Upgrading garage fuse box to breaker box using existing 10/2 wire with ground

Question

The sub-panel in my detached-garage is a 4-fuse box that is supplied by two 10/2 cables with ground. The 10/2 feed cables are routed through underground plastic conduit. Each 10awg feeder hot wire is connected to separate 20amp breakers in the main 150 amp service panel. My goal is to upgrade to a larger subpanel in garage using the existing feed cables. (As pulling larger wire through the existing conduit is not an option.) I only need to add one or two 20amp circuits. I believe I'll need to change the two 20amp breakers in the main to a larger two pole breaker and that I'll need to install a ground rod for the garage service, as neutrals and grounds are currently going back to the main service.

Question 1) what size breaker should I install in the main to accommodate the
10awg wire without over doing it? Is 30amp the limit?

Question 2) what size subpanel is appropriate for a 10awg feed?

Answer 1

Given your fairly precisely stated 15/16" OD of the pipe, the only pipe anywhere near that size is EMT "Electrical Metal Tubing". But you say it's plastic. I can't find any plastic pipe near that dimension. Everything is <7/8" or well over 1", nearly 1-1/16". I'll proceed assuming EMT, since my wire reco has a lot of slack.

It also sounds like you consider the 30A an undesired restriction, and would prefer to exceed it. That's just as well, because the cables now in the pipe are totally unworkable, and must be removed. I won't get into a codevio list, because that would be tiring, insult your father's work, and just cheese you off. Suffice it to say, there is no way I can possibly see to make use of the NM cables in the pipe.

You can run up to #6 copper, for 70 amps.

That 3/4" pipe will support up to 6 AWG wire - that is to say, three #6 THWN-2 individual wires, and one #8 bare ground wire. (even though EMT is a valid grounding path, I would not trust it outdoors). That will put you at 29% fill, which is perfectly reasonable for 4 wires in a pipe.

(this works for all pipes except 3/4" schedule 80 PVC; for that, a #8 ground wire won't fit due to conduit fill limitations; the #10 ground limits you to 60A. But then, if it was schedule 80, I supremely doubt you got two #10 cables in there!)

That means you'll be able to support the load you have planned, plus a pretty good sized EVSE, wood shop, whatever will come up for you.

Given the short run of the wire, I don't see any reason to bother with aluminum wire (#6 Al would limit you to 50A) or with shrinking the wire to save a few bucks. Having 70A service at the garage would be sweet.

The 10/2 NM cables have to go

Unfortunately they will be annoyingly difficult to pull. This is what you are up against.

2 x 10-2 inside EMT: 2 x 10-2 inside Sched 80 PVC:

It's actually worse than it looks in the drawing. It'll be bind-up city. This is why oval cables are treated as a round wire of the large dimension.

But steady on; get that wire outta there, and the three #6 will go in happy as a lark. Or smaller wire will obviously be easier still.

As to your questions:

Feed breaker size

What breaker for the existing 10-2 cable? None. That cable can't be supported for a variety of reasons.

What breaker for new 3x#10 THHN wires? 30A. Only. Because of NEC 240.4(D).

What breaker for larger THHN wires? Since you are 100% in conduit, and using THWN-2 wires, look at the 75 degree C column in Table 310.15(B)(16) and round up to the next larger breaker. So 8Cu/6Al = 50A. 6Cu = 65->70A. Larger wires won't fit.

Subpanel size: No place to scrimp!

What size subpanel is appropriate for a 10 AWG feed? Subpanels are about breaker spaces, first and foremost. That is the defining characteristic of a panel. A lot of people decide they want to scrimp on the panel spaces, so they get a smaller panel. This saves them the price ... of a latté. Then later, they have a $1000 problem because the panel is full. So gospel around here is Think Big. Think at least 16-18 space, and a lot more if you reasonably expect to fill that. Spaces are cheap, in fact, they make other things cheaper. Look for combo packs which include a main breaker and some "bonus breakers" to save yourself some money.

