Home Forums Hunting & Shooting Hunter's Campfire Electrician Question

I would talk to someone at your local utility or wholesale
house with some electrical engineering smarts. Give him your info about distance and amperage requirements and they should be able to give you a wire size. Nothing wrong with aluminum as long as it's sized accordingly.

I did the same thing in my shop, about the same distance. #3 will work for 100A. Copper individual wires in conduit. I don’t have the charts in front of me, the NEC codebook has the appropriate charts. You may want to size up 1 or 2 sizes due to length

My shop is roughly the same difference from the house BUT is down by the road where the service line is at. Full ampacity available with a separate meter and able to add anything (within reason) is nice. Cry once.....

You have 200A in the house but will pulling from that cause a problem in the house? Electric furnace? A/C? All electric? Our house is all electric with 200A. I pulled 100A to a garage I built, then another 100 to a shop for my wife. She does art glass and runs 3 kilns, all at the same time on occasion. In the garage, I do some welding. Of course, not nearly everything runs at the same time. That's the big issue - what might the total load be at any 1 time? While you're welding, will you have clothes in the dryer and a roast in the oven? We've never had a problem yet.

2/0 Aluminum URD

Check w/Utility for construction cost of new Service & go from there

Another consideration...Generator (either existing or future)
IF you need/ want backup power to shop bdg.

When I would install power to a customer's shop or other location, I would consider the load calculation before anything else is done- just like designing the service for the home to begin with.

As long as you aren't maxing out your existing 200 Amp panel at the moment (or using more than 50% at any one time) , you can run a 100 amp feeder to the shop without consequence. But the fact is, 90% of people will never use more than 50 Amps @ 240V in their shop at any one time even using a heavy use tool like a welder. You're only going to be able to use one or two tools at any one time, along with lights and possible a small heat source, so your load will be minimal at any one time.

To your question- 100 amp feed can easily be fed by #3 THHN/THWN . Voltage drop calculations include load estimates along with distance and the fact is you will seldom pull enough load for load drop to be a factor. If you would feel more comfortable #2 would be more than sufficient. For conduit for these conductors a 1" will handle 3- #3 conductors. Might want to jump up to 1 1/4" PVC for #2. Be sure to have a ground rod at the shop building and isolate the neutral conductor from the grounding/bonding bar at the panel as this will be a subpanel. It is also legal to pull a separate ground conductor with the feeders but I have found this to not be effective in breaker function at these distances.

Aluminum would also be fine as a feeder, but you would need to bump up the conduit size to accommodate the larger sized conductors. The cost difference will basically offset the difference in wire cost in the end.

^^^This^^^
When I was an electrician, many folks had no idea of "non-concurrent loads" and overbuilt as a result.
For that length of run, however, I would go one size conduit larger - for ease of pull.
Not sure, anymore, though - I've been out of the loop for 30 years (damn - that's 10 code changes!)

There are several ways to approach this. So...I'll comment on the specific parameters you provided.

a. You have a 200 amp service at your house

b. You want to feed a 100 amp service at your garage

c. Your garage is 275 feet away


As others have stated - voltage drop is a determining factor. However, under your paramaters - you will be running a "feeder circuit" to your garage - not a branch circuit. Your allowable voltage drop is 2% NOT 3% as someone has stated.

The NEC puts it this way: Voltage drop on a branch circuit is 3% with a voltage drop of 5% overall.

A branch circuit is defined as the conductors "from the last overcurrent device to the load." Each circuit "inside" your garage is a branch circuit. The circuit running from your house to the garage is a "feeder circuit."

5% overall voltge drop: 3% for the branch + 2% for the feeder = 5%

If you are running a 240V Feeder to your garage, 2% voltage drop is...4.8V

Here is a simplified equation for figuring voltage dorp: I x R x L

Where: I = Current R = Resistance L = Feet of Wire Note: A circuit has two conductors - if the load is 275 feet away - you will use 550 Feet of Wire

The Details

I would not use conduit. A 275 Foot pull is VERY difficult.

I would use "triplex" direct burrial cable - and I would opt for Aluminum

Your trench must be a min of 2 feet to the top of the cable...so be safe...trench 30 inches deep.

You must protect direct burial cable to a min distance of 18" below the ground at each end where the cable emerges from the trench. No need to protect direct burial cable in-between.


Caculation

Note: The Min Size Alu Conductor For 100 amps is: #1 However, because of voltage drop, your actual wire will be larger.

#1 100 x 0.000829 x 550 = 45.6 Volts Dropped - Can't use #1

2/0 100 x 0.000523 x 550 = 28.8 Volts Dropped - Can't use 2/0

250 kcmil 100 x 0.0002778 x 550 = 15.3 Volts Dropped - Can't use 250 kcmil

500 kcmil 100 x 0.0001391 x 550 = 7.69 Volts Dropped - Can't use 500 kcmil

What's the point?

