Mike Holt Enterprises Electrical News Source

NEC Questions & Answers based on 2017 NEC - November 2017


Figure 01


For EC&M Magazine

By Mike Holt, NEC® Consultant

Here's the follow-up to yesterday's newsletter.
This includes the answers to the questions sent, so you can see how you did.

Note: The answers to these questions are based on the 2017 NEC.
Underlined text indicates a change in the rule for the 2017 NEC.

Q1. What uses are permitted by the NEC for Service-entrance cable, (Types SE and USE)?
A1. Uses permitted [338.10]:
Service entrance cable used as service entrance conductors must be installed in accordance with Article 230 [338.10(A)].

Service-entrance cable can be used for branch circuits or feeders:

(1) Where the circuit conductors are insulated [338.10(A)(1)].

(2) If the insulated conductors are used for circuit wiring, and the uninsulated conductor is only used for equipment grounding purposes. Figure 01

Ex: In existing installations, uninsulated conductors may be used for the neutral conductor if the uninsulated neutral conductor of the cable originates in service equipment.

(3) Where the SE cable isn’t subjected to conductor temperatures exceeding its insulation rating.

(4) The installation of SE cable used for branch circuits or feeders must comply with (a) and (b).
(a) Interior Installations. SE cable used for interior branch-circuit or feeder wiring must be installed in accordance with the same requirements as Type NM Cable, Part II of Article 334, excluding 334.80.

Where installed in thermal insulation, the ampacity of conductors 10 AWG and smaller, must be sized in accordance with the 60°C (140°F) conductor temperature rating in accordance with Table 310.15(B)(16). For conductor ampacity correction and/or adjustment, the conductor temperature rating ampacity is to be used.

CAUTION: Underground service entrance cable (USE) isn’t permitted for interior wiring because it doesn’t have a flame retardant insulation. It’s only permitted in interior wiring when listed as both a cable (USE) and a conductor, such as RHH, in accordance with Table 310.104.

(b) Exterior Installations. The cable must be installed in accordance with Part I of Article 225, supported in accordance with 334.30, and where run underground, the cable must comply with Part II of Article 340.

Q2. What uses are permitted by the NEC for rigid metal conduit (Type RMC)?
A2. Uses Permitted for Type RMC [344.10]:

(A) Atmospheric Conditions and Occupancies.
(1) Galvanized steel and stainless steel rigid metal conduit is permitted in all atmospheric conditions and occupancies.
(2) Red brass rigid metal conduit is permitted for direct burial and swimming pool applications.
(3) Rigid aluminum conduit is permitted if approved for the environment.

(B) Corrosive Environments.
(1) Galvanized Steel, stainless steel and red brass elbows, and couplings can be installed in concrete, in direct contact with the earth, or in areas subject to severe corrosive influences if approved for the condition.
(2) Rigid aluminum conduit must be provided with supplementary corrosion protection approved by the authority having jurisdiction if encased in concrete or in direct contact with the earth.

In wet locations, support fittings, such as screws, straps, and so forth, installed in a wet location must be made of corrosion resistant material or protected by corrosion resistant coatings in accordance with 300.6 [344.10(D)].

CAUTION: Supplementary coatings (asphalted paint) for corrosion protection haven’t been investigated by a product testing and listing agency, and these coatings are known to cause cancer in laboratory animals. If practical, contact with dissimilar metals should be avoided to prevent the deterioration of the metal because of galvanic action. Aluminum fittings and enclosures are permitted to be used with galvanized steel intermediate metal conduit where not subject to severe corrosive influences [344.14].

Q3. Is liquidtight flexible metal conduit (Type LFMC) required to be trimmed after being cut where the termination fittings have a part that is inserted into the end of the raceway?
A3. Cut ends of liquidtight flexible metal conduit must be trimmed both inside and outside the raceway to remove rough edges from the cut ends [350.28].

Q4. What is the NEC requirement for securing and supporting of liquidtight flexible metal conduit (Type LFMC)?
A4. Liquidtight flexible metal conduit must be securely fastened in place and supported in accordance with 350.30(A) and (B).

(A) Liquidtight flexible metal conduit must be securely fastened by a means approved by the authority having jurisdiction within 1 ft of termination, and must be secured and supported at intervals not exceeding 4½ ft.

Where cable ties are used for securing liquidtight flexible metal conduit, the cable ties must be listed and identified for securement and support.

Ex 1: Liquidtight flexible metal conduit isn’t required to be securely fastened or supported where fished between access points through concealed spaces and supporting is impracticable.

Ex 2: If flexibility is necessary after installation, unsecured lengths from the last point where the raceway is securely fastened must not exceed:
(1) 3 ft for trade sizes ½ through 1¼
(2) 4 ft for trade sizes 1½ through 2
(3) 5 ft for trade sizes 2½ and larger

Ex 4: Lengths not exceeding 6 ft from the last point where the raceway is securely fastened can be unsecured within an accessible ceiling for luminaire(s) or other equipment. For the purposes of this exception, listed fittings are considered a means of securement and support.

(B) Liquidtight flexible metal conduit installed horizontally in bored or punched holes in wood or metal framing members, or notches in wooden members, is considered supported, but the raceway must be secured within 1 ft of termination.

Q5. What uses are permitted by the NEC for Electrical Metallic Tubing (Type EMT)?
A5. Uses Permitted for Type EMT [358.10]:

(A) Electrical metallic tubing is permitted exposed and concealed for the following applications:
(1) In concrete in direct contact with the earth in accordance with 358.10(B).
(2) In wet, dry, or damp locations.
(3) In any hazardous (classified) location as permitted by other articles in this Code.

(B) Electrical metallic tubing is permitted in Corrosive Environments where [358.10(B):
(1) Galvanized steel. electrical metallic tubing, elbows, and fittings can be installed in concrete, in direct contact with the earth, or in areas subject to severe corrosive influences if protected by corrosion protection and approved as suitable for the condition. See 300.6(A).
(2) Aluminum electrical metallic tubing must have approved supplementary corrosion protection where in concrete or direct buried.

Support fittings for Type EMT, such as screws, straps, and so on, installed in a wet location must be made of corrosion resistant material [358.10(D)].

Author’s Comment:
If installed in wet locations, fittings for EMT must be listed for use in wet locations and prevent moisture or water from entering or accumulating within the enclosure in accordance with 314.15 [358.42].

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