Why is non-metallic cable (NM) cheaper than THHN?

Question

I'm looking at the cost of wire. A 500' spool of THHN costs roughly $75. I will need three of these to run hot/neutral/ground to outlets, etc. Three spools will cost $225. On the other hand, I can buy two 250' coils of cable and do the same work, but for only $125. Isn't cable just THHN wrapped into an outer sheath?

I'd much rather run THHN through my conduit than use cable (I understand cable cannot be used in conduit...well, it's cross-sectional area is too high to make it feasible), but the cost has me stumped.

Is there some technicality or spec that makes THHN superior to what comes in cable?

Answer 1

All of these other answers primarily address the complexity of creating the wire, but cost really is due to a host of market forces such as base manufacturing cost (influenced by complexity of design and local labor wages), supply chain cost/availabilty, distribution costs, and economies of scale (how much of each wire type people buy and from where).

Don’t believe me? I encountered a situation at a big box store where a bare copper wire was more expensive than same gauge THHN wire: Is there a reason to prefer bare wire for Grounding Electrode Conductor?

THHN obviously requires far more processing than a bare copper conductor, so explain that one to me as something other than market forces.

Answer 2

Pricing is local, so things may be different in your area, and pricing has been all over the place in 2020. Supply has been short, and demand has been high, prices have been high, crazy high on some things.

But for today, right now on the Home Depot web site, just as a reference, 2x250' 12-2 Romex is $176.00, and 3x500' #12 THHN is $171.00.

They sell a lot more Romex, so you get that sheath pretty cheap.

https://www.homedepot.com/p/Southwire-250-ft-12-2-Solid-Romex-SIMpull-CU-NM-B-W-G-Wire-2-Pack-28828258/206647943

https://www.homedepot.com/p/Cerrowire-500-ft-12-1-Black-Stranded-THHN-Wire-112-3651J/202304641

Some places don't sell much THHN and don't pay much attention to their pricing. There are a lot of hardware stores don't sell conduit but have the same six rolls of THHN on the shelf for ten years, hoping someone someday will come through and not care / not notice the ridiculous price. In the meantime they sell a pallet of Romex every weekend.

Now if you're shopping at an electrical supply house - their pricing to the general public, not account holders, generally makes no sense at all. Pricing to electrical contractors, commercial / industrial / institutional customers, etc. with house accounts and negotiated pricing is totally different and makes sense occasionally.

Answer 3

Keep in mind if you're running metal conduit, you don't need a ground wire, so it's only 2 spools.

Also I think $125 is a great price for the Romex, but $75 is not a very good price for THHN. Could you be buying from a shop where Romex is a high-volume commodity, and THHN is an odd specialty?

You are certainly looking at fine-stranded (not 7-strand) THHN wire, which is more expensive to make and a great deal easier to work with - so apples and oranges.

Also, keep in mind the THHN is cross-labeled THWN-2. That means it can be run in wet locations. NM-B can never be run in wet locations because of the paper packing and the fact that it will damage the simple insulation on many manufacturer's NM-B.

Further, in wires large enough for 240.4(D) not to apply, you are allowed to run 75C thermal limits with THHN... so you can get 50A out of #8 instead of 40A... and 65A out of #6 instead of 55A.

Answer 4

THHN ( Thermoplastic High Heat-resistant) isn't a brand name, it's a rating given by it's fire code. And indicates how long it takes to break down under load.

Where as Romex has a "protective" cover designed for pulling through a house. THHN is more for applications where a more strict fire code requirement exists or is warranted.

As far as your cost problem goes, I'd recommend calling your local codes office to explain the situation and ask them what's best. As for safety, electricity doesn't give second chances, better to be overly safe and expensive than cheap and dead.