Under the parameters you stated - You Cannot Size Your Wire Large Enough To Satisify a 2% (4.8 V) Voltage Drop


I would get with your utility company. Explain what you want to do. And let them provide you with options.

Looking from where I sit - a separate service at your garage is the only real option.

A licensed Electrican from your local area may have different/better advice for you.

Respectifully,
GB

Don’t forget to add the length of service from transformer to house into the equation. If existing service to house is pretty long adding another 275 feet might cause too much voltage drop.

THIS ^^^^

You have to meet code minimum. Remember if there ever would be a fire do to wiring issues and you not doing it right can mean a denial of claim by insurance company. May cost more $ upfront but doing it right never hurts. This is no place to look cheap.

These posts are exactly why you need to understand voltage drop and load calculations. Using the feeder breaker size for your voltage drop calculation is not necessary nor accurate. A load calculation of what loads you would be using at any time would probably come up with about 30 Amps, including lights and using your largest tool at any one time- the welder. Once you have these parameters it makes a lot more sense to use wire suited for the calculated load and voltage drop. As a matter or course, as I said before 99% of guys who have a shop could easily get by with a 50-60 amp feeder to the shop panel and never have a problem. About the biggest load most shops have at any time is a welder or air compressor and they seldom pull more than 30 amps on the feed unless you are running a very large professional machine. Using these parameters you could easily install a 60 Amp feeder and pull the appropriate feeder for the load and distance.

How you feed underground is up to you- direct burial cable or PVC conduit is all a matter of preference. Follow code requirements for depth of burial, which is usually 24" under a non drivable surface- 36" under a drivable surface like a gravel driveway . 275 feet is a pretty good pull for most guys, but upsize one conduit size and it should be doable with a bit of help. Vacuuming in the pull line will probably be harder than the pull itself.

About the best suggestion so far is to get a local electrician to come out and tell you what you need and follow his advice. Even if you pay him a few bucks for an hour or two of his time it will be money well spent.

If one is going to guesstimate how much load they are going to use at one time - it is important that the feeder overcurrent protective device be sized accordingly.

So, if one estimates their load will be no more that 50A and one sizes the wire for 50A - then one must install a breaker rated 50A.

The Feeder OCPD limits the load.

A conductor sized for 50A must not be connected to a breaker rated 100A (in general). The breaker will pass 100A. It cares not wither or not the conductors are rated for.

Several guys providing very good information

Thank you

I didn't consider direct-burial cable, in spite of having installed miles of it. After I moved back into this area, I used conduit exclusively, due to pocket gophers digging into, and chewing into underground cables - I simply won't do it. Your situation very likely is much different.

If you go with the 2/0 copper thhn check out this price compared to what you listed, its where I buy most of my wire for my business.

https://www.wireandcableyourway.com/search/?q=2%2F0+stranded+thhn





The voltage drop calculator does show the 2/0 as needed for the voltage drop. You also can consider direct burial URD aluminum

I can't speak for the OP, but provided I purchased and installed the pole and box, PG&E only wanted $17K for a 100A ag panel install. My pole to be no more than 85' from the pole where they'd hang the transformer.


$17k buys one heck of a lot of gasoline for my 240v inverter...

I’m an electrician. Sheister is correct. Voltage drop is your enemy here more than load. Definitely get with your local power company to have a separate service installed. Preferably with a transformer near the garage.

Underground mine electrician here, if you’re anticipating needing 100 amp load being fed from your homes existing service you should have another drop installed with its own meter. That 100 amps being pulled thru your homes breaker box could cause problems in the home.

That's what I ran out to my shop, typical trailer run stuff.

As far as conduit, depends on code I guess, I believe ours is 18in deep you have to run conduit, 24in or more you don't, something like that.

I had to get creative to add that 100amp breaker to my box inside the house, took some moving around. I'd imagine most 200amp boxes in older houses are pretty much full. I was never drawing more than 35 amps at a time out in the shop. Being the length and the tools he wants to run, I'd definitely entertain the new service idea. You don't want to worry about what you are running the house and out in the shop at the same time.

I agree with GeorgiaBoy except that I would run the triplex URD in conduit - 1/0 AL should be enough. True, the URD is made for direct bury but could be a nightmare for maintenance if you plan to have it over the long term. URD direct buried will fail - maybe not for many years or maybe in a few years - depending if you "knick" the insulation at installation, soil type, rocks, etc. Conduit is little more expense and a little more forgiving.

FYI - I have a ranch camp that is 620' from the transformer. The underground cable is direct buried 1/0 alum. My voltage fluctuates between 119/238 and 121/241 under load. It serves a 220 AC window unit, freezer, refrigerator, elect 40 gal hot water heater, and of course inside and outside lights. I did get the power company to change the bandwidth on a set of distribution regulators to where the secondary voltage at the transformer averages 126/252. That is at the high end of their tariff requirements.

Not suggesting someone do what I had to do but just saying 275' is very doable with out building a substation. You can do it with no problem without adding another meter.