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I currently have a 50 amp sub panel in my guest house. I need to upgrade to a 100 amp panel. The main panel is a 200 amp panel about 300 feet away from the sub panel. The electrician told me I need #4 wires from the main to the sub. He also quoted me $6500. So, I am gonna try to do it myself because that seems like a lot of money for a sub. The wiring I have looked at has 4 wires inside the insulation. Is this the wiring I need to run from the main to the sub? Also, the wiring seems to be about $7.75.ft at Lowes. I want to make sure I am buying the correct wiring for the job. Thanks!!!

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    Smells like you are looking at copper wire, which is a foolish expense. Aluminum (a size or two larger) will carry the current and costs a lot less - and has never been a problem for feeders - only a combination of bad choice of alloy and fixture connections that did not work well caused all the bad press you have heard about aluminum wire in houses (on 15 & 20A branch circuits.) Put in conduit unless you are running overhead, and get aluminum wires, not a cable. You need 4 wires; hot, hot, neutral ground, #4 AWG is too small (if pulling 100 amps) even in copper.
    – Ecnerwal
    Jul 8, 2021 at 17:58
  • Key questions: 1 - How much total power do you expect to need at any one time (e.g., you won't need heat & a/c at the same time) in the guest house? 2 - What kind of wire/cable/conduit do you currently have? Jul 8, 2021 at 18:00
  • @Ecnerwal Definitely. 2-2-2-4 aluminum at Lowes (which is not necessarily the best choice, but is quoted by OP) is currently $1.50 per foot SER wet-rated. So $7.75 must be copper. Jul 8, 2021 at 18:01
  • Are you in an area that prohibits aluminum wiring? (Some old local codes haven't gotten the memo) Jul 8, 2021 at 23:50

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Your actual load would be needed to give a better answer but @300’ 75 amp draw 2 awg aluminum 5.5% voltage drop would be my recommendation if you are going to need 100Amp go with 1/0 aluminum @5.2% voltage drop, copper at that size and distance is crazy expensive and not needed for a feeder. I would use pvc conduit it saves 6” of digging depth over direct burial a 1000’ spool for the 2 hot’s and neutral then a smaller ground is how most electricians will pull it.

As far as the voltage drop code only recommends 3% & 5% in a case like yours you will probably not ever see more than 2-3% because you would have to be drawing the full amount to see that large of a drop on either wire size.

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    DEFINITELY CONDUIT
    – JACK
    Jul 8, 2021 at 18:59
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I need to upgrade to a 100 amp panel. The main panel is a 200 amp panel about 300 feet away from the sub panel. The electrician told me I need #4 wires from the main to the sub.

He's wrong on three counts.

First, using COPPER for a heavy long feeder isn't even stupid. Once you get to about 30A it's time to investigate whether aluminum is a better choice. (30A copper wire is about the same price as 90A aluminum wire). At 100A it's a no-brainer.

Second, #4 COPPER is only good to 85A not 100A. He is misunderstanding NEC 310.15(B)(7). It doesn't apply to everything with "house" at the end of the name, only things which are legal and proper domiciles (ADUs) equipped and permitted for people to actually live there - bedroom, competent kitchen with range, etc. If it's not a legal domicile, minimum size for 100A is #3Cu or #1Al. If it is a legal domicile, #2Al (normally 90A) will suffice.

Third, that size completely ignores voltage drop which will be considerable on a 300' cable run.

I wouldn't call this guy back. That seems like the "I don't wanna do it" price or "I got more customers than time so I'll charge markups on everything" price. I'm getting that a lot from A/C contractors these days lol.

Also, the wiring seems to be about $7.75.ft at Lowes. I want to make sure I am buying the correct wiring for the job.

If it costs more than 1/3 that for all 4 wires, it's the wrong stuff for the job.

There's nothing wrong with Ed Beal's wire sizes. I'm pretty bold with allowing voltage drop, Ed's a bit bolder -- but if Ed recommends it, it's a good choice.

You really need to compute voltage drop on your actual/expected loads, not breaker trip. (In fact you're not allowed to plan to use more than 80% of breaker trip).

Honestly if cost were no object I'd go with 2/0 aluminum. It's a commodity size, and will reduce voltage drop to nothin' since it's 2 sizes larger than the legal minimum.

As far as burial method, it must have 24" of cover if direct buried. However direct burial has some issues - the cable can get penetrated by rocks as the ground settles (or is tamped down by vehicles).

There's a very strong case to be made for conduit from panel to panel. I would run 2" conduit since you can pull in #2-1AL easy enough, and a pro could pull in clear up to 250 kcmil if needed. Run a second 1" conduit for datacomm - it's not allowed in power conduits unless it's non-conductive fiber-optic, which AT&T is doing these days. Once you're in a full-run conduit, you get to use THWN or XHHW wire, which is easier to handle. You can buy all 1 color of wire and mark it with tape.

If you use up to #2Al, a #8 bare copper ground (or #6 insulated Al ground) will suffice. If you bump for voltage drop, you'll need #6 copper or #4Al ground.

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