Can you guys check my math?

[b-boy]

I'm in the process of running power out to an external building.

My 30x40 pole barn is ~250ft from my home. This includes an internal run of approximately 40 ft to my main panel. My main panel is 200A. It already contains a breaker for an old barn on my property. The barn has 60A running to it. I will be disconnecting that and using the freed slots for the new subpanel.

Ideally, I'd like a 100A panel in the garage. I'd like to run a 90A breaker in my main panel to supply the sub panel.

I'll run 4-wires (THHN/THWN) through a 2-inch PVC conduit that was just buried. My sub-panel will be unbonded, with separate neutral and grounds. I'll have a grounding rod with #6 copper wire running from the out-building as well.

Based on my understanding, I'm planning on using #3 THHN/THWN between the panels. The entire length of the run will be inside conduit. It looks like it will run me close to $1,000.00 for the wire.

Does this sound correct? Anything I've missed?

One question - what size ground wire can I use for this?

 

[matt_i]

I think you need 2 grounding rods driven 6ft apart.

I just ran the number with #3 Cu, at Home D the 500' spool is ~$450.

So, it would be important to see if the 250 ft can actually do the job for each conductor without going over that length by 1 inch. If so, you could get a couple of 500' spools and just eat the cost of the ground wire also being #3 vs. sourcing another 250' wire by-the-foot for the ground.

For example, also at Home D, a #6 awg green/ground by-the-foot is 0.93/ft so $232...vs $225 for the #3 black that you could glean from the remainder of the spool....

There are probably other ways of doing this for less but I'd solidify the exact length first. For lack of other resources I'd pull a string thru it, account for excess needed in terminating, and then measure that exactly.

 

[b-boy]

I found #3 THHN/THWN online, delivered, for $.99 per foot. I can order any length, so I'll definitely add a few extra feet to the order just in case. I'd rather pay an extra $50 than run short mid-job.

Pulling a string is a great idea. I never though of that.

 

[sberry]

Use number 2 alum, under 1.50 a foot for all 4 wires and use the 60. HD has 2 2 2 4

 

[mm08822]

So here is the bad news......250' distance makes for lots of voltage drop. It means you either need to adjust your plan or open your wallet wider.

90A load at 250' fed with #3 CU will not provide proper voltage at load due to feeder resistance. Distance of ~ 170' is max for 3% drop.

So determine if you really need 90A (load calc) or up-size the wire. 90A of simultaneous load is a lot of power used in a small shop. Wanting a large panel (# ckts) does not mean you have to feed it at its main cb rating. e.g. - 200 panel could be fed with 90A feeder, etc.

with 90A load:
#2 Cu good for 220'
#1 Cu good for 270'

2/0 Al good for 245'

2 - 2/0 Al, xhhw, 1 - 1/0 Al xhhw, 1 - #4 xhhw will fit in 2". Pulling it may be another story.

 

[75gmck25]

I had a much shorter run (about 100 feet total), and I used 2-2-2-4 AL, which was readily available.

I used SER inside the house and dual rated MHF in conduit outside. I ended up buying the MHF wire from a supply house because the local HD couldn't verify their MHF was dual rated for RHW/THW so I could run directly into the sub-panel. However, even with shipping I don't think I spent more than about $150 total for wire.

I fed a 100 amp sub-panel with 60 amps from the main, although for my 100 foot distance I could have used a 90 amp breaker in the main panel.

Bruce

 

[b-boy]

I'm going to pass on the AL.

My Dad is a retired metallurgist. He specialized in copper/brass alloys, and how they were used in building construction.

When I mentioned AL, he started lecturing me. I'd love to save a few bucks, but then I'd have to hear about it for the next 10 years. Plus, he's helping me.

 

[sberry]

He knows a little too much about it. Most of the world's power runs down alum wire, it's what the poco uses. This would be a good chance to educate him. Under 400$ for the wire and at that distance would consider direct burial.

 

[pattenp]

I'm going to pass on the AL.

My Dad is a retired metallurgist. He specialized in copper/brass alloys, and how they were used in building construction.

