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I've read many good answers here regarding what types of cables (conductors) can/cannot be buried. I've come across blanket statements like "SER cannot be buried, even in conduit." I want to install a subpanel in my workshop (100' from main to sub) and plan to use 2-2-2-4 Al. A friend did almost the exact same job and used SER type XHHW-2, buried in conduit as recommended by an electrician. Alarms went off in my head because it is SER. However, XHHW-2 can be buried in conduit, so my question is: just because the cable is called SER does that preclude its use outside (i.e. buried in conduit) even if it is labeled as XHHW-2 (i.e. does the XHHW-2 designation change anything)? I'll paste a shot of the cable below to capture all the markings.

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EDIT Just to close the loop on this. I went back to the store and peeled back a bit of insulation and looked at the wires. I saw no markings on the conductors and the ground was not insulted. A non-insulted aluminum ground cannot be buried regardless.

tnknepp
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    Why do you want to pull cable through conduit in the first place? That sounds like a nightmarish pull job... – ThreePhaseEel Oct 05 '20 at 11:43
  • @ThreePhaseEel I take pleasure in pain and suffering! Honestly, I want the added protection of the conduit in case somebody digs (with a shovel) without checking first. That's all. To that end, I bought 2" conduit to ease the job. – tnknepp Oct 05 '20 at 12:10
  • It would be ***very unusual*** in my experience, but by any chance are the individual conductors inside the cable sheath **marked** XHHW-2? – Ecnerwal Oct 05 '20 at 14:34
  • @Ecnerwal I assume they are, but I did not check. I did not purchase the cable, so I did not strip away the outer sheathe sheath. – tnknepp Oct 05 '20 at 14:39
  • You know your #2 Al is only good for a 90A breaker. That is because it is feeder not a whole house service. – Harper - Reinstate Monica Oct 05 '20 at 19:51
  • @Harper-ReinstateMonica I've read different ratings for #2. Whether right or not, "most" references show it rated to 100A. That being said, I plan on running 75A to the workshop. That should be plenty to spare. – tnknepp Oct 06 '20 at 10:17
  • @tnknepp -- 75A isn't a standard breaker size :P – ThreePhaseEel Oct 06 '20 at 11:46
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    @tnknepp "most" references???? *It's not a matter of opinion*. 310.15(B)(16). #2 aluminum, 3 numbers there. By default you use the 60C column, but you almost certainly meet the qualifications to use 75C. That says **90A** final answer.. What "most references" are thinking of is a different rule, 310.15(B)(7), which allows an 83% derate (favorable) for **a whole house's service**. That means 83A cable suffices for a 100A **service**. So #2 is acceptable for a whole house's service, but not for a feeder like yours. – Harper - Reinstate Monica Oct 06 '20 at 16:35
  • @Harper-ReinstateMonica That's a great explanation. Thank you. For my length of cable (100 ft from box to box) can I bumpt this up to 90A, or it is long enough to force me to stay at 70A? – tnknepp Oct 06 '20 at 16:48
  • @tnknepp Depends. In the USA you can breaker to 90A and disregard voltage drop. Over in the People's Republic of Canada, you must "nanny breaker" so you get less than 3.75% voltage drop at breaker trip amps (or to be more precise, 3% at 80% of breaker trip). However, at only 100', that won't matter. I don't even bother checking the voltage drop calc until 180'. – Harper - Reinstate Monica Oct 06 '20 at 17:20

4 Answers4

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SE cables simply can't be buried, conduit or no conduit

Why you're seeing "SER can't be buried" on this site is because the rule's right there in section 338.12(A) of the National Electrical Code (irrelevant point 1 snipped):

338.12 Uses Not Permitted.

(A) Service-Entrance Cable. Service-entrance cable (SE) shall not be used under the following conditions or in the following locations:

...

(2) Underground with or without a raceway

You're better off pulling individual wires instead

Even disregarding the restriction above, you're still better off pulling individual wires through the conduit, simply due to how stiff that cable will be to pull. You'll have to send the cable to the scrapper and get new wire for your pull, though, since the wires inside cables aren't generally marked for usage outside the cable. Furthermore, a 2AWG Al conductor is limited to 90A by NEC Table 310.16 when landed on the 75°C capable terminations found in modern distribution equipment; the 100A figure you may see in other sources is only usable when you can apply the 83% rule for dwelling unit services or feeders found in NEC 310.15(B)(7), which is not the case here since you're feeding an outbuilding. Given that you're putting a fat conduit in, being limited to 90A at the moment isn't too big a deal now since you can upgrade later without needing an excavator, but you'll probably want to go to 1AWG Al so that you can have a full 100A feeder to your workshop subpanel.

GO BIG OR GO HOME

Another common mistake with subpanels is to try to pinch pennies by downsizing the subpanel. In particular, breaker spaces count for far more than amps do; while a 100A or 125A, main breaker panel is fine for most detached outbuildings, it should be a 24 space panel at a minimum in order to have sufficient room for expansion, and a 30 space panel is not at all out of place here; given that this is a workshop, splashing out on a 40 space, 200A panel might even be worthwhile. (The extra main breaker amps in the subpanel don't matter since the subpanel main breaker is simply a convenient shutoff switch in this situation.)

