Can this subpanel's feeder be used for a replacement panel?

Question

As described in my other question about this barn's flooring, there's an electric feeder supplying an out-of-service subpanel and circuits in a barn that is hardly enclosed anymore. I want to know if I can keep using this feeder cable, with all new subpanels and circuits in the barn.

The feeder cable is marked "10-3 WITH GROUND TYPE UF 600V SUNLIGHT RESISTANT E25682F (UL)". That starts at the main panel, connected to a standard 2-pole 30A breaker. It runs at least 170ft from the main panel to the subpanel, leaving the main basement through a hole in the cinderblock wall >2ft below grade. This electric cable emerges out of the soil in a room of the barn.

As for functionality: I tested the cable by turning the breakers on at the main panel, while the subpanel was turned off. The wires from this cable behaved as I'd expect an in-tact cable to, albeit with rust to scrape off terminals on the subpanel: both hots read apx. 120V compared with the neutral and ground, hots read 240V compared with each other, neutral and ground read 0.04V compared with each other, and all voltage readings were stable (did not flicker or change between readings once I got good contact). That seems promising.

As for code: I am in PA, USA. It looks like it is compliant with the code to use UF cable buried 2ft or more, and it is allowable to put UF cable in a conduit of sufficient size.

I would like to keep using this cable, with a few improvements:

• Replace the 30A 2-pole breaker with a GFCI version for added safety

• Detach this cable from the existing subpanel, tape it off, then slide metal conduit over the cable so that within the barn it is safe from abrasion. (I plan to use this room as a small workshop.)

• Replace the subpanel with something and all circuits in the barn. I understand this 10/3 UF cable is sufficient for each wire to carry 30A loads maximum, meaning the barn subpanel can have a maximum of 30A 120V or 240V power supply. I expect to have just (3) 20A 120V circuits: lights, tool outlets used one at a time, and electronics outlets that would at most power a small electric space heater.

Does that sound reasonable? Are there other improvements I should make or ways I should test this, particularly its grounding capabilities, to ensure it is safe for continued use?

Some photos

Here is the cable leaving the basement, just 5-10ft from the main panel. The hole in the cinderblocks has a piece of PVC with which a heavy duty water hose runs through (not connected to anything on either side for now). The white cable runs to the barn's subpanel, exiting the house's basement along the outside of that PVC piece. There's a black cable in this photo that does not go to the barn.

Here's the cable and subpanel in the barn

Here's a close-up of the cable coming up out of the ground in the barn

Answer 1

I think your plan sounds fine. You should dig down as far as practicable; Code actually requires conduit-like protection for the entire stub-up (from 24" burial depth, the curve/sweep, up to exiting the ground) and you might as well carry on all the way to the new panel.

Three 20A circuits are fine. Note that 170' is not a great worry for 240V circuits, but will cause more voltage drop for 120V circuits. The more the circuits are balanced the better it will be. It's a pity; if the person who paid for the 10/3 had known 2-2-2-4 MH feeder was about the same price, they could've had unlimited power out there.

If you're planning to run the space heater regularly, that thing will cost you about a quarter an hour to run, counting repeatedly replacing it (they don't last). You may be better off with one of the modern, "works to low temperature" mini-split heat pumps, some of which are made for DIY. (They use the A/C gage set, but you are testing your line work with inert nitrogen instead of handling freon. The unit ships with extra refrigerant to fill the condenser and linesets.) They run 200-600% the efficiency in heating mode because of heat pump magic. However they also provide A/C, which is undesirable if you're trying to minimize your summer electric bill :) No seriously, they're not bad at all, with far better SEER ratings than clunky old traditional A/C's.

Answer 2

Because this is only a 30A circuit, you can put in a GFCI at the feed and protect everything in the barn. However, I would highly recommend using a regular breaker in the feed panel and instead using GFCI breakers in the subpanel (for 240V circuits or for hardwired 120V circuits (e.g., hardwired lighting) and GFCI receptacles for 120V receptacle circuits (just the first receptacle in each circuit with the balance run from the GFCI-protected load terminals.

There are two advantages of doing it this way:

• 170' is a long way to go if you have a GFCI nuisance trip (hands a bit wet when plugging something in or whatever).
• If a tool or heater has a ground fault and trips the receptacle circuit, the lights (on a separate circuit) stay on.

As far as heating, Harper's advice is correct, as usual. But if you really must use electric resistance heating (giant toaster), get 240V heaters. They are designed to be permanently installed and cost more than the throwaway 120V portable heaters but:

• They last longer, by design
• They can pump out a lot more heat
• Because they run at 240V rather than 120V, they work much better in terms of voltage drop.
• Because they run at 240V rather than 120V, for the same amount of heat generated they impact your feed much less. For example, 8A @ 240V = 1,920W. That's more heat than you can get at 120V (because the standard 120V stuff maxes out at 12A/1500W) and leaves a 22A balance (not counting voltage drop) on each leg instead of 30A on one and only 18A on the other)

They do cost more, but over lifetime usage (i.e., replace less frequently) should do OK. The only big catch is that they are permanently installed - you can't move them around the barn like you can with the 1500W 120V portables.