National Electrical Code Tips: Article 725, Class 1, Class 2, and Class 3 Circuits. Part 3.
- What are the size and use requirements for Class 1 circuit conductors? You can use sizes as small as 16AWG and 18AWG, if the loads they supply don't exceeed the ampacities given in 402.5 and if you install them in a raceway, approved enclosure, or listed cable [725.49(A)]. If you use conductors larger than 16AWG, they can't supply loads greater than the ampacities given in 310.15. If you use flexible cords, they must comply with Article 400.
- What are the insulation requirements for Class 1 circuit conductors? It must be rated for at least the system voltage and at least 600V. If the conductors are larger than 16AWG, they must comply with Article 310. For conductors 18AWG and 16AWG, you can use one of the types of insulation listed in 725.49(B). You can use other types and thicknesses, but only if the conductors are actually listed for Class 1 circuit use.
- As you might expect, there's a limit to how many Class 1 circuit conductors you can put in a raceway. If the raceway has only Class 1 circuit conductors, you can determine this limit per 300.17. You can use the ampacity adjustment factors of 310.15(B), but only if the conductors carry continuous loads in excess of 10% of the ampacity of each conductor [725.51(A)]. Note the use of aggregate ("the conductors") and individuation ("each conductor"), else you will not understand and correctly apply this requirement.
- You can run power-supply conductors and Class 1 circuit conductors in the same raceway, if you meet the requirements of 725.48 and you determine the number of conductors per 300.17 [725.51(B)].
- If you have the arrangement just discussed in Point 4, you must apply the ampacity factors of 310.15(B)(3)(a). But there are two caveats, which we discuss in the next two Points.
- One caveat for applying the ampacity factors of 310.15(B)(3)(a) to power-supply conductors and Class 1 circuit conductors in the same raceway is you must apply those to all conductors where the Class 1 circuit conductors carry continous loads in excess of 10% of the ampacity of each conductor and you have more than three conductors in total [725.51(B)(1)].
- Another caveat for apply the ampacity factors of 310.15(B)(3)(a) to power-supply conductors and Class 1 circuit conductors in the same raceway is you must apply them to the power-supply conductors only, if the Class 1 circuit conductors do NOT carry continuous loads in excess of 10% of the ampacity of each conductor and you have more than three power supply conductors [725.51(B)(2)].
- Cable tray is often a good way to run Class 1 circuit conductors. Just make sure you comply with 392.22 and 392.80(A) if you go that route [725.51(C)].
- If you have any Class 1 circuit(s) extending aerially beyond one building, ensure it complies with Article 225.
- Part III of Article 725 covers the requirements for Class 2 and Class 3 circuits. Note that we jump from 725.52 at the end of Part II to 725.121 at the start of Part III.