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Figure 01
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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 does the Code require when Type NM cable is installed through or parallel to framing members?
A1. Type NM cable installed through or parallel to framing members or furring strips must be protected against physical damage from penetration by screws or nails by maintaining 1¼ in. of separation of the cable to the framing member or furring strip, or by a suitable metal plate in accordance with 300.4(A) and (D) [334.17]. Figure 01
If Type NM cables pass through factory or field openings in metal framing members, the cable must be protected by listed bushings or listed grommets that cover all metal edges. The protection fitting must be securely fastened in the opening before the installation of the cable.
Q2. What does the NEC require when Type NM cable is installed in accessible attics or roof spaces?
A2. Type NM cable installed in accessible attics or roof spaces must comply with 320.23 [334.23].
Author’s Comment:
• On the Surface of Floor Joists, Rafters, or Studs [320.23(A)]. In attics and roof spaces that are accessible, substantial guards must protect cables installed across the top of floor joists, or across the face of rafters or studding within 7 ft of the floor or floor joists. If this space isn’t accessible by permanent stairs or ladders, protection is required only within 6 ft of the nearest edge of the scuttle hole or attic entrance.
• Along the Side of Framing Members [320.23(B)]. When Type NM cable is installed on the side of rafters, studs, or floor joists, no protection is required if the cable is installed and supported so the nearest outside surface of the cable or raceway is at least 1¼ in. from the nearest edge of the framing member if nails or screws are likely to penetrate [300.4(D)].
Q3. When does the Code allow Type NM cable to be spliced using NM Cable interconnection devices?
A3. (B) NM Cable Interconnection Devices. A box or conduit body isn’t required where a listed nonmetallic-sheathed cable interconnector device is used for any exposed cable wiring or for concealed cable wiring in existing buildings in accordance with 300.15(H) [334.40].
Author’s Comment:
• According to UL (QAAV), interconnection devices have been investigated for equivalency to Type NM cable in insulation and temperature rise, and for capability to withstand fault currents, vibration, and mechanical shock that may occur during transport. The interconnects are intended for use with 14 AWG and 12 AWG solid or stranded copper conductors.
Q4. What are the Code rules regarding the ampacity of conductors in Type NM cable?
A4. Adjusted and corrected conductor ampacity for Type NM cable is based on the 90°C insulation rating per Table 310.15(B)(16); however, conductors are sized based on the 60°C termination temperature rating in accordance with Table 310.15(B)(16) [334.80].
Example: What size Type NM cable is required to supply a 9,600W, 240V, single-phase fixed space heater with a 3A blower motor?
Solution:
Step 1: Determine the load for the heater:
I = W/E
I = 9,600 VA/240V
I = 40A + 3A blower
I = 43A
Step 2: Size the conductors at 125 percent of the total current load [110.14(C)(1) and 210.19(A)(1)]:
Conductor Size = 43A × 1.25
Conductor Size = 53.75A
Conductor Size = 53.75A, 6 AWG, rated 55A at 60°C [Table 310.15(B)(16)]
Conductor/Protection Size = Load × 1.25
Conductor/Protection Size = 43A × 1.25
Conductor/Protection Size = 53.75A
Answer: A 6 AWG conductor rated 55A at 60°C [Table 310.15(B)(16)], protected with a 60A overcurrent protection device [240.6(A)].
If multiple Type NM cables pass through the same wood framing opening that’s to be sealed with thermal insulation, caulking, or sealing foam, the allowable ampacity of each conductor must be adjusted per Table 310.15(B)(3)(a).
Author’s Comment:
• This requirement has no effect on conductor sizing if you bundle no more than nine current-carrying 14 AWG or 12 AWG conductors together. For example, if three 14/2 cables and one 14/3 cable (eight current-carrying 14 THHN conductors) are bundled together in a dry location, the ampacity for each conductor (25A at 90°C, Table 310.15(B)(16)) is adjusted by a 70 percent adjustment factor [Table 310.15(B)(3)(a)].
• Adjusted Conductor Ampacity = 25A × 0.70
• Adjusted Conductor Ampacity = 17.50A
Where more than two NM cables are installed in contact with thermal insulation without maintaining spacing between cables, the allowable ampacity of each conductor must be adjusted per Table 310.15(B)(3)(a).
Q5. What is the NEC rule regarding the insulation temperature rating of the conductors in Type NM cable?
A5. NM insulated conductors must be one of the types found in Table 310.104(A) and the insulation must be rated 90°C (194°F) [334.112].
Informational Note: Type NM cable identified by the markings NM-B meets this requirement.
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