Home Improvement → Conduit Size for 6-3 NM

What size plastic conduit should I use for a 20' run of 6-3 NM w/ground if I leave the outer plastic jacket on the cable?

It will be a feed to a subpanel, so does it need to be derated since it is a 3 conductor cable?

Tim

I would strongly advise against running 6/3 romex or UF through PVC conduit for a lot of reasons.
If you insist on doing it, you are going to want to upsize the conduit drastically to cope with the wires "unwillingness". I'd say 2". If it's just a short stub with no bends, then 1".
The proper way would be to install individual conductors correctly rated for the environment in which they will be installed (typically THHN/THWN for outdoor and buried applications.

No need to derate as the neutral and equipment ground in your 6/3 cable aren't counted as a current carrying conductors as per 310.15(B)(5)(a) and 310.15(B)(6) In the unlikely event your service is fed from two phases of a 208 wye (as whizkid mentions is common in NYC) the neutral is counted as a current carrying conductor as per 310.15(B)(5)(b) but even in this case you only have three current carrying conductors in the cable and derating doesn't apply as per table 310.15(B)(3)(a)

Neutrals are always counted as current carrying conductors.

Always??? Really???

310.15(5) Neutral Conductor

(a) A neutral conductor that carries only the unbalanced current from the other conductors of the same circuit shall not be counted when applying the provisions of 310.15(B)(3)(a)

That's an exception that's rarely met. We are talking about a single phase or *possibly* two phase system here.
See (b) and (c) right below.

My original reply covered that contingency and mentioned (b) specifically. (c) doesn't apply since there are only three insulated conductors in a 6/3 cable making it's (legal) use on a 4-wire 3-phase wye circuit impossible. More than likely the OP's system is a vanilla 120/240 single-phase where the neutral isn't required to be counted for the purpose of current adjustment.

NM cables are not allowed to be in conduit, as such. That is not a 'listed use'. You are limited to the use of conduit for 'nipples' under 24". You can use conduit for mechanical protection, but you have to derate if over 24". Curiously, if you use a "cable shield":

»tinyurl.com/98s3qvc

then you don't need to derate. Go figure...

I'd say that 6/3 CU will go though 2 - 90* and 1 - 45* in the 1-1/2" range, if the derate for your install allows that. Feed from the end that has the LEAST amount of bends and pull from the end that has the MOST bends. Make up the pull-head as small as possible, tape smoothly and use plenty of lubricant. Have someone push the cable while you pull.

I faced this same problem early in my career, and an AHJ told me that I could strip the NM jacket off and use the single conductors IF the individual conductors were marked (the same as single conductors). Some are and some aren't...YMMV.

how will one be sure it is perfectly balanced? this isn't a connected load, it is a sub.

The neutral carries the unbalanced portion of the total circuit load. If the load is balanced across the 240V line, the neutral current approaches zero. As the load becomes more unbalanced, the neutral current is inversely proportional to the that of the lightly loaded phase conductor. Whether the load fed by the 3 wire circuit is static or a sub panel, the circuit theory is the same.

Basic split phase theory and calculations

»www.allaboutcircuits.com ··· 0/1.html

I probably should have been a bit more clear in the scope. This feed will connect to a 60A nonfused disconnect safety switch and be feed by a 60-AMP breaker in the panel.

The panel is in the garage and the conduit will be run along the garage wall to protect the NM until it enters the basement.

Safety switch will be feeding resistance heaters in a 9.6kW electric furnace so it should be pretty balanced with little or no neutral current. In fact, I am not even sure the furnace has a neutral connection, but it probably does for the control transformer. The heater elements are spec'd at 40A load @240VAC so I am well below the rating of the cable.

The pull should not be too difficult - famous last words- as it is 20' straight horizontal, a 90*, 5' up and finally a LB with a short nipple into the side of the panel. I had thought about the "unwillingness" of the 6-3 to bend so stripping the sheath off was a backup plan.

Interesting about the AHJ comment that if the wire is marked, it can be stripped and used for conduit application. In the conduit no one can see the wire markings but it could be disallowed. How is this different than the wire being exposed for 18" or so in the panel and not being labeled? That is legal, but when it is hidden it has to be marked??? I don't see the logic here. Can some one enlighten me?

You might want to verify your furnace but I've not seen an air handler with strip heaters that requires a neutral. The control transformer primary and blower motor are 240V . If this is accurate for yours, you can use 6/2. Have you considered taking the advice of nunya about running conduit the entire length of the circuit and using THHN/THWN? It would allow smaller conduit, be much easier to pull, eliminate the questions that surround the compliance of cable in conduit and probably be less expensive.

The AHJ here won't allow stripping the sheath off of the cable for use in conduit. Of course, they have to catch it but most local inspectors can tell the difference between a THHN conductor that was manufactured as a single conductor and one that was cabled. Their argument is the cable was approved and listed for a particular use as it was originally manufactured and stripping the sheath for use in conduit is an unauthorized modification and thus renders it unlisted and in violation of 110.3(B)

Of course, YMMV

110.3(B) Installation and Use. Listed and labeled equipment shall be installed and used in accordance with any instructions included in the listing or labeling.