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Here's my situation:

  • Meter base is ~115' (conduit length) from my new 200 amp panel
  • 2 1/2" conduit is run end to end under ground
  • the last ~15' is in a crawl space and pops up directly into my panel
  • I have a ufer ground that also pops up into my panel.
  • I'm in Washington state and up against 2020 NEC

Can/should I use URD? Or should I use 3 THHN/THWN conductors? And why? Do the jackets need to be different colors? Planning on 4/0 Aluminum conductors.

Thank you for the assist!

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    Conduit is complete end-to end? If not (i.e. it ends in crawlspace) just complete the conduit so it DOES go all the way end to end and use XHHW as Eddie suggests (the more common insulation type at that wire size .vs. THWN, and also arguably better, tougher, more flexible.) Pulling cable in conduit sucks, and is pointless pain to add when there is conduit. If you need to add 15 feet of conduit, that's much less pain than pulling cable in conduit... At 4AWG and larger, jackets can be and usually are only black, mark the neutral with heat shrink (or tape, but heat shrink tubing won't fall off.)
    – Ecnerwal
    May 12 at 12:23
  • I haven't finished the conduit in my crawlspace yet. Thanks for the tip on marking the neutral! May 12 at 17:02
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    Just make sure you have enough access points (LBs or junction boxes, typically, though other things can count) so that the run is pullable (360 degrees max turns between pulling points, 270 is better to aim for if practical.)
    – Ecnerwal
    May 12 at 17:04
  • oh! that makes so much sense. I wondered by the LBs. Thank you! May 12 at 17:57

1 Answer 1

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I would go one size up in Aluminium, to 250 kcmil. 4/0 Aluminium is technically under the 200A at 75ºC in the ampacity chart. There may be a carve-out in the NEC for service feeders specifically but I'd still use the slightly bigger wire.

There's no reason to use URD cable as far as I can tell, since you have conduit.

Consider XHHW or RHW-2 wire instead.

You need to ensure the meter base and panel are bonded so that the base is grounded as well. There are a few ways this can happen-- you can have the bond via the neutral if the meter base doesn't have an insulated neutral. If the meter base has an insulated neutral, and your conduit is metal then you may simply need a bonding bushing in your panel to bond the panel to the meter base via the conduit. If you have PVC conduit and an insulated neutral in the meter base then you'll need to run an additional grounding conductor between the meter base and the panel.

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  • I'm still confused as to why many places show 4/0 Aluminum XHHW max amperage equal to 225. May 12 at 17:09
  • @jacobdubail Feel free to link to a place where it shows that. May 12 at 17:14
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    Not per NEC tables, which is what you have to go by, and the 75 C column at that (because the terminals are only rated for 75 C.) There is the "service feeder exception" Eddie mentions getting it to 200, but I'd tend to agree with ignoring that exception and getting the 250 kcmil (or two of those and a smaller neutral as permitted if the various conditions specified are met.) Here's one version of the table: usawire-cable.com/pdfs/nec%20ampacities.pdf
    – Ecnerwal
    May 12 at 17:28
  • The big box stores list 4/0 XHHW at 225 amps, for some reason: lowes.com/pd/4-0-Black-Stranded-AL-XHHW-Wire/5004314225, but that's max amperage :shrug: I appreciate the link to the NEC table. That's helpful. Now to find 250 mcm/kcmil wire at an affordable price ;) May 12 at 18:05
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    @jacobdubail I don’t know why a big box store would do that but you have to use the ampacity charts as written in the code book. Bottom line is that the code sets the rules and the code sets the ampacities. It’s also important to note that the code does leave room for the manufacturers spec to take precedence BUT a big box store is a retailer. The actual Southwire ampacity chart shows the 4/0 wire at 180A in the 75° rating. assets.southwire.com/ImConvServlet/imconv/… don’t trust the retailer. May 12 at 18:11

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