouts. In which case you are only responsible for the 250ft from the meter to the house, and can use that distance. And then if you find the voltage at the meter is too low, you'd have to get the utility to address it by upsizing their portion or changing taps.

2) should I use 120V or 240v?

You'd want to do each separately. For 120V, use the maximum calculated unbalanced load (neutral load), possibly with a further reduction to reflect that the NEC load calculations are very conservative. For 240V, use the full calculated load.

Augie address question 3.

Cheers, Wayne

 

[ggunn]

That's partially a regulatory question. If you consider the utility transformer fixed, then you should use 500 ft. But if the utility owns the 250 ft from the transformer to the meter, then typically the utility is regulated to provide a minimum voltage at the meter or thereabouts. In which case you are only responsible for the 250ft from the meter to the house, and can use that distance. And then if you find the voltage at the meter is too low, you'd have to get the utility to address it by upsizing their portion or changing taps.


You'd want to do each separately. For 120V, use the maximum calculated unbalanced load (neutral load), possibly with a further reduction to reflect that the NEC load calculations are very conservative. For 240V, use the full calculated load.

Augie address question 3.

Cheers, Wayne

But understand that for 240V you need to account for the fact that Vd occurs in both lines, so 500' will be in the equation for the round trip distance.

 

[wwhitney]

But understand that for 240V you need to account for the fact that Vd occurs in both lines, so 500' will be in the equation for the round trip distance.

Round trip distance applies in both the 120V and 240V cases I specified.

Cheers, Wayne

 

[ggunn]

Round trip distance applies in both the 120V and 240V cases I specified.

Cheers, Wayne

What if the 120V loads are balanced so that there is no current from them in the neutral?

 

[wwhitney]

What if the 120V loads are balanced so that there is no current from them in the neutral?

Then their combined load is covered by the 240V case. For the 120V case, I specified "use the maximum calculated unbalanced load (neutral load)."

Cheers, Wayne

 

[electrofelon]

That's partially a regulatory question. If you consider the utility transformer fixed, then you should use 500 ft. But if the utility owns the 250 ft from the transformer to the meter, then typically the utility is regulated to provide a minimum voltage at the meter or thereabouts. In which case you are only responsible for the 250ft from the meter to the house, and can use that distance. And then if you find the voltage at the meter is too low, you'd have to get the utility to address it by upsizing their portion or changing taps.


You'd want to do each separately. For 120V, use the maximum calculated unbalanced load (neutral load), possibly with a further reduction to reflect that the NEC load calculations are very conservative. For 240V, use the full calculated load.

Augie address question 3.

Cheers, Wayne

Yeah I would want clarification on the transformer to meter conductors. Specifically are these customer supplied and specified? Might want to make sure the POCO doesn't have any rules on that distance. Here one POCO won't let you go over 200 feet to a meter, but they don't care what you do after the meter.

I usually use around 65 amps for a typical Resi VD calc. I might bump that up to around 100 now because of EV's. Using the 65A assumption, 250AL will do it. If you bump up to 100A you get 350AL. 250URD pulls fine in 2-in conduit, but if you go to 350 you'll need 2.5" and that's a big price jump. 200 amp meter sockets except 350. 200 amp load centers usually take up to a 300, but will fit a 350 compact, FYI. I might do something like 350 to the meter and then 250 after the meter.

 

[suemarkp]

Generally for residential, if you size per your load calc, or maybe upsize one size, you should be fine. You wil rarely if ever draw what the calculation demands you install. Now on the utility size of the meter, they tend to skimp on size and if that distance is long they need to upsize.

 

[TheWantonElectrician]

Yeah I would want clarification on the transformer to meter conductors. Specifically are these customer supplied and specified? Might want to make sure the POCO doesn't have any rules on that distance. Here one POCO won't let you go over 200 feet to a meter, but they don't care what you do after the meter.

I usually use around 65 amps for a typical Resi VD calc. I might bump that up to around 100 now because of EV's. Using the 65A assumption, 250AL will do it. If you bump up to 100A you get 350AL. 250URD pulls fine in 2-in conduit, but if you go to 350 you'll need 2.5" and that's a big price jump. 200 amp meter sockets except 350. 200 amp load centers usually take up to a 300, but will fit a 350 compact, FYI. I might do something like 350 to the meter and then 250 after the meter.