ThreePhaseEel
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  • Thanks for the help. Thankfully I have NOT purchased any wire/cable yet. I'm doing a thorough read on what is needed before I move forward. I'd rather be overly cautious than not (electricity only give you one screw up). I already have a 100A 24-space panel (48 circuits...should I ever decide to torture myself) and I "only" need 8 of those spaces for now (famous last words). – tnknepp Oct 06 '20 at 10:27
  • Couple more things. Can I use mobile-home feeder wire? I've seen that recommended a lot. Also, XHHW-2, THWN are fine for this application, right? I have gotten a lot of contradictory information online and from talking to electricians so it will be nice to get a straight, correct answer. Oddly enough, every electrician (only a couple) I've talked to has been all too happy to help until I mention pulling a permit then people lose interest. – tnknepp Oct 06 '20 at 10:45
  • @tnknepp -- good on you for not buying the wire yet, both THWN and XHHW-2 are suitable here, although the larger sizes of Al are generally most available as the latter; mobile-home feeder could also work, but may be a tougher pull than an individual bundle of wires, and is only available in a limited set of sizes – ThreePhaseEel Oct 06 '20 at 11:46
  • @tnknepp Yes, grats on not buying the cable. Harper's Law: buy the cable *last* :) And nice work on that 24 space panel too. – Harper - Reinstate Monica Oct 06 '20 at 17:14
  • @Harper-ReinstateMonica Thanks. I figured that he ONE thing I'll mess up on is the ONE thing I cannot return: the cable. – tnknepp Oct 06 '20 at 17:19
  • Thanks @ThreePhaseEel. I took your advice on going big. My panel has 24 slots and with everything I can imagine putting in the shop, and then some, I will only use 14 of these slots, max. I have room to grow. Also, I upgraded to #1 Al (XHHW-2) so I can run the full 100 A to the shop. I just got my permit application accepted today with and bought my wire, so I'm pumped up a bit and had to boast. Thanks for the advice. – tnknepp Oct 23 '20 at 19:50
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Buried cable, in or out of conduit, must be rated for "wet location". That's the problem with SER, it's not rated for "wet".

Regardless of what your friend did, use the correct cable. His installation is incorrect in spite of being "advised" by an "electrician".

jwh20
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  • That's what I'm inclined to believe/do. It's the XHHW-2 designation on the cable that has me confused. THAT can be buried, but SER cannot. It seems the label is contradictory. – tnknepp Oct 05 '20 at 11:21
  • XHHW-2 specifies the TYPE OF INSULATION not the construction of the cable. – jwh20 Oct 05 '20 at 11:25
  • Right, but an XHHW-2 insulated wire can be buried in conduit, right? I'm just not understanding why a group of XHHW-2 wires can be individually buried, but once they are wrapped together as a cable and have SER printed on the outer sheathe they cannot. Not arguing here, it just doesn't make sense. – tnknepp Oct 05 '20 at 11:30
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    Again the XHHW-2 specifies the insulation type not the construction of the cable that makes it suitable for different applications. SER is not constructed for underground use. You need a UF or USE cable or THWN/THWN-2 wire if I understand your application properly. You might want to consult your local electrical supply shop as they would be in a better position to get you the right cable for your job considering local code requirements. – jwh20 Oct 05 '20 at 11:43
  • This is very helpful. Thanks. If I run individual XHHW-2 conductors in conduit then I'm OK, right (suspecting you will say no, just looking for clarification). I ask, because XHHW-2 is considered, as I understand it, to be better than THWN-2 for handling moisture and "toughness". – tnknepp Oct 05 '20 at 12:26
  • I think I just got it. The XHHW-2 is rated for burial (in conduit) when run as single conductors. However, when you bundle them into a cable you must assign a new rating to the CABLE, regardless of what's in it. Therefore, if the cable's sheathing is not rated for burial then the cable cannot be buried regardless of what the individual conductor rating it. Is that right? – tnknepp Oct 05 '20 at 12:41
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    If the wire has (and is marked with) a W in the rating, it can be used in a wet location. THWN, THWN-2, XHHW, XHHW-2, RHW, etc. are all fine. All exterior conduit is a wet location. And individual conductors are the right choice in conduit (they pull better/easier, and conduit is far better than a cable jacket for protection.) – Ecnerwal Oct 05 '20 at 14:59
  • @Ecnerwal How then do you resolve the conflict between XHHW-2 and SER. SER says NO to wet locations. I believe SER negates XHHW-2 when both are present. – jwh20 Oct 05 '20 at 15:03
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    I was referring to WIRE, not Cable, since the OP seemed to be moving to using wire, as makes sense. – Ecnerwal Oct 05 '20 at 15:05
  • Your comment above implies that the wire must be rated (specifically) THWN or THWN-2, when it is in fact the case that (for wires) any wet rating is acceptable (unless temperature effects require a particular temperature grade of wet rating. Which is unusual in home project buried applications.) – Ecnerwal Oct 05 '20 at 15:13
  • @Ecnerwal Ok, I think we're on the same page then. I was still in "cable" mode. – jwh20 Oct 05 '20 at 15:38
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    @jwh20 -- SER can be used in wet locations aboveground just fine, it simply can't be buried at all (conduit or not) – ThreePhaseEel Oct 05 '20 at 22:45
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SE cables can't be buried (per NEC 338.12), but the conductor type indicates the suitability for wet, damp, or dry locations, and the temperature rating of the insulation which is needed for the starting point (from table 310.16) when calculating ampacity when applying various derating calculations such as ambient temperature, terminations, and conduit fill.

NoSparksPlease
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I would talk to your AHJ but you might be able to strip off the outer jacket and just pull the XHHW-2 through your underground conduit. I do this frequently, run SER across an attic than skin it out through conduit when it leaves the structure and just pull the inner conductors your conduit with xhhw-2 insulation

JacobHess3N
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  • Once you strip off the outer jacket, you void the UL listing. The OP has already stated that the individual wires are not marked "XHHW" . – JACK Nov 22 '22 at 13:04