The second characteristic is bus amperage / main breaker trip. "Going Big" probably mooted that, as we're way beyond the 30A-bussed or 60A-bussed micro-panels. So if your panel has a 100A main breaker, or a 125A bus, that'll work out just fine. The main breaker is only there to be a disconnect switch (since this is an outbuilding), it's not there to be a breaker, so we don't care what size it is.

Hurry up and pull that permit before NEC 2020 kicks; that'll require an exterior disconnect switch also. If you're in Massachusetts, too late.

"But I want the nearby breaker to be the same size so it'll trip locally instead of at the house!" That trick never works, and you shouldn't even be trying. First, both breakers see exactly the same amount of current, so it would be a 50/50 chance even if they were identical. Second, they're not identical - main breakers tend to have a more forgiving trip curve. Third, if that happens often enough to care, the feeder is too small and must be enlarged. (see why I'm recommending #6? :) Feeder must be at 25% above expected practical load.

Answer 2

First problem: those 10/2 cables ate your conduit fill

Your first problem was that for some reason, you used cables in this conduit run originally, instead of individual wires; since a cable must be treated having a round cross-section for fill purposes due to cable twist, stuffing cables down your conduit is what left you with no room for anything else. To wit: a 3/4" Schedule 80 PVC conduit has a usable fill area of 105mm2 (if it's 3/4" Schedule 40 PVC, or 3/4" EMT, it's bigger inside than the Schedule 80, but not by much), and a 10/2 W/G UF cable, measuring at 11.684mm on its major axis, takes up 107mm2 of fill area, overstuffing the conduit then and there!

Worse yet, if they're NM cables instead of UF (the latter is notable because its jacket is almost always a distinctive light grey), than they have to go anyway, as NM cable is not usable in wet locations, such as the inside of underground conduit (if you don't believe me, see NEC 300.5(B)). Using it there not only violates NEC 334.12(B) point 4, but leads to failures down the line as the paper separator in NM will wick moisture into junction boxes and drip, drip, drip it all over the electrical bits inside.

Once we address that though, this becomes a lot nicer

Once we rip those errant 10/2 cables out, though, this becomes a much nicer job. We can then fit 3 6AWG stranded copper THHN wires with a bare stranded copper 10AWG ground down this conduit, for a total of 103.4mm2 fill; while a tight pull, it's certainly not an impossible one, and much easier than wrangling those terribly miserable cables!

That gives us a nice, healthy 60A feeder for our money here, that we can then protect with a 60A, 2-pole breaker in the main panel. Once that's all straightened out, we can then move onto replacing the subpanel; since this subpanel is in a separate structure, we need a main breaker in the subpanel to serve as a cheap shutoff switch. Furthermore, 60A bussing is far too limiting in subpanel sizing, so we go to the next major size up, which is 100A or 125A. However, amperage is not as large a concern as the number of spaces the panel supports (not "circuits", which is rather a lie): 24 spaces is the bare minimum I would use in this application, with 30 space panels being preferred for this.

Grounding this all

First off, we need to make sure the subpanel has matching ground bars fitted to it, so there's a place to land the ground wires; with that done, we then can pull the bonding screw out of the panel (most likely a green screw, somewhere in the neutral assembly, the labeling or instructions for the panel will say where it is), which separates ground and neutral at the subpanel, as there can only be one neutral-ground bonding point in your system.

Furthermore, in order to return lightning-induced currents back to terra firma, you need to put in a grounding electrode system at the garage, as you allude to with your mention of a ground rod. For a system such as yours, a pair of 8' ground rods, driven 8' apart, and connected to each other and to the panel with 6AWG bare copper wire, will normally do the trick.

TORQUE ALL LUGS TO SPEC

Last but not least, in order to make sure this install doesn't come undone on you the way Greg Biffle's infamous lugnuts came undone on him, you will need to torque all the terminal screws on breakers, neutral/ground bars, and panel lugs to their specification torques using an inch-pound torque screwdriver or torque wrench. This is a requirement of the 2017 NEC in 110.14(D), and also simply a good idea, as mistorque is one of the most common reasons mechanical connections fail in service.