When I mentioned AL, he started lecturing me. I'd love to save a few bucks, but then I'd have to hear about it for the next 10 years. Plus, he's helping me.

Nothing wrong with using aluminum wire if installed properly. The aluminum alloy used today is a far cry from aluminum used years ago for wire. Your dad needs to catch up with the times.

 

[sberry]

The 2 2 2 4 also has the ground wire sized up for v drop on long runs. I have direct bury alum in for 30 years now, not a problem.

 

[wyliesdiesels]

I'm going to pass on the AL.

My Dad is a retired metallurgist. He specialized in copper/brass alloys, and how they were used in building construction.

When I mentioned AL, he started lecturing me. I'd love to save a few bucks, but then I'd have to hear about it for the next 10 years. Plus, he's helping me.

Your dad is 40+ years behind on technology.

The aluminum alloys used today for building wire are not the same problem prone alloys used in the 60s and 70s.

Heres some reading homework to give him:

http://www.southwire.com/commercial/AluminumBuildingWireHistory.htm

This one below goes into the specifics on the alloys:

http://www.southwire.com/commercial/ALBuildingWire30YrsOfReliability.htm

Should be right up his alley if he is a metallurgist.

And at the lengths youre talkin, it will be a lot more than a few bucks!!

 

[b-boy]

Well, Al will be considered. I'm looking at $1200 for #2 Cu THHN/THWN.

Assuming the info below, do I use the 75C rating for my application?

Size: 2 AWG
Number of Strands: 19
Cross Section Area (mm2): 33.60
PVC Insulation Thickness (Conductor): 1.020 mm / 0.040 inches
Nylon Jacket Thickness: 0.150 mm / 0.006 inches
Outside Diameter: 9.53 mm / 0.375 inches
Weight: 0.254 lbs per ft
Allowable Ampacity: 95 Amps at 60ºC / 115 Amps at 75ºC / 130 Amps at 90ºC

 

[pattenp]

For the type of wire you are looking at you will use the 75C rating.

 

[mm08822]

As said 75C rating, but in your case it doesn't matter b/c wire diameter, length and load keep your ocp at 90A to limit the vd to an acceptable amount.

#2 Cu at 250' gives 3.5% vd, which is pretty good. I doubt you will ever be pulling 90A.
Consider pricing scenarios also using reduced neutral and #6 egc vs. all #2. That will save a few $.

 

[Jeeper89]

Keep in mind the max wire size the breaker will accept. Example a CH290 breaker will only take up to 1/0 wire Cu or Al. Unless there is a way to adapt this down that I am unaware of that rules out the 2/0 aluminum. I ran into this issue and ended up going with a CH2150 breaker and 3/0 aluminum wire, but I only had to go 55 feet. It's not fun pulling three 3/0 wires and a 4 AWG copper inside 2" conduit. I had too many bends in my run, I got it done but it took me nights of fighting after work.

 

[benjamintmiller]

I just ran a 100 amp service about 250 feet, similar to what you're doing.

The short answer is that you want 1/0-1/0-1/0-2 aluminum cable. It is commonly sold as URD (underground residential distribution), but usually is also marked USE or USE-2, and that secondary marking makes it legal.

I spent about $450 on 250' of it, and direct buried it, but you can also put it in conduit.

There is nothing in the NEC that mandates low voltage drop, but 3% to the building and 5% to the farthest outlet is a common value. It probably won't hurt to be a bit above that -- keep in mind that voltage drop is at maximum load, which you will likely never see.

 

[ard]

Under 400$ for the wire

I'm looking at $1200 for #2 Cu THHN/THWN.

Welp, look at the bright side: if you personal fortunes take a turn in the future, you have $800 worth of copper you can 'withdraw'....

