Exposed NM-cable must be protected from damage, so it is not allowed to run across the ground in a crawl space or unfinished basement. And the photo above is an example of how not to do it.The National Electrical Code [NEC 334.15(C), NEC 300.4(D), and NEC 334.30(B)] allows these installation methods: 

1) Where the cable is run at angles with joists in unfinished basements or crawl spaces, it can be secured directly to the bottom of floor joists if not smaller than two #6 AWG or three #8 AWG.

2) Smaller NM-cables must be run through holes in the joists or on running boards.

3)  NM-cables installed on a wall must be protected by listed conduit or tubing.

4) NM-cables running parallel to floor joists must be secured at least 1-1/4” from nearest edge of joist.

5) NM-cables must be supported and secured by listed methods and products, such as staples, designed and installed so as not to damage the cable, at intervals not exceeding 4-1/2 feet, and within 12-inches of every cable entry into an enclosure. The length between cable entry and the closest cable support cannot exceed 18-inches.

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Here’s links to a collection of answers to our most-frequently-asked questions about NM-CABLE (ROMEX®):

• What is the maximum spacing requirement for securing NM-cable (nonmetallic-sheathed cable)? 

• Is it alright to just put wire nuts on the end of unused or abandoned NM-cable or wiring? 

• Can NM-cable (Romex®) be used to make a cord and plug to connect an appliance?

• What is the color code for NM cable (Romex®) sheathing?

• What are the code requirements for NM-cable (nonmetallic-sheathed cable or Romex®) in an attic? 

• Can you run exposed NM-cable (Romex) in a garage? 

• What are the requirements for NM-cables entering an electric panel box? 

• How many NM-cables allowed in an NM-connector clamp?