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I am adding a home run from the main panel to the garage.

On the outside of the house, it will be pvc.

Where I come through the exterior wall to the main panel, I'd like to use metal flex, for simplicity of installation.

The main panel is in an unfinished basement.

Is it OK to switch from pvc to metal flex inside the unfinished basement?

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    If you need flex for simplicity, why not use non-metallic flex? – Tyson May 26 at 13:32
  • I take it this metal flexible conduit is running all the way from the PVC/flex coupling to the panel? Also, were you planning to use FMC (greenfield) or LFMC (liquidtight), and what wires are you trying to fit through it? – ThreePhaseEel May 26 at 14:11
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It can be done, however you will need to:

  1. Ensure all sections of metal conduit are bonded to ground. If you are going all the way to the panel with the metal flex, the terminating coupling at the panel should take care of it.

  2. Use appropriate methods to transition between conduit types (e.g. transitioning via a junction box).

  3. Adhere to conduit fill rules for both types of conduit. You may need to up-size the trade size for flexible conduit relative to rigid conduit such as PVC based on the conduit fill rules in the NEC. Generally fills in flexible conduit are a little more restrictive than rigid conduit.

  4. Possibly transition back to rigid conduit at different points or otherwise build enclosures to protect the flexible conduit because no type of flexible cable is subject to physical damage. While "subject to physical damage" is not defined by the NEC and therefore subject to inspector interpretation, anything not protected by a protrusion (e.g. wooden guard strips on either side of the cable assembly / conduit which stick out further than the cable/ conduit) at a height less than 8 ft is commonly considered subject to physical damage. So you may need to add protection for the flex conduit at specific points--minimally at the entrance to the panel. Here is the "subject to physical damage clause" specific to flexible metal conduit, but similar clauses exist for all flexible cable assemblies and conduit:

From NFPA-70:2014 (2014 NEC):

ARTICLE 348 Flexible Metal Conduit: Type FMC

348.12 Uses Not Permitted. FMC shall not be used in the following:

(7) Where subject to physical damage

One additional note: Anywhere your PVC is subject to physical damage, it needs to be schedule 80 PVC. Schedule 40 is fine for underground use, and around me the big box stores only carry schedule 40 electrically-rated PVC, so you may have to go direct to an electrical supply house.

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    Thanks! Must the transition be via a junction box, or can I simply put a threaded adapter on to a pull 90, and attach metal flex to that? (this would be indoors, in the unfinished basement) – samsmith May 26 at 15:12
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    It would need to be a fitting listed for use to transition between the two types of conduit. A fitting may exist in the size you need, but I have never looked for one myself and would imagine this isn't a very common transition, so it would probably be a more expensive specialty fitting if it exists. A junction box and fittings for both types of conduit are pretty common and therefore likely to be the cheaper and more readily available option. I would still recommend doing a quick search though because you might get lucky. – statueuphemism May 26 at 15:27
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So long as the metal flex is bonded to ground (presumably at the panel you are going to) this is fine.

Where mixing PVC and metal gets difficult is when you have to manage to ground an otherwise isolated piece of metallic raceway - which can be done, it's just an extra hassle.

  • Correct, the metal flex will be grounded via direct attachment to the main panel. – samsmith May 26 at 15:13

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