NM Cable, MC Cable and Flexible Cords for Apprentices: NEC 334, 330 and 400 Support, Bends, Ampacity and Cord Limits

Tsai Electro is an independent learning site. Not affiliated with TDLR, PSI or NFPA. We explain the ideas in our own words and do not reproduce NEC text. Check every rule and table value in your own 2026 NEC, follow the cable manufacturer’s instructions, and follow your supervisor and the local inspector.

Which article covers what

Product	Article	Where you usually see it
NM cable (NM-B)	334	Houses and other buildings where the code permits it
MC cable (metal armored)	330	Commercial work, exposed runs, places where NM is not permitted
Flexible cords	400	Pendants, appliance connections, portable equipment

NM cable: where it can and cannot go

• Uses permitted. NM is permitted in one- and two-family dwellings and their attached or detached garages and storage buildings, in multifamily dwellings, and in other structures where the code allows it (NEC 334.10). The building’s construction type matters, so read the full list.
• Uses not permitted. The code lists places where NM is not allowed, such as wet or damp locations and embedded in poured concrete (NEC 334.12). Read the full list before you answer a “permitted” question.
• Exposed work. Exposed NM closely follows the building surface and is protected where it is subject to physical damage (NEC 334.15).
• Bends. The radius of the inner edge of a bend must be at least 5 times the cable’s diameter (NEC 334.24).
• Support. Secure NM at intervals no greater than 4½ feet and within 12 inches of every box, cabinet or fitting (NEC 334.30). Cable run horizontally through bored holes in framing spaced within that distance counts as supported. Check the box rules too, because some boxes without cable clamps have their own securing requirements. The section has conditions for unsupported lengths, such as fishing cable in finished walls, so read them in your NEC.

NM ampacity: the 60°C limit

The conductors in NM-B have 90°C insulation, but the cable’s ampacity is limited to the 60°C column of Table 310.16 (NEC 334.80). The 90°C rating can be the starting point for correction and adjustment factors, as long as the final ampacity does not exceed the 60°C value. That is why a derated NM circuit often ends up at the same answer as one with no derating.

MC cable: support and uses

MC cable is generally secured at intervals of no more than 6 feet. Cables with four or fewer conductors sized 10 AWG and smaller are also secured within 12 inches of every box, cabinet or fitting. The section has other conditions and exceptions, so read it in your NEC (NEC 330.30). Its uses permitted and not permitted are in 330.10 and 330.12. Check whether the MC you are installing has armor listed as an equipment grounding conductor, or carries a separate one.

Flexible cords: listed uses only

Cord types are listed in Table 400.4 with their uses and ratings. The code lists where cord is permitted, such as pendants, connecting portable appliances, and appliances that are frequently interchanged (NEC 400.10). It also lists where cord is not permitted, unless the code specifically allows it elsewhere (NEC 400.12):

• As a substitute for the fixed wiring of a structure.
• Run through holes in walls, ceilings or floors.
• Run through doorways, windows or similar openings.
• Attached to building surfaces.
• Concealed by walls, floors or ceilings, or above suspended ceilings.

Two more rules apply where cord is used:

• Splices. Cord is used in continuous lengths without splices or taps, with a limited repair allowance for hard-service cord when the repair keeps the cord’s insulation and usage characteristics (NEC 400.13).
• Pull at joints and terminals. Cord is connected so tension is not passed to joints or terminal screws, for example with a suitable strain relief fitting (NEC 400.14).

Cord ampacity has its own tables in Article 400. Do not use Table 310.16 values for cords. For choosing a cord for portable use, see our extension cord guide.

Worked example 1: how many staples on a 30-foot NM run

An NM cable runs 30 feet exposed along a framing member between two boxes. Assume no bored holes support it and no exception applies. What is the fewest staples that meets the general support rule?

1. One staple within 12 inches of each box, assuming neither box needs a closer staple. Put each one 1 foot from its box, measured along the cable. The distance left between those two staples is 30 − 1 − 1 = 28 feet.
2. The maximum spacing is 4½ feet. 28 ÷ 4.5 = 6.2, so you need at least 7 spaces.
3. 7 spaces need 8 staples, counting the two near the boxes. Spacing: 28 ÷ 7 = 4 feet, which is under 4½ feet.
4. Check: with 6 spaces, 28 ÷ 6 = 4.67 feet, which is over 4½. So 8 staples is the minimum.

Worked example 2: derating 12 AWG NM

Four current-carrying 12 AWG copper conductors in NM-B are bundled so the 80% adjustment for 4 to 6 conductors applies. No temperature correction applies. What is the final ampacity?

1. Start from the 90°C column for 12 AWG copper: 30 A.
2. Adjust: 30 × 0.80 = 24 A.
3. The NM 60°C limit: 12 AWG copper at 60°C is 20 A.
4. The final ampacity is the smaller value: 20 A.

The derating did not pull the answer below 20 A because it started from the 90°C value. Overcurrent protection is a separate check: for 12 AWG copper it is also generally limited to 20 A by the small conductor rule in 240.4(D).

Check yourself

1. Maximum spacing between supports for NM cable? 4½ feet, and within 12 inches of each box.
2. Can flexible cord run through a wall as permanent wiring to a fixed appliance? No. Cord is not a substitute for fixed wiring and is not run through walls.
3. Which temperature column limits NM cable ampacity? 60°C.

FAQ

Sources checked October 2026

• PSI Candidate Information Bulletin, Texas electrician examinations (updated September 15, 2026): https://test-takers.psiexams.com/api/content/bulletin/1682
• NFPA 70, National Electrical Code, 2026 Edition, Articles 310, 330, 334 and 400