[tmreeves]

I will install a long buried cable to a subpanel in a remote barn. Online wire-size calculators require me to input if my circuit is 120 or 240 V, and this parameter produces different wire-size results. MY QUESTION: My cable will have two hot leads (each 120, allowing 240 V at barn), but my subpanel will also be used to split out single 120 V lighting circuits. Do I input 120 V or 240 V in wire-size calculators? Thank you, Tom.

 

[roughneck]

You’ll use the 208/230 for input voltage into the sub panel.
The sub panel will have 2 hots, a neutral and ground. 4 conductors total.
What calculator are you using? Your sizing the circuit to fit the sub panel your installing.

 

[Kevin_Essiambre]

How far is the building you're trying to get power to from the source of the power?

What size of circuit are you wanting to feed this building? 100A? 60A? 30A?

Sent from my new phone. Autocorrect may have changed stuff.

 

[Jim Port]

You would use the 240 volt in your calculation.

 

[CodeMatters]

Agree with post 4.
When the sub-panel is lightly loaded it may be mostly/all on one leg
or the other,but being lightly loaded means that voltage drop won't
be problem.
It's reasonable to assume (and accepted method) though that as the
load increases it approaches balance and therefore it's seen as a
240V load.

 

[tmreeves]

Thanks to all…very helpful.

To roughneck: I've used two calculators, but the posting rules don't seem to let me state the links (one is on the askmeDIY site.

To Kevin_E: It’s pretty far…180 feet. A particular amperage is not required…would like 60A, but cost will likely move me lower. I would like to use direct buried UF (Underground Feeder) cable, and avoid conduit. The largest caliber I can find in UF (with 4 conductors needed for separate neutral & ground) is 6 gauge. This looks to be $3 or $4 per foot.

To Jim Port & CodeMatters: I’m happier with the results 240 volt gives me, although I don’t understand how two conductors each with 120V looses less over distance than one 120 V conductor.

 

[Datawise]

I would advise all of my clients to install conduit; it is a no brainer. In your case I would advise that you install a 2" DB TypeII duct.

If you want 60 amps at 240 volts with a 3% voltage drop (180 feet) you need to use #4 copper wire.

Cheers
John

 

[rjniles]

I would advise all of my clients to install conduit; it is a no brainer. In your case I would advise that you install a 2" DB type duct.

If you want 60 amps at 240 volts with a 3% voltage drop (180 feet) you need to use #4 copper wire.

Cheers
John

Suggesting copper that size at that distance sounds nice untill he sees the bill. Aluminum is the reasonable choice.

Sent from my RCT6A03W13E using Tapatalk

 

[Datawise]

Suggesting copper that size at that distance sounds nice untill he sees the bill. Aluminum is the reasonable choice.

Sent from my RCT6A03W13E using Tapatalk

Agreed. I just did the calculation in CU. Could have done it in AL, but it will probably be #2 AL.

Yes it is #2 AL wire which gives a 2.64% VD for 60 amps.

Cheers
John

 

[rjniles]

Before you use a wire size calculator you need to do a load calculation to determine what amperage is required.

 

[Kevin_Essiambre]

Before you use a wire size calculator you need to do a load calculation to determine what amperage is required.

How would one do a load calculation for it when OP isn't even sure how much amperage he wants?

Datawise did the calculation at a 60 amp load, to figure worst case scenario if he does 60 amps to the building, which is perfectly fine by code.

Sent from my new phone. Autocorrect may have changed stuff.

 

[carmusic]

use AL, 3-5 times cheaper than copper so you can even prepare for 100amps!

 

[seharper]

Do I input 120 V or 240 V in wire-size calculators?
It’s pretty far…180 feet. A particular amperage is not required…would like 60A, but cost will likely move me lower.

240V, but you also change some other stuff.

The default 3% is wire-salesman bullpuckey. It's not in Code at all. Put 5.5% and see what it says.

Also change the ampacity to your more realistic actual expected load. The usual mistake is to use the breaker trip number (e.g. 60A), but even that is wrong, because you're not allowed to plan to use more than 80% of that. (e.g. 48A for a 60A trip). So putting more than 80% is simply an error.

When you factor for 80%, you will not need a wire-size bump at 180' and can just use the statutory minimum wire size. For instance the common/standard 60A circuit with 4 AWG Al wire, at 180' your voltage drop is only 3.29% at 48A actual. That is totally fine, even by the wire salesman's standards.

Why does lowering amps help? Because voltage drop is caused by amps actually flowing at that moment. No amps / low amps = no/low voltage drop. A solitary 120V tool flowing 12A on that hypothetical #4 Al will only drop 1.64%. If you added a 120V 12A dust collector *on the other leg*, that would actually reduce voltage drop to 0.82%.

I would advise all of my clients to install conduit; it is a no brainer.

One exception: if theft is a serious risk. Theft from conduit is easy, just bash into one end, tie it to the trailer hitch and drive away. Stealing direct burial line buried to 24", a bit harder.

you need to use #4 copper wire.
[or] AL, but it will probably be #2 AL.
Yes it is #2 AL wire which gives a 2.64% VD for 60 amps.

I think you miscalculated on that one. But hey, if you're going to #2 Al, you might as well go a half-size larger and go #1 Al. That'll put you at 100A, as carmusic proposes.

Remember, voltage drop is not a code requirement, it's considered a preferable practice (by wire salesmen lol). That means if you bump to #1 aluminum wire *for voltage drop reasons*, you are perfectly welcome to "call it" a 100A circuit and put a 100A breaker on that, since #1 Al is the correct wire for a 100A circuit. And then just limit your draw to 60A lol.

