I built a garage with guest house over the past year or so and my dad and I did all the electrical work. We have some light electrical experience and we had real electricians that we could call up to bail us out if we got in any real trouble. So, it all turned out fine, but there were a few things I wish I knew when I started out.

The subfeed

The subfeed from the house to the garage was the part that went worst by far. A combination of big, expensive, hard-to-get equipment and a lack of experience really set us up for failure here¹. I’ll maybe write about the entire experience at some point.

The 350 mcm debacle

We ran 350 mcm aluminum since we were going a long distance with 200amps². If I had gotten 3/0 or 4/0 copper instead of the 350mcm, everything would have been better. I no longer care that it is way more expensive.

• All the breakers and off-the-shelf polaris splice connectors usually work up to 250 mcm. I had to hunt for stuff that would fit.
• We were pulling through 3” conduit. It fits, but it’s not easy.
• 4/0 is way easier to bend than 350mcm

	Price $/ft	Weight lb/ft	Voltage drop	Price increase
350 mcm AL	$3.24	.388	3.1%	-
4/0 Copper	$6.96	.731	2.97%	53%
3/0 Copper	$4.29	.603	3.53%	32%
250 mcm AL	$3.11	.286	3.99%	-4%

Bottom line: 250 mcm Al is a no brainer, and I think the smallness of 3/0 may have been worth a 32% price increase (about $800).

A few more hard-fought subfeed lessons

• Southwire provides an excellent tool for calculating what size conductors you need.
• I should have gotten 3 conductors cut to length instead of cutting a single wire in thirds myself. It’s much easier to pull if you can keep it on the spool and in order to get all 3 conductors together we needed to pull it off the spool (and then we didn’t have a good way to put it back on the spool)
• You have to run a ground with a subfeed. We had just run the 3 conductors since the building was grounded on the other side and we were running through plastic conduit. This is technically safe but definitely not allowed and hasn’t been for a long time. Fortunately we had run a pull string along with the conductors and didn’t have too much trouble pulling the ground through.
  • The best idea we had during the entire project (credit to my dad) was to pull a pull line with the conductors.
• Once you’re in the wall, you can use service entrance cable which is basically like giant romex. Way easier to work with than wrestling conductor through conduits that need to snake through walls

General electrical tidbits

• It’s always easier to build conduit up than tear it down
• It’s always easier to cut wire than splice it
• When placing several runs in the same vertical space, use standoffs instead of trying to wedge a bunch of wires next to each other on a 2x4.
• Wire is insanely expensive, it’s worth it to shop around even with shipping for any decently sized job. Wirenco was the best for most of what I needed at the time of my project.

Footnotes

1. We did want to have a real electrician do most of this, but most of the work (if you know what you’re doing) is grunt work and we couldn’t find anybody that was willing to slot us in. So instead I put together a plan with the local electrician and then failed to really understand how to execute it. In retrospect, the plan wasn’t bad, but it needed more experience in the field. ↩

2. I am not sure how I ended up with 350mcm instead of 250 or 300. 250 would have been fine (about a 4% drop). I think there were a few factors at play:

   • My electrician initially thought that the voltage at the main house had probably dropped a bit and therefore thought I should minimize drop to the guest house. However, I measured it at the house and there was barely any drop. I should have adjusted my priors at this point.
   • I did want to minimize voltage drop. Appliances and the EV charger in the garage were all 240v and I wanted them to work as well as possible.

   All that being said, 4% would have been fine and I would have had aluminum conductors that would have fit into off-the-shelf panel hardware. ↩