I am running a new line from my main panel on the pole out to a sub panel in my shop. The sub panel will be 100amp. I have a 180' run and have buried 1-1/2" sch40 conduit. I'm planning on using 1/O alum wire, but can't figure out if I need to use "SER" cable ( looks like it comes with a ground) or "URD" and use a additional wire for the ground?

If I use the "URD"... what size & type of wire do I need to use for the ground? Is THHN good enough or do I have to use XHHW ?

  • Neither of those is wires, they are cables. Cables are multiple wires (usually) inside a sheath. They are a royal PITA to pull through conduit, for the same reason four 2x4’s glue-laminated are much stiffer than four 2x4’s unglued. Jul 10, 2020 at 17:22

3 Answers 3


SER cable isn’t legal underground, so cross that off the list.

URD also called Mobile Home feeder, comes in 3-wire or 4-wire, so it’s an option. It’s legal for direct burial underground, but not for use indoors without conduit as if it was Romex or something.

So the real question is, what happens at the ends of the conduit? Does the conduit go straight into the panels at both ends? Or does it just sorta end, and then ??? to get over to the panel? I do not recommend the latter, as that ??? can get rather messy with wires this large.

If it’s conduit all the way

Then don’t use cable. Because the wires are bound to each other, cable is excessively stiff and a real pain to “pull”. You can buy individual wires, either THWN or XHHW. (All THWN is dual-rated THHN). You simply buy three full-size wires for hots and neutral, and one appropriately ground-sized wire for the ground.

Note that URD is simply THHN or XHHW wires glued together. If your conduit is continuous, then gluing the wires together is bad, and works against you. Get the wires individually.

You need #1 Al. Do you need a wire size bump?

Voltage drop is proportional to actual load, and cannot see the number on the breaker handle. Therefore we do not calculate voltage drop based on breaker trip. We calculate it based on the actual, expected, or permitted load on the wires. For instance you’re only allowed to plan for 80A on a 100A circuit, so you’d never, ever calculate voltage drop based on 100A. Never!

At less than 180’, I don’t even bother crunching the numbers to see if a wire size bump is worthwhile. It’s not. So you’re right on the cusp. Anyway, we don’t do wire size bumps for distance. We do them for distance and load. So let me ask - how much thinking did you do about loads?

  • Did you sort-of “arm-wave it” like “meh, everybody gets 100A, it’s a common size”? Or
  • was there a gory-details load calc like “47A compressor, 23A plasma cutter, blah blah, need to provision 93A” sort of thing?

In short, if it’s the second one, and you’re close, I’d do the bump to 1/0 Al wire. Otherwise fuggedaboutit, just stick with #1 Al.

If you bump hots, bump ground too.. #1 Al maxes out at 100A, and that means a #8Cu / #6Al ground is sufficient. However, if you do a wire size bump to 1/0, that maxes out at 125A, and that means you MUST have a #6Cu/#4Al ground. When you bump the hots, bumping the ground also is mandatory per Code. That’s because ground wires have voltage drop too, and theirs is worse, because they only carry fault current!

Don’t “Nanny Breaker” yourself. If you decide on a wire size bump to 1/0 AL, you might as well use the 125A breaker that you are entitled to. That way, you can the entire 125A of your feeder on the occasions when you do need it and are willing to “endure” the “scary, terrible” 3.7% voltage drop, woo woo. You don’t need a “100A nanny breaker” to protect yourself from voltage drop. Unless you are in Canada, because they say you do.

Did we mention BIG panel? Buying extra spaces at install time is extremely cheap. Adding spaces after you run out is extremely expensive. As such, we really like to see people splurge on excessively sized panels (in terms of numbers of spaces). If a 125A feed breaker seems like a buzzkill because your panel is only 100A, that implies a too-small panel. Take it out and swap it now, while it’s still super easy to do. That’s one mistake NOT to double-down on.

  • If the 100 amp breaker is rated 100% like square d you could rate that at 100 amps , but it would be rare to use that much.
    – Ed Beal
    Jul 10, 2020 at 18:44
  • @EdBeal -- no, 100%-rated breakers are only really seen in industrial work Jul 10, 2020 at 23:39

Much of what is sold as "THHN" is actually "XHHW2", but yes you need "XHHW" or "XHHW2" if it's going under the ground.

You'll need both a neutral and a ground conductior in addition to the phase conductors, so a service entrance cable is unsuited. you can use URD or SER (so long as you have enough conductors) but they will be harder to pull as they are designed for direct burial and so have a thicker sheath.

my research suggests that #6 aluminium is sufficient for your ground.

see also this question: What wire size for 100 Amp sub panel 100ft in length form 200 Amp main panel?

  • You recommend #6 AL without any explanation of this choice, while another answer recommends 1/0 AL or even 2/0 AL. That's a VERY wide difference of OPINION!
    – jwh20
    Jul 10, 2020 at 11:36
  • 2
    @jwh20 "#6 aluminium is sufficient for your ground", and "#6 AL ground would be legal" is not a VERY wide difference. Jul 10, 2020 at 12:59

If the cable is SER and not also USE or URD then it is not permitted underground. THHN is not wet location rated, underground is wet location even in conduit, but most THHN cable is also rated THWN, which is wet location rated. THWN or XHHW would be fine in conduit for all the circuit conductors. If you use a cable from the panel in 10' or longer conduit then the code requires the cable be secured in the cabinet, but a cabinet or connector Listed (UL or other NRTL) for that purpose seems to be as elusive as Bigfoot.

1/0 conductors and #6 AL ground would be legal in the US, but at that length the resistance of the wire may cause some voltage loss if operating at full load. 2/0 AL and #4 ground would fit in your 1.5" pvc and would alleviate the voltage loss issue.

  • You recommend 1/0 AL or even 2/0 AL without any explanation or references, while another answer recommends #6. That's a VERY wide difference of OPINION!
    – jwh20
    Jul 10, 2020 at 11:35
  • @jwh20 The OP already stated in his answer that he planned on using 1/0 aluminum for the energized conductors,it appears he already researched the minimum size required, a reference for his research would be redundant. I will add the cause of voltage loss in my recommendation to upsize the wires. Jul 10, 2020 at 14:14
  • Almost all thhn is dual rated thhn -thwn, the thhn I got yesterday south wire brand was thhn, thwn-2, MTW and possibly one other. I have not seen a single rated thhn for 20 years. But I agree SER is not allowed underground.
    – Ed Beal
    Jul 10, 2020 at 14:21
  • @EdBeal I understand probably all THHN is now THWN, but it's a lose/lose situation, if I say THHN is OK or not OK I still get kicked in the nads. Jul 10, 2020 at 15:00

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