Poco is responsible for transformer to meter. They won't do further than 270ft or something like that. I don't think I can see their wire size without opening the meter enclosure, but I could check voltage at the meter and assume that would give a good idea of what kind of Vd I'm seeing at the meter. My rough load calcs have 83A @120V and 135A @ 240v (no EV). It's a small, 1 story house (52'×26') with an unfinished basement that I included in the general lighting sqft calculation.

I had been thinking of running URD, but thought it could be directly buried (no conduit) - am I wrong? I would sleeve it as it goes to a 200A disconnect at the house.

 

[electrofelon]

Poco is responsible for transformer to meter. They won't do further than 270ft or something like that. I don't think I can see their wire size without opening the meter enclosure, but I could check voltage at the meter and assume that would give a good idea of what kind of Vd I'm seeing at the meter. My rough load calcs have 83A @120V and 135A @ 240v (no EV). It's a small, 1 story house (52'×26') with an unfinished basement that I included in the general lighting sqft calculation.

I had been thinking of running URD, but thought it could be directly buried (no conduit) - am I wrong? I would sleeve it as it goes to a 200A disconnect at the house.

I would probably run 250 URD. Not sure what your soil is like but direct bury around here is not a good idea. You can sand it, and it should hold up ok, but i usually find the labor and hassle of sand to be a wash with just getting conduit.

 

[TheWantonElectrician]

I would probably run 250 URD. Not sure what your soil is like but direct bury around here is not a good idea. You can sand it, and it should hold up ok, but i usually find the labor and hassle of sand to be a wash with just getting conduit.

What do you mean by "sand it"? The soil here is sand - probably terrible for grounding, but decent for digging.

 

[tortuga]

That's partially a regulatory question.

The OP is in Wisconsin which if I am not mistaken is one of the few last states that is not on the 2018 IECC yet, so voltage drop is still optional. In states where it is now required by the energy code you use the calculated load per article 220 (VA or watts) and the system voltage in 220.6 (240V) starting at the 'service point' per the NEC, typically the meter.

 

[TheWantonElectrician]

The OP is in Wisconsin which if I am not mistaken is one of the few last states that is not on the 2018 IECC yet, so voltage drop is still optional. In states where it is now required by the energy code you use the calculated load per article 220 (VA or watts) and the system voltage in 220.6 (240V) starting at the 'service point' per the NEC, typically the meter.

I'm here for code requirements, as well as best practices.

 

[electrofelon]

What do you mean by "sand it"? The soil here is sand - probably terrible for grounding, but decent for digging.

ok, here the soil is very rocky so typically one adds a bed of sand below and on top to help the direct bury conductors survive.

 

[tortuga]

I'm here for code requirements, as well as best practices.

These days the code requirement in most states is "The total voltage drop across the combination of feeders and branch circuits shall not exceed 5 percent." if you size the feeder for 3% your generally OK. Where we typically run into troubble is long runs of #14.

 

[mtnelect]

This direct burial cable thing never worked for me, only conduit, it's just me. In Southern California, the Southern California Edison Company is making a big effort to underground everything. In my area, Foothill Boulevard was undergrounded, several years ago. It was a big improvement; all those power lines were an ugly sight.

 

[electrofelon]

The OP is in Wisconsin which if I am not mistaken is one of the few last states that is not on the 2018 IECC yet, so voltage drop is still optional. In states where it is now required by the energy code you use the calculated load per article 220 (VA or watts) and the system voltage in 220.6 (240V) starting at the 'service point' per the NEC, typically the meter.

Technically you are correct, but I think enforcement of whatever it says about voltage drop will likely be sparse. Heck, a lot of Washington doesnt even enforce their own energy code

 

[Fred B]

ok, here the soil is very rocky so typically one adds a bed of sand below and on top to help the direct bury conductors survive.

This is not just nice rounded stone but many time very sharp fractured shale or bluestone. Even with a sand bed wouldn't recommend DB too much frost heave, Conduit is the way to go. Seen even with sand bed these sharps cause penetration to DB Wire.

 

[augie47]

Technically you are correct, but I think enforcement of whatever it says about voltage drop will likely be sparse. Heck, a lot of Washington doesnt even enforce their own energy code

That's commonplace in a lot of our areas, The State demanded every jurisdiction adopt the latest energy Code but the local inspectors either are unaware of many provisions or simply choose to ignore them.