Installing a 100A panel in detached garage. NEED HELP!

Question

I am a carpenter contractor with a wide knowledge of the trades. I am confident in my abilities to tackle this job, but want to make sure it's done RIGHT! I used to be terrified of electrical work, not so much anymore. I've got a 2 car detached garage that previously had 1 20A breaker running from the main. Powered the lights and 2 outlets. There is already what I believe is a ¾ conduit buried going from the house to the garage. Has 4 12 or 10 gauge wires coming off the main. The main panel I am running everything off of is a 100A panel and going to a 100A panel in the garage.. The main in the house has 1 15A breaker and nothing else.

My question here IS - 1. Can I get away with leaving the ¾ conduit in place and use that. 2. What is the smallest wire I can run to FIT in the ¾ conduit to power the sub panel. 3. Can I use individual THHN between 3 AWG and 8 AWG to run to the pannel. 4. Can I get away with a 60A breaker in the main?

Any help would be appreciated! If this was your house, how would you do this?

Answer 1

The #1 non-safety mistake novices make it chintzing out on the subpanel, and not getting enough spaces for future needs. Then later they want to add a circuit and their panel is full.
24 spaces is not excessive for a craftsman. 240V tools happen, and you want to be able to power them.

The #2 mistake is using copper wire unnecessarily for feeder. Aluminum heavy feeder is fine in most cases (not here though).

Yes, you can use the conduit if it’s fit for use. With THHN wire the conduit route must be complete from end to end, and must be built complete before any wires are pulled in.

The largest wire size you can fit three of in 3/4” conduit is 6 AWG.

• 6 AWG aluminum THWN is good for 50A and can use a #10 copper or #8 aluminum ground.

• 6 AWG copper THWN is good for 65A and can use a #8 copper or #6 aluminum ground. Since 65A breakers are not made, you can breaker at 70A.

You can always use a smaller breaker than required. 60A breakers are cheaper than 70A.

This is one case I would use copper if it's in the budget, because of the limited space in the pipe.

Note that at #6 wire, the wires must be the "right" colors:

• white or gray for neutral
• green or bare for ground
• anything else for the hots. The hots can be the same color.

Answer 2

there's no reason you can't use a 60A supply to run a 200A panel, but you'll be limited to 60A of loads operating at any time.

It looks like 60A is going to need #6 copper or #4 aluminum,

https://www.cerrowire.com/products/resources/tables-calculators/ampacity-charts/

The thinner copper will be easier to pull. and the #4 may not actually fit depending on the type of 3/4 conduit.

https://solutions.borderstates.com/resource/conduit-fill-table/

Answer 3

Generally You can re-use the conduit. The maximum feed through an existing 3/4" sch. 40 PVC conduit using copper THWN is 3@#6 and a #8 ground being fed by a 70A breaker.

Some Details That Can Matter The NEC limits maximum conduit fill for over 2 wires to 40% of cross sectional dimension conduit. Each wire and conduit type have different dimensions, so actual type of conduit and wire is relevant. There are 80 pages of charts in the code book dedicated to determine the maximum number allowed, but it's easier to use an online calculator.

So using the calculator to determine the maximum wire size you could feed through existing 3/4" PVC using THWN conductors you are looking at a 40A to 70A feed. You put in 3/4", Sch80 PVC, you refer to NEC Table 310.16 75°C column see #6 copper is rated for 65A, so you enter 3@#6 and a #8 THWN ground, and oops, 46% fill, too much. Go back up and change to Sch 40 which has same external dimension, but slightly larger internal dimension, and get 37% fill, you are ok. So the actual type of conduit is relevant. Even with Sch 40 if you're not experienced pulling wire it would be a good idea to consider sticking with a #8 feed. Don't even consider trying to pull UF, USE, or any cable assembly through this (or any) conduit, it requires a different calculation that results in larger conduit being needed.

Buried conduit is considered wet location and the rating of THHN is technically not wet rated, but all the THHN wire I have seen on the market is multirated as THWN also, which is wet location rated.

The actual maximum breaker size is dependent on rating of the terminations, modern panels and circuit breakers are rated for 75°C and are higher rating than 60°C terminations found in old equipment like fuse panels. This rating needs to be confirmed.

#6 THWN is 75°C rated for 65A, which isn't a standard breaker size listed in the NEC, so you are allowed to round up to the next size larger breaker. Although your wire may be THWN-2 no terminations are rated for 90°C, that number is provided for when de-rating is required, which doesn't apply to your question.

Normally local jurisdictional authorities will let you add a 40 to 70A breaker into your service panel without much consideration for what's existing, but when you go to upgrade the service panel they will take into account what you have added to he calculation for service size. Any question on Code issue is also subject to adoption or amendments adopted and interpreted by the local authority.

I didn't consider calculating maximum feed with metal conduit as ground, first because I think it unlikely, and second it's hard to verify the conduit is intact and sufficient for use as the equipment ground. I also didn't calculate using aluminum because I was trying to give you the max available without head spinnninng variables.

As others have noted a 40 to 70A feeder supplying a 100A panel is fine since your sub-panel must be rated for at least the size of the breaker feeding the sub-panel. If you do need to replace a panel bigger is usually only a small increase in price and prevents possible need for upgrade later.

Your statement "The main in the house has 1 15A breaker and nothing else" doesn't really seem like it could mean what is seems to mean.