I am replacing my 55 year old, 100 amp, 12 space outdoor service entrance panel with a new 20 space 200 amp outdoor panel. Since the new panel is 12" above the rim joist, I was thinking of using PVC emt elbows to get the branch circuits into the box. Code says I can run romex in conduit for short sections for protection.

How much NM cable for what size conduit (I have a 2" knockout and 9 @ 3/4" knockouts). And what about strain relief when the cable enters the panel?

This is what I'm trying to do. current (ha) box

The new combo panel with underground service will be just to the left of this panel. There is a 6-3 cloth covered romex (soon to be replaced) in the large conduit. How do I get the other romex circuits into the new box?

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    So your main service breaker panel is outside?? How are the circuits run now?? Also, I hope you are not just replacing the panel. To go from 100A to 200A you need to do a COMPLETE service upgrade. And an outdoor panel DOES NOT make for an easy DIY job. Sep 7, 2015 at 18:15
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    It depends on what size conductors you're using. 12 AWG nonmetallic sheathed cable will be larger than 14 AWG NM cable. There's also a difference between x/2 cable, and x/3 cable. To get an accurate answer, you'll have to tell us how many and what size cables you're working with.
    – Tester101
    Sep 7, 2015 at 18:28
  • I guess if what you have now is acceptable to the LAHJ <eek!> you can probably just connect it any old which way, While it would certainly be an upgrade to get that into conduit rather than completely exposed to the weather, exterior conduit is a wet location, and NM is not rated for wet.
    – Ecnerwal
    Sep 9, 2015 at 11:52
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    How did this even pass a home inspection when buying/selling? This looks like EC&M's "Code Violation Quiz"... Sep 9, 2015 at 13:16
  • I know 20 spaces may feel like an upgrade, but you'll want way more than 20 spaces unless you plan to feed a number of subpanels. I would put a main shutoff switch here and run power in that hole, then have a 42 or 60 space panel inside the house. Apr 23, 2017 at 21:19

1 Answer 1


For conduit fill calculations you have to treat romex/NM as a ROUND wire of the same size as its largest dimension (ie, the long axis of the oval.) Oddly enough this means that 14-2 is larger than 14-3 for conduit fill. As @SpeedyPetey notes nipple (less than 24") fill is 60%, not 40%, and is a LOT of wire to get into a conduit; but I don't think that's the major issue here if NEC compliance is a desired upgrade. The wire type and location is the main issue.

NM cannot be run in exterior/underground conduit since it is not rated for wet locations. With an outside service box that alone might mean you can't do this and meet code - at which point you'll need a major junction box or a lot of minor junction boxes inside, and some wet-location-rated wire until you are inside.

(And now that I've seen the pictures...)

My understanding of NEC (which does not seem to have been particularly involved in your current service, with exposed NM running outside) is that you would have to run a wet-rated wire (THW, XHHW, etc.) until you get inside the house, where you would connect to the interior type wire you currently have exposed to weather and sunlight. NEC defines (and reality frequently confirms) all exterior conduit as a wet location.

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    Conductor fill for short sections is not the same as regular conduit fill. Under 24" you'd be hard pressed to overfill a sleeve or nipple. Sep 7, 2015 at 23:32
  • The problem is he can't get from the panel to the hole that way. He will need some sort of outdoor friendly raceway... Conduit comes to mind... And now it will be more than 24 inches. Apr 23, 2017 at 21:22

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