I would like to use direct buried UF (Underground Feeder) cable, and avoid conduit. The largest caliber I can find in UF (with 4 conductors needed for separate neutral & ground) is 6 gauge. This looks to be $3 or $4 per foot.

That's correct. UF cable stops at 6 AWG. At that point you switch to other direct-burial types such as URD or MH feeder.

6 AWG is the wobble-point at which you should seriously be evaluating aluminum at 2 sizes larger (numerically smaller). There is nothing wrong with aluminum wire as feeder, however Code requires it be the new AA-8000 alloy, a rule made out of an abundance of caution. The lugs on the subpanel will be aluminum anyway, so this averts a dissimilar-metal problem.

I don’t understand how two conductors each with 120V looses less over distance than one 120 V conductor.

Because voltage drop *doesn't actually happen* as a percentage, it happens as an absolute volt drop. So if you have 2.4 volts drop at 120V, you just dropped 2%. If you have 2.4V drop at 240V, you only dropped 1%. Further, if you can run the same load at 240V instead of 120V, that means current drops in half. So the voltage drop reduces by half *again*.

Subpanel rating: If you lived in Wyoming with 80 mph posted freeway limits, would you want R-rated (85 mph max) tires on your car? Probably not, because you don't want to run your car tires at absolute redline. So why would you put a 60A rated or even 100A rated subpanel on your 60A circuit?

Subpanel main breaker rating: A subpanel in an outbuilding does not need any main breaker at all. It needs a disconnect switch. Usually the cheapest way to get a disconnect switch is get one with a main breaker. The main breaker size does not matter; it's only a disconnect switch. So if you find a panel you like with a 200A main breaker, that is fine.

Subpanel spaces. I see it all the time where people overspend on wire (weren't you ready to buy #4 Cu!?) and then scrimp on the subpanel. Then they immediately run out of spaces and go "woe is me!" The cost difference for more spaces is trivial at the time you are buying the panel. So go stupidly huge on spaces. Get the largest panel that won't get you kicked out of bed. Gosh, so you miss out on a couple of pizzas. This means when you get a wild hair to develop a wood shop, 240V saw (2 spaces) got em. 240V dust collector (2 spaces) got em. 240V planer got em. 240V jointer got em. 240V radial arm (2 spaces) got em. *fwiff* we just went through 10 spaces just like that. See, when I say get a 30-space panel, I'm not insane.

 

[Jim Port]

You only need to load a breaker to 80% for a continuous load of over 3 hours. Less than that can be at 100%.

 

[Kevin_Essiambre]

I think you miscalculated on that one. But hey, if you're going to #2 Al, you might as well go a half-size larger and go #1 Al. That'll put you at 100A, as carmusic proposes.



Remember, voltage drop is not a code requirement, it's considered a preferable practice (by wire salesmen lol). That means if you bump to #1 aluminum wire *for voltage drop reasons*, you are perfectly welcome to "call it" a 100A circuit and put a 100A breaker on that, since #1 Al is the correct wire for a 100A circuit. And then just limit your draw to 60A lol.

Except Datawise is probably using our Ontario code for the calculation, which DOES have code requirements for voltage drop (Canadian Electrical Code has it too).

Sure, you can bump that up to #1 to get 100 amps.... except our code would require a voltage drop calculation to keep it within 3%, so it may not be 100 amps.
@Datawise

Sent from my new phone. Autocorrect may have changed stuff.

 

[rjniles]

How would one do a load calculation for it when OP isn't even sure how much amperage he wants?

Datawise did the calculation at a 60 amp load, to figure worst case scenario if he does 60 amps to the building, which is perfectly fine by code.

Sent from my new phone. Autocorrect may have changed stuff.

He must have some idea how he plans on using the barn. To guess 60 amps when he needs 100 will not work. And to guess 60 if he needs 30 is overkill.

 

[Kevin_Essiambre]

He must have some idea how he plans on using the barn. To guess 60 amps when he needs 100 will not work. And to guess 60 if he needs 30 is overkill.

When in doubt use the current of a standard arc welder...

If he isn't sure how he'll use it, it's pointless to try to do a voltage drop calculation. Run a conduit and pull #10 for now. When you want to upgrade pull the #10 out and pull in 4/0.

Sent from my new phone. Autocorrect may have changed stuff.

 

[seharper]

Except Datawise is probably using our Ontario code for the calculation, which DOES have code requirements for voltage drop (Canadian Electrical Code has it too)

Interesting. What percent drop is it? 3% is a misquote out of NEC.

 

[Kevin_Essiambre]

Interesting. What percent drop is it? 3% is a misquote out of NEC.

Here's what our canadian code says on it.

Sent from my new phone. Autocorrect may have changed stuff.

 

[Datawise]

Since there is no requirement to put your location on this site, it is impossible to determine which code rules apply. Having said that, there are some pretty common practices that work on both sides of the line.

Also vernacular is important; if somebody says they need / want XXX amps then you have to determine if that is the usable amps or the "overcurrent" setting. Most people (non-professionals) think that if they have a xxx amp panel, that they have the same number of amps available; of course that is not true.

The last issue is that part of the conversation needs to be about future-proofing your installation. Conduit is a great example; in many cases it does not make sense to install a minimum sized conduit. The most expensive part of that installation is the digging; why anyone would put in a small conduit or only one conduit is mind boggling. Conduit is very inexpensive compared to digging up the ground multiple times.

Cheers
John