Unusual old work discovery

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74sparks

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I'm digging into a conduit run from a prior electrician, just beginning the autopsy now, and discovering some creative peculiarities. Full report to come.

But for a quick check, when wire is marked "TYPE MTW OR THWN-2 OR THHN" what is its temperature rating when used in underground nonmetallic conduit? I believe THWN-2 carries a 90C rating wet or dry, but MTW and THHN would drop to 65C for a wet location, such as buried PVC conduit. So, when carrying multiple and conflicting ratings, does the highest or lowest govern?

Part of this run two hardwired loads (ceiling fan and overhead lighting) connected via AWG-14 (same "MTW OR THWN-2 OR THHN" wire, to a 20A breaker. If they were arguing for a 115% rating at 50F ambient on a 90C wire, then AWG-14 would technically pass, even with the 70% derating for 9 wires in a conduit. But if this wire rating drops to 65C for wet location, then we have a violation.
 
THWN-2 is a 90°C conductor in wet locations so that is what you can use for your calculations.
 
Cool. So the highest of the labeled ratings always applies for wire type/insulation class?

What do you think of them leaning on the 115% ambient prorating to make AWG-14 pass at 20A?
 
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infinity said:
Article 240 would limit the #14 conductors to 15 amps for your application.
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I thought so. I had been hunting for an exception to 240.4 that would allow for it in this case, but not finding it. You just saved me some time. Thanks!

Conduit runs from basement, through unknown crawl space (inaccessible), and then underground through yard. Not sure how that would impact ambient de/pro-rating, but without some help from a cooler ambient rating, I'm looking at up-sizing all circuits to AWG-10 to stay with 20A breakers on the planned increase in CCC count. Conduit presently carries 9 CCC's, but we were hoping to increase to 12 CCC's in this 1.5" tube. It looks like my predecessor had assumed a 50C ambient.
 
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Traced all wiring back thru breaker, and then out to load, and here's how it all shakes out:

conduit150 to yard existing 2011.png

It turns out the three conductors I called out last night appear to be the only conductors in violation, although the jump from AWG-12 with 9 CCC's to AWG-10 with only 2 CCC's in the next tube had me wondering what they were thinking there.

I've never seen 12 VAC low-volt lighting run in the same conduit as 120V and 240V circuits, but it's all THHN and all AC power, so I guess there's no issue. The 12 VAC is protected with a 12.5A / 250 VAC pushbutton breaker on the load side of the transformer.
 
74sparks said:
I've never seen 12 VAC low-volt lighting run in the same conduit as 120V and 240V circuits, but it's all THHN and all AC power, so I guess there's no issue. The 12 VAC is protected with a 12.5A / 250 VAC pushbutton breaker on the load side of the transformer.
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As long as the entire wiring method for the 12VAC including the power supply is installed per a Chapter 3 wiring method, it is OK. Think of it like this... I have reclassified the low voltage wiring to class 1 and/or power and lighting wiring, regardless of the voltage. The transformer and all wiring has to be enclosed, basically.
 
74sparks said:
Cool. So the highest of the labeled ratings always applies for wire type/insulation class?
Click to expand...
I think of it differently. THHN isn't a wet location wiring method, so it wouldn't "drop" to 65 degrees because it just isn't allowed in a wet location, period. As THWN-2 it is wet location rated at 90 degrees, so that is that. If the wire had only carried a THHN rating, it couldn't be used outside.
 
Thanks, guys!

One other odd thing about this old work is that they ran it all into a Quazite open-bottom PC style box (PC1212BA12 Tier-15), but it appears all or most of the wire nuts used for joining wires in this box are regular Ideal wire nuts, not rated for underground junctions. Everything is still working after 28 years, but the few I tried disconnecting were seized in place.

There's enough length there that cutting off all junctions and re-doing with wet location wire nuts won't be a problem.
 
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