THHN vs Romex behind drywall - why the difference

[timoct22]

THHN is supposed to be enclosed in conduit when behind drywall. Romex does not have the same requirement. Romex is "basically" THHN encased by a secondary sheath. Does this sheath really make that much difference such that THHN must be in conduit but Romex does not? Does that thin layer of plastic really make that big of adifference?
Context for the question: I am connecting a manual transfer switch to a subpanel. In order to minimize drywall work, I am mounting the transfer switch to the surface of the drywall. The subpanel is flush mounted (even with the surface) to the drywall. I am running the wires from the transfer switch through MC to a square junction box to provide a more 'finished' look. My plan was to run the wires through the back of the junction box the six inches to the subpanel inside the drywall without any conduit. If it were Romex, it would be allowed. THHN needs a conduit. Doesn't make sense to me.

 

[Dustydazzler]

THHN is what we call Singles , which has to be put in something like a conduit. There is one exception and that is some singles come with double insulation ( 2 layers ) which can be run outside of a conduit like from the electric meter to the fuse box.

 

[Lucien Nunes]

Your situation, with a six-inch run of cable, is not a typical example of cable installation inside drywall. Normally, a length of 12-2 snaking around from outlet to outlet is exposed to extra mechanical damage, e.g. at staples, by carpenters' tools, by pulling through awkward holes etc. The sheath of NM offers a buffer that can afford to be worn down a little, without compromising the insulation itself. If the sheath becomes completely penetrated or ripped, any non-electrician can see that there is a problem and the cable needs replacing or repairing, even while the insulation remains safe.

In contrast, as soon as THHN is subjected to wear and tear, the insulation starts to be compromised. It is intended to withstand installation in properly executed, code-compliant conduit or raceway but no more. Leave it unenclosed at the mercy of the builders, and who gets to say when the amount of damage to the insulation is 'too much' to be safe?

It's not practical within an electrical code such as the NEC, to grant exemptions to every possible situation where the risk of damage is lower than normal, such as yours. Hence for every type of cable there has to be a simple and consistent set of rules about what conditions it can be installed in.

If you cut a hole in the sheetrock behind where the J-Box is going, can you not aim some FMC with a connector already attached, down into the hole in the panel?

 

[pc1966]

In terms of USA regulations (the NEC "code") I would always defer to one of the American members of this forum such as @Megawatt

However, based on the basic idea of what you are doing, that of main supplies with a very high fault energy present, I would always prefer the cables to be in conduit, trunking, or other metal (i.e. non inflammable and hard to accidentally penetrate) containment.

 

[timoct22]

Thank you for the thoughtful responses. Much appreciated.

 

[Megawatt]

THHN is supposed to be enclosed in conduit when behind drywall. Romex does not have the same requirement. Romex is "basically" THHN encased by a secondary sheath. Does this sheath really make that much difference such that THHN must be in conduit but Romex does not? Does that thin layer of plastic really make that big of adifference?
Context for the question: I am connecting a manual transfer switch to a subpanel. In order to minimize drywall work, I am mounting the transfer switch to the surface of the drywall. The subpanel is flush mounted (even with the surface) to the drywall. I am running the wires from the transfer switch through MC to a square junction box to provide a more 'finished' look. My plan was to run the wires through the back of the junction box the six inches to the subpanel inside the drywall without any conduit. If it were Romex, it would be allowed. THHN needs a conduit. Doesn't make sense to me.

You need to check the code book, all THHN or any single strand wires must be installed in a raceway.

 

[pc1966]

You need to check the code book, all THHN or any single strand wires must be installed in a raceway.

I had to look that up and it seems USA raceway is UK trunking!

 

[Dustydazzler]

I had to look that up and it seems USA raceway is UK trunking!

I love U.S terminology

Raceway
Pony Panels
Grounding
Roughing wiring
PigTailing

 

[pc1966]

PigTailing

Squeal like a piggy! Squee! Squee!

 

[Megawatt]

I had to look that up and it seems USA raceway is UK trunking!

It’s the same thing my friend

 

[Megawatt]

I love U.S terminology

Raceway
Pony Panels
Grounding
Roughing wiring
PigTailing

Let’s not forget Pecker heads on motors