Mike Dolt Enterprises
Mike Dolt Enterprises
User: Will Wire, Location: California: NEC 2020
Asked on: Thursday, January 2, 2025 at 5:59 pm EST
Answered on: Thursday, March 6, 2025 at 6:52 am EST
Question: Hello,I am confused on how to calculate the Circular Mils to determine the size of the wires for a commercial lighting circuit on the outside of a Storage Unit building.I have seven 120 Volt LED lights with Drivers that draw 1.2 amps each. The total amperage draw is 8.4 amps. It is a continuous load and must be increased 125% for 10.5 amps. I will install a 20 amp circuit.The lights are equally spaced. The furthest light is 370 feet away. I want no more than three percent voltage drop on the branch circuit.I am aware of the CM formula, just not sure which amperage value to use.Do I calculate the size (CM) of the wire based on 1.2 amps, or 8.4 amps, or 10.5 amps at 370 feet?Thank you for your time.
You're on the right track! Let's break down which amperage value to use for your circular mil calculation.
The correct amperage to use is 10.5 amps.
Here's why: Continuous Load and 125% Factor
You've correctly identified that the 8.4 amp load is continuous. NEC Article 210.19(A)(1) requires you to size the conductors at 125% of the continuous load. This gives you 10.5 amps. This ensures the wiring is adequately sized to handle the sustained heat generated by a continuous load.
Voltage Drop Calculation
Voltage drop is directly related to the current flowing through the conductor. The higher the current, the greater the voltage drop for a given conductor size and length. You're trying to limit the total voltage drop in the circuit, which is affected by the total current flowing.Using the 1.2 amp value would underestimate the total voltage drop. The voltage drop from the panel to the first light would be minimal. But, the accumulated voltage drop from the panel to the last light is where you will have to overcome the most voltage drop.
The 20 Amp Circuit Breaker
While you're installing a 20 amp circuit breaker, this doesn't directly influence the CM calculation for voltage drop. The breaker protects against overcurrent, but the wire size you choose must be adequate to limit voltage drop at the actual operating current (10.5 amps in your case).
The Circular Mil Formula and your values K = Resistance of one foot of conductor, one circular mil in area. For copper it is 12.9 I = Amperage of the circuit: 10.5 D = Distance in feet from the panel to the furthest light, multiplied by two: 740 VD = Voltage Drop, the lights are 120 volt lights. You want no more than three percent, so use 3.6 as your value.
CM = (2 x K x I x D)/VD (2 x 12.9 x 10.5 x 370)/3.6 = (270,786)/3.6 = 75,218 CMWire Size Based on the CM value, you will need to use #4 AWG copper conductors. Please check a wire ampacity chart to ensure #4 AWG meets the ampacity requirements.
Important Considerations NEC Requirements
Always double-check your calculations against the latest edition of the National Electrical Code (NEC). Local codes may have stricter requirements.
Temperature Correction
The ampacity of conductors is affected by ambient temperature. If the lights are in a particularly hot environment, you may need to adjust the ampacity accordingly (NEC Table 310.15(B)(1)).
Conduit Fill
Ensure that the conduit you're using is properly sized for the number and size of conductors you're installing (NEC Chapter 9, Table 1).
Professional Advice: When dealing with electrical installations, especially commercial ones, it's always best to consult with a licensed electrician. They can ensure the installation is safe, code-compliant, and performs as expected.Good luck with your project! Flag for review