Question 1. I am trying to find a Code article or reference to the use of SE cable in conduit underground. Chapter 9 states that for multiconductor cables, you have to use the actual dimension. This is not a problem, because you just measure the outside diameter. My question is, Is USE cable listed for use in conduit underground? — R.P.

Answer 1. The use of Type SE or USE in conduit either underground or aboveground is not prohibited by the NEC. However, neither of these cable types is listed for such use so the decision to accept or reject is up to the authority having jurisdiction (AHJ).

The following information may be helpful in considering such a decision. There are two serious concerns with this type of installation. The first is damage to the cable or individual conductors as they are pulled into the raceway. The other is the added resistance to heat dissipation. Both concerns can be addressed to some extent by using a conduit one size larger than required in Chapter 9. This reduces the friction of the pull, making it less likely that the conductors will be damaged during installation, and provides a larger air space to dissipate the heat buildup in the conductors under operating conditions.

In general, such an installation is poor practice and should not be permitted where there are more than two ninety-degree bends in the run of raceway or where the circuit is expected to run at full load (80 percent of rated ampacity) for any length of time. The sole advantage to running SE cables in conduit underground is the ability to replace them, if necessary, without excavating. Of course, the use of THHW conductors in conduit provides this same advantage with none of the drawbacks identified above and is probably more economical as well. — T.E. Trainor, CMP-7

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Question 2. Does this requirement include NM wiring in the basement run between floor framing and upwards through the subfloor for outlets, etc., on the first or second floor? — Rick Bowling

Answer 2. Section 336-6 [334.15 in NEC 2002 ] relates to installations where the nonmetallic-sheathed cable is exposed to physical damage. If the NM cable comes through a floor into a wall, it is not exposed and no additional protection is required. If the NM cable comes through a floor (any floor) into an open area, it is exposed to physical damage and additional protection is required. — Tom Trainor, CMP-7

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Question 3. I am trying to find a Code article or reference to the use of SE cable in conduit underground. Chapter 9 states that for multiconductor cables, you have to use the actual dimension. This is not a problem, because you just measure the outside diameter. My question is, Is USE cable listed for use in conduit underground? — R.P.

Answer 3. The use of Type SE or USE in conduit either underground or above round is not prohibited by the NEC. However, neither of these cable types is listed for such use so the decision to accept or reject is up to the authority having jurisdiction (AHJ).

The following information may be helpful in considering such a decision.

There are two serious concerns with this type of installation. The first is damage to the cable or individual conductors as they are pulled into the raceway. The other is the added resistance to heat dissipation. Both concerns can be addressed to some extent by using a conduit one size larger than required in chapter 9. This reduces the friction of the pull, making it less likely that the conductors will be damaged during installation, and provides a larger air space to dissipate the heat buildup in the conductors under operating conditions.

In general, such an installation is poor practice and should not be permitted where there are more than two ninety-degree bends in the run of raceway or where the circuit is expected to run at full load (80 percent of rated ampacity) for any length of time. The sole advantage to running SE cables in conduit underground is the ability to replace them, if necessary, without excavating. Of course, the use of THHW conductors in conduit provides this same advantage with none of the drawbacks identified above and is probably more economical as well. — T.E. Trainor, CMP-7

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Question 4. In 336-6(b), Exposed Work [NEC 1999 ], it says, "…Where passing through a floor, the cable shall be enclosed in rigid metal conduit, intermediate metal conduit, electrical metallic tubing, Schedule 80 PVC rigid nonmetallic conduit, listed surface metal or nonmetallic raceway, or other metal pipe extending at least 6 in. (152 mm) above the floor."

Does this requirement include NM wiring in the basement run between floor framing and upwards through the subfloor for outlets, etc., on the first or second floor? — Rick Bowling

Answer 4. Section 336-6 [334.15 in NEC 2002 ] relates to installations where the nonmetallic-sheathed cable is exposed to physical damage. If the NM cable comes through a floor into a wall, it is not exposed and no additional protection is required. If the NM cable comes through a floor (any floor) into an open area, it is exposed to physical damage and additional protection is required. — Tom Trainor, CMP-7

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Question 5. I seem to be frequently involved in discussions over the use of cable ties as a supporting means for MC cable. Section 330.30 does not clearly state that cable ties can be used as a support method. I am aware that cable ties are clearly defined as an approved method in the following code locations: 320.30, Armored Cable; 334.30, Nonmetallic-Sheathed Cables; 338.10(B)(b), Service-Entrance Cable; and 340.10(4). Could you give us your take on this subject? — G. C.

Answer 5. Section 330.30 of the 2002 NEC requires Type MC cable to be supported and secured at intervals not exceeding 1.8 m (6 ft). Section 300.11 also applies and requires cable assemblies to be securely fastened in place. Section 320.30 allows cable ties to be used to secure Type AC cable and does not allow cable ties to be used to support Type AC cable. Sections 334.30, 338.10(B)(4)(b), and 340.10(4) also reference cable ties for securing but not for supporting cables. Cable ties are specifically mentioned in 2002 Code articles for securing cable type wiring methods but not as the supporting means. — Gaylen Rogers, CMP-7

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