Mike Dolt Enterprises
Mike Dolt Enterprises
User: K2X, Location: Colorado Springs
Asked on: Wednesday, March 30, 2011 at 9:54 am EDT
Answered on: Tuesday, May 14, 2024 at 3:28 am EDT
Question: I doing an estimate for a small grow house, (yes, we're legal here), and I have a question on the remote cord and plug hid ballasts. The ballasts draw 10.2 amps @ 120v or 5.1a @ 240 v. The boss says i have to use 2008 210.21(b)(2) which limits a 20 amp breaker to 80% or 16 amps plus he wants it reduced again using 210.20 (a), continuous load, to 12.8 amps. This means I can only run 2 ballasts on a 20 amp 240v circuit instead of 3 ballasts. ..I've done commercial lighting for years and 16 amps was always the number we used for hid's on a 20 amp circuit so i'm a little confused on what the code is actually calling for in this case?Thanks for any help.
NEC 2008 210.21(b)(2)
This section states that a branch circuit rated 20 amperes or less shall not supply a continuous load that exceeds 80% of the branch circuit rating. In this case, 80% of 20 amps is 16 amps.
NEC 2008 210.20(a)
This section states that a continuous load shall not exceed the rating of the branch circuit that supplies it.
Calculation
Based on your provided information, the ballasts draw 5.1 amps @ 240v. Using 210.21(b)(2), the maximum number of ballasts that can be run on a 20 amp 240v circuit is:16 amps / 5.1 amps = 3.14 ballastsHowever, according to 210.20(a), a continuous load shall not exceed the rating of the branch circuit. Therefore, you can
only run 2 ballasts on a 20 amp 240v circuit.
Conclusion:
The NEC requires you to use the lower of the two calculated values, which in this case is 2. Therefore, you can only run 2 ballasts on a 20 amp 240v circuit. Flag for review