Electrical – Is USE-2 or RHW-2 individual wire fit for direct burial or open-joist use

electricalwiring

I see companies selling USE-2/RHH/RHW-2 individual wires. Many claim they are fit for direct burial or fixation to joists. They are individual wires, and lack an outer jacket like SE-R or MH feeder has.

Can that be right?

If all that's true, it would seem like a nice solution to applications where conduit is desired for part of a run but individual wires for the rest.

Is the jacket extra tough to be able to endure direct exposure to soil/sand/crushing force? How does it cope with the "all related wires must be in the same cable or conduit" rule of thumb?

If not, then what is the advantage of this cable over, say, XHHW-2 or THWN-2? Why would my distributor go to the expense of stocking both?

Best Answer

USE-2/RHH-2/RHW-2 can be direct buried, but otherwise requires a raceway

The rules for USE-2 wire and cable can be found in NEC 338.12(B):

(B) Underground Service-Entrance Cable. Underground service-entrance cable (USE) shall not be used under the following conditions or in the following locations:

(1) For interior wiring

(2) For aboveground installations except where USE cable emerges from the ground and is terminated in an enclosure at an outdoor location and the cable is protected in accordance with 300.5(D)

(3) As aerial cable unless it is a multiconductor cable identified for use aboveground and installed as messenger- supported wiring in accordance with 225.10 and Part II of Article 396

This, practically, limits single-rated USE-2 wires and cables to direct burial applications, or use as a multiplex cable-in-conduit outdoors. (You can't use it indoors or for general aboveground work, even when in a raceway!)

However, when the wire has a RHH-2/RHW-2 rating as well, that changes the picture. Now, the wire is rated for use in raceway, as the table 310.104(A) entry for this type of conductor specifies a flame-retardant insulating material. However, it still cannot be used strapped to joists bared, as this violates NEC 300.3(A):

(A) Single Conductors. Single conductors specified in Table 310.104(A) shall only be installed where part of a recognized wiring method of Chapter 3.

Exception: Individual conductors shall be permitted where installed as separate overhead conductors in accordance with 225.6.

and NEC 398.10 only permits the use of open wiring on insulators in industrial and agricultural occupancies:

398.10 Uses Permitted. Open wiring on insulators shall be permitted only for industrial or agricultural establishments on systems of 1000 volts, nominal, or less, as follows:

(1) Indoors or outdoors

(2) In wet or dry locations

(3) Where subject to corrosive vapors

(4) For services

while NEC 394.10 only permits limited use of knob-and-tube to extend wiring in existing buildings, and 394.12 point 5 also prohibits the use of K&T within insulation:

(5) Hollow spaces of walls, ceilings, and attics where such spaces are insulated by loose, rolled, or foamed-in-place insulating material that envelops the conductors

Even though you can use it in conduit, you probably shouldn't

Even though tri-rated USE-2/RHH-2/RHW-2 can be legally used in conduit jobs, it's probably a bad idea to use it. Why? It has a thicker insulation than THHN or XHHW-2, which uses up more precious conduit fill area than necessary and makes the wire stiffer and harder to pull as well. It's also slightly more expensive than XHHW-2, making it a rather pointless choice to make.

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