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.
I'd say that there are too many unknowns in regards to what your local jurisdiction mandates for burial and separation of utilities for anyone to answer with any certainty in your situation. What might be legal in one town may be illegal in the next town over, so the only sure way of knowing how you can legally and safely bury your utilities is to talk to your local permitting authority. I'm sure they have plenty of documentation for you to look through which should answer all of your questions.
If you just choose to blindly proceed without following guidelines and proper permits and inspections, you will most certainly not get a certificate of occupancy for your garage apartment, and likely you'll need to dig it up and do it again. If you're in the rare situation where there are no permitting authorities, I still suggest following guidelines of a nearby authority for utility burial, just for safety's sake.
If nothing else, for any non-metal lines you run in trenches not in common with a metal line, bury with it a marking wire so that utility locators can easily find your buried utility lines in the future.
Scott
Best Answer
Tesla's website is just an opinion. The controlling document is first NEC, and second the UL-approved instructions that come with the EVSE.
What matters is the ampacity on the (once again) UL approved nameplate on the charger, not some sales literature about what the car is supposed to draw. I suspect the nameplate actually says 39-40A. For any EVSE, this number must be derated 125%, so we arrive at 49-50A.
Your cable will include 2 hots and a neutral. It is a complete mystery to me why an EVSE needs neutral, but the upside of a 14-50 is if you ever get an RV, it can plug in there too.
NEC says a NEMA 14-50 recep can be on either a 40A or 50A breaker. The wire must be large enough for the breaker chosen.
OK, stop and evaluate the existing conduit. It is certainly better to re-use the existing conduit rather than have to lay a second one. In fact, you need to know what's there now, because you are not allowed to have 2 same-voltage circuits to the same outbuilding. If the existing conduit is big enough, it may be better to install a subpanel and pull cable big enough to support the EVSE and also the other garage loads - typically 10A more than the EVSE will suffice.
Worst case let's say the existing conduit is 1/2". And let's say your EVSE's nameplate really is 32A - which must be provisioned to 40A. And let's say you fit a subpanel supporting a 40A EVSE breaker and two 15A breakers for garage circuits. You only need a 50A feeder for that (some oversubscription is allowed), and that's three #8 THHN wires and a bare #10 ground, and those just fit inside a 1/2" conduit.
Obviously if the existing conduit is bigger you have more size freedom. 3/4" = #6 THHN wires (good for 65A). 1" = #3 THHN (good to 100A).
Note that THHN is allowed to run at higher temperature than NM/UF cable (75C vs 60C), and so it's allowed higher ampacity (in NEC 310.15(B)(16)). That's how we're running 50A on #8.
You need 18" of cover. So for 1" PVC you'll need a 20" trench. 1" PVC is overkill, but overkill makes pulling easier.
A real electrical supply house should be able to easily accommodate all your needs. Those are all over the place. You drive past 5 of them on your way to work. You don't see them because they're not in malls, they don't have 1000-space parking lots, they don't run ads on TV or have inserts in the Sunday paper. Their prices are better than big-box, and they'll work with you if you call them (most of them, anyway). They won't give you much help, but (huge upside) they won't give you BAD help like the staff at big-box who really know nothing.
Also electrical supply is cheaper for 90% of stuff, if they will give you trade prices. Some will give novices "list" prices; those guys are being jerks. I like to open the conversation with how I'm fed up with paying Home Depot's high prices :) (which is true and they know it).
Also Home Depot sells #8 and #6 THHN both by the spool and by the foot. You need a white neutral, but both hots can be the same color. If you're trying to cram #8 into 1/2" conduit the ground must be #10 and bare and copper, or it won't fit inside legal fill limits.
If you run #6 THHN you must use a #8 ground, for arcane reasons. #8 ground is good all the way to 100A (#3 copper).
Well, you have better instincts than most. I'm not worried about you doing anything unsafe. But still, "trying to fill knowledge holes using Google". Which doesn't work; Google doesn't tell you what you didn't know to ask. But you're here, so you'll get that if you keep reading and asking.
Harper's Rule: Buy the wire last. You'll learn more that will further educate you; don't pull the trigger too early and buy the wrong thing. Doesn't matter; the feeder goes in AFTER the conduit and panel work is complete and buried. (cannot assemble conduit around wires).
Cable inside conduit is a nightmare. NM cable isn't allowed outdoors (can't handle water) and UF is very flat, so it requires absolutely huge conduit. However THHN requires conduit the entire route panel to panel/socket.
As far as lowering the cost, the magic word is aluminum wire. It is your friend for large feeders like this. The only reason I discussed copper above is because of the desire to cram feeder into existing conduit, which is obviously a huge win if you don't have to trench it lol.
With aluminum, just use 2 sizes larger - #4 instead of #6 etc. If you use #4 or larger wire, it is no longer necessary for neutral to be white, it can be black like the others and mark it with white tape. Ditto ground (green tape).