Base Conductor Size
Start out by using Table 310.15(B)(16), and applying any required corrections, to determine what size conductors you'll need. For your situation, we'll assume we can use the 75°C column, that you want to use copper conductors, and there's no other corrections required. So in your case, if you want to install a 50 ampere panel, you'll need at least 8 AWG copper conductors. If you want a 60 ampere panel, you'll need 6 AWG copper conductors.
Voltage Drop
Once you have the base conductor size selected, you'll want to calculate the voltage drop across that size conductors for the length of the feeders. The first step here will be to use Table 8 from chapter 9 of the NEC, to determine the resistance of the conductors you've selected.
In your case, 8 AWG stranded copper wire has a resistance of 0.778 ohms per 1000 ft. 6 AWG stranded copper wire has a resistance of 0.491 ohms per 1000 ft.
Next you'll use the following formula, to calculate the voltage drop across the feeders.
V = L * 2 * R * A
Where:
- V = Voltage Drop
- L = Distance along the wire from one breaker to the next.
- R = Resistance per foot of wire.
- A = Current running through the conductor.
For a 50 ampere circuit, 130 ft. long, using 8 AWG stranded copper conductors, the calculation looks like this...
V = 130' * 2 * 0.000778 * 50 A
V = 260 * 0.000778 * 50 A
V = 0.20228 * 50 A
V = 10.114 V
10.114 V is 4.2% of 240 V. The NEC recommends having a voltage drop less than 3%. To achieve this, you're going to have to use larger conductors.
6 AWG stranded copper conductors have a resistance of 0.000491 ohms per foot, which means the voltage drop would only be 6.383 volts or 2.7%.
For a 60 ampere circuit 130' long, 6 AWG stranded copper conductors would have a voltage drop of 7.6596 volts or 3.2%. While 4 AWG stranded copper would be 4.8048 volts, or 2%.
Conductor Type
Once you know what size conductors you need, you'll have to determine what type of insulation the conductors should have. Since you're burying the conduit, you'll need a wire rated for wet locations. The popular choice in this situation, would be to use THWN wires.
Wire Size
Now that you know what size conductors, and what type of wires you'll use. Then next step is to determine the physical size of the wires, and how much space they'll take up in conduit. For this, you can use Table 5 from chapter 9 of the NEC. There you'll find that 6 AWG THWN wires have an area of 0.0507 square inches, while 4 AWG THWN wires have and area of 0.0824 square inches.
Conduit Fill
Using the size of one wire, you can figure out the area required for all four wires.
0.0507 * 4 = 0.2028 in.sq.
0.0824 * 4 = 0.3296 in.sq.
Use Table 1 from chapter 9 of the NEC, to determine the allowable conduit fill percent. Since you'll have more than 2 conductors, you can fill the conduit to 40%.
Conduit Type
If you know what type of conduit you're using, you can use Table 4 from chapter 9 of the NEC to look up the area fill values for various sizes of conduit.
Conduit Size
Since you've decided to use Schedule 80 PVC, you'll simply find that table in Table 4. Then look down the 40% fill column, until you find an area large enough for all your wires.
In your case four 6 AWG THWN conductors, will require 1" Schedule 80 PVC. While four 4 AWG THWN conductors, will require 1 1/4" Schedule 80 PVC.
Conduit Size Alt.
If you don't feel like calculating wire/conduit area, and all the wires are the same size, you could use Table C.9 from Annex C of the NEC to look up the conduit size required. There you'll find that you can fit five 6 AWG THWN wires throug 1" Schedule 80 PVC, and that you can fit six 4 AWG THWN wires though 1 1/4" Schedule 80 PVC.
tl;dr
- For 130' long 50 ampere feeder, use four 6 AWG stranded copper THWN conductors though 1" Schedule 80 PVC.
- For 130' long 60 ampere feeder, use four 4 AWG stranded copper THWN conductors through 1 1/4" Schedule 80 PVC.
NOTES:
- This answer contains some of the tables used in this answer.
- If you don't feel like doing any maths, you can surely find a calculator online to do all the work for you.
Correct. An Outbuilding needs a main shutoff switch even if it has one breaker. It does not need a main breaker, but it absolutely needs a shutoff switch.
It is unclear whether a "connected by roof" building is an outbuilding or not. Code plainly says it's not an outbuilding and does not need a main switch. Your local inspector is the final word on the subject.
Spend some time in the Eaton price book, and you find out that if you optimize for "cheap" or "compact", the best way to get a main shutoff switch is to get a panel with a big switch that by wild happenstance is a circuit breaker too. We don't care about that, we just need a shutoff switch. We only care that its "circuit breaker" trip value does not unnecessarily limit us. If your supply is 100A, then any main breaker 100A or larger will suffice.
This thing is not a breaker for us, and coordinating the breaker trip is hopeless - expecting this local breaker to trip first for our convenience violates Murphy's Law.
The one special characteristic this main "switch" might have is GFCI -- using an oversize hot tub panel is one way to provide necessary GFCI protection to every garage circuit at once, at the cheapest cost. The problem is, hot tub panels are woefully small, though this is helped by our ability to use double-stuff breakers if AFCI is not needed.
And "small panel" is death to a project like this. A person who brings 100A of service to their garage means to run some 240V loads. Those go through breaker spaces like congressmen through taxpayer dollars. Most loads in a garage need to be GFCI with some AFCI, so these will be full-space breakers.
The upshot is, don't even think of a panel less than 24-space...
Best Answer
When wires get this large, using copper is just throwing money away. 2AWG aluminum is a very appropriate choice for this application. Whatever ooga-booga you may have heard about aluminum, never applied to the large conductors like this, and doesn't apply to the new AA-8000 alloys.
How about EMT steel conduit? Use that, and you don't need to pull a ground wire. The EMT is the ground. (Not only that, it's ground for any other circuit that needs to be grounded back to that panel, in case you are retrofitting grounds in an older home).
If possible, I would use 2" conduit. I know the tables say 1.5" is adequate, but this assumes you have an electrician's truck chock full of pulling tools and I'm guessing you don't. The more oversize you go, the easier the pull, and the less likely you'll need to hire the guy with the truck. And guess what, he won't work with you unless you hire him for the whole job. I managed to hire one just for the pull, but he was a friend of my family, and even he needed to delay the job a day so he could buy another tool he didn't have. Anything you can do to ease the pull is a good idea.
Watch the bends. Every bend makes the pull much harder. When I lay conduit, I allow myself only one 90 degree bend between accessible points (Code allows 4, but you'd need a power winch and a bunch of lube to pull 3x#2AL around 4 of them! Yikes!) Whenever you use a junction box or conduit body at a 90 degree bend, that replaces the bend with an accessible point! Quite a bargain! It also makes the turn sharper and neater. If it were me, on a big one like this, I'd put an access point at every single 90... which would make the pull easy, and could make it workable with smaller pipe. I still wouldn't go smaller than 1-1/2".
Food for thought (i.e. where to use a junction box instead of a conduit body), you can have up to 4 circuits in a single conduit. (literally, 9 conductors, but grounds don't count, and neither do neutrals on split-phase 120/240 circuits and MWBCs, so you've only used 2 of 9.)