 

[sberry]

Since there is already buried conduit the number 2 would pull that distance thru 2 inch. It would keep close to the v drop values at 60A. We don't know what you expect to run but I have aired a couple dozen garages and a couple 3 house feeds at that distance with a 60 breaker and never had a trip call and a couple were busy places with 5 hp comps on them.
My own house is fed from my parents with 50 at 250 ft, maybe a bit more. I have gas appliances though but use a little AC and electric heat. Voltage still near 120 when its all going including microwave.
As was mentioned above, v drop is only a concern at max load and nominal is above 120 these days, it's just not much an issue. I ran a welding shop 1 winter from 250 of 2 on 60 minus air comp on another circuit, ran a 250 and 180 mig plus lights no problem, never tripped.

 

[malibu101]

Keep in mind the max wire size the breaker will accept. Example a CH290 breaker will only take up to 1/0 wire Cu or Al. Unless there is a way to adapt this down that I am unaware of that rules out the 2/0 aluminum. I ran into this issue and ended up going with a CH2150 breaker and 3/0 aluminum wire, but I only had to go 55 feet. It's not fun pulling three 3/0 wires and a 4 AWG copper inside 2" conduit. I had too many bends in my run, I got it done but it took me nights of fighting after work.

For info to all--
Yes there is a legal way to change wire size to fit into a smaller lug.
They are called pin adapters.
However they are pricey for what they are and need a special crimper.

Example- https://commerce.ilsco.com/e2wShoppingCatalog.aspx?parentId=3100012329

 

[ard]

For info to all--
Yes there is a legal way to change wire size to fit into a smaller lug.
They are called pin adapters.
However they are pricey for what they are and need a special crimper.

Example- https://commerce.ilsco.com/e2wShoppingCatalog.aspx?parentId=3100012329

Cool. Had wondered about that. Someone had a 75A breaker to a shop that was 300ft away and due to voltage drop needed to upsize to a wire that would not fit the breaker.

Although I expect the tool for that ilsco is $$$.

Anyone know of a split bolt or other type of adapter??

 

[malibu101]

Cool. Had wondered about that. Someone had a 75A breaker to a shop that was 300ft away and due to voltage drop needed to upsize to a wire that would not fit the breaker.

Although I expect the tool for that ilsco is $$$.

Anyone know of a split bolt or other type of adapter??

Yeah, the crimper is WAY more than anyone would spend for a couple of crimps.
But to bring out a contractor to do a couple of crimps would probably cost a couple hundred bucks to show up and an hours labor. Although I understand why their cost.

I don't know of anything else that is NEC compliant. But I don't know everything. If we're being lawyers.
I too wonder- Is there another way of fitting an upsized wire into a lug that is "too small"?
If you have allowable space- Is a split bolt (or Polaris-type connector) from an upsized wire to a lug-fitting wire size allowable?

 

[wyliesdiesels]

Theres no reason why a split bolt or polaris connector would not work. Just make sure that wire from connector to breaker is rated for breaker rating.

 

[sberry]

Can we find a harder way? Maybe a more expensive one?

 

[b-boy]

Just to clarify, I'm planning on a forced air furnace, a welder, a large compressor, and a few tools (table saw, band saw, etc), as well as the standard 20A outlets and LED lights. At this point I'm open to downsizing the panel if it's cost effective.

I always like having more than I need, but I already spent a lot running the conduit, water, and gas. I don't have a problem spending a few extra bucks, but this is starting to get into some serious money.

 

[sberry]

The air comp is the greedy item and in a small shop there is no rule you cant stop or have to start a weld while its running. Many 5 hp draw less than 25A and modern welders use under 25 screaming wide open. Even together they wont trip while the rest of the stuff is running. You don't need or want to downsize the panel, you want 100A main breaker 20 space. You can make do with a more common economical wire. Even a cheaper breaker, there is a price jump after 60, they are so common and you already have one, it takes number 2 wire. I paid under 1.50 for wire, I have heard as low as 1.17 on this board as best I can recall not all too long ago.
I have 4 of these circuits leaving my building of my own, I put one in for a Bud 30 yrs ago who ran a mechanic shop, 2 men had long wire and a 60, never ever even notice it was wired anything but next to the service. I even stick welder with a buzzer from it time to time and nothing he had was energy friendly. Air on 24/7 on demand.