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I have an oil fired heating system with 4 zones heated with baseboards. I recently noticed that one of the zones stopped working because its thermostat wire in the basement was frayed. I removed that part of the wire an spliced in a new one. It is 18 gauge wire (2 wires) for a 24V relay. I cannot find any code that specifies:

  • whether a junction box is needed
  • ... and if so, what type of junction box is required

Has anyone had experience with this? Thanks in advance.

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    If for some reason you don't use a box, at least don't use random twist nuts dangling in air as is so common. Use UR/UG etc splice connectors and use zip ties for strain relief. And make sure your splice doesn't drop into a wall.
    – jay613
    Dec 2, 2023 at 20:33

2 Answers 2

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The NEC says:

90.3 Code Arrangement. This Code is divided into the introduction and nine chapters, as shown in Figure 90.3. Chapters 1, 2, 3, and 4 apply generally. Chapters 5, 6, and 7 apply to special occupancies, special equipment, or other special conditions and may supplement or modify the requirements in Chapters 1 through 7.

Assumption is your thermostat control system, like most, is a class 2 control circuit by definition in Article 725. So rules in Chapter 3 apply, as modified by Chapter 7.

Box requirement is found in chapter 3

300.15 Boxes, Conduit Bodies, or Fittings - Where Required ...Where the wiring method is...nonmetallic-sheathed cable, or other cables, a box or conduit body shall be installed at each outlet point, switch point, conductor splice point, conductor junction point...

So the general requirement is a box is needed for splices, but the Article 725 which modifies chapter 3 articles says:

725.3 Other Articles In addition to the requirements of this article, circuits and equipment shall comply with the articles or sections listed in 725.3(A) through (E). Only those sections of Article 300 referenced in this article shall apply to Class 2 and Class 3 circuits.

300.15 isn't referenced in 725, so a box isn't required.

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A junction box is not required for low voltage.

However, using a junction box may make the splice more reliable in the long term, by protecting it and securing the wires so it's not stressed.

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