How to choose the cable size for an electrical run

Choosing the right cable type and size for a given electrical run isn't rocket science but it does involve working with the rules, regulations and wire characteristics tables found in the National Electric Code (NEC). Solving this problem also requires the ability to work a few simple algebraic formulas. Factors like the length of the cable run, the specific resistance of the conductor, the load current being carried by the cable, continuous or non-continuous duty load, and the maximum allowable voltage drop, all come into play when selecting the size of cable to be used.

Determine the type of loads that the cable will be carrying electricity to, whether they are continuous duty loads, noncontinuous duty loads, or a combination of the two. Article 100 Definitions of the NEC defines a Continuous Load as "A load where the maximum current is expected to continue for 3 hours or more." An example of such a load would be De-icing equipment." Noncontinuous loads can be defined as any load where current is expected to flow continuously for less than 3 hours at a time; i.e., a refrigerator/freezer.

List the "Full Load Amperes" (FLA) or "Branch Circuit Selection Current" for all the loads to be served. This data will be found on the load's "Nameplate." A load rated in watts on its nameplate, like a baseboard heating unit, requires you to compute the FLA by dividing the Wattage Rating by the voltage; i.e., the FLA for a 240-Volt, 5,200 Watt heater equals 5,200/240 = 2.17 Amperes (A).

Compute the minimum conductor ampacity for the continuous duty loads by adding up separately all their FLA and Branch Circuit Selection currents and then multiplying the larger of the two sums by 1.25 per NEC Article 210.19(A)(1). Example: Total FLA = 30 Amperes Total Branch Selection Current = 25 Amperes Minimum conductor ampacity = 1.25(30) = 37.5 A

Add up the FLA for all the noncontinuous loads and add that sum to the minimum conductor ampacity computed for the continuous loads in the last step to find the actual minimum conductor ampacity for the circuit.

Using the total minimum conductor ampacity computed in the above step, select the cable size based on NEC Article 310, Table 310.16. At fist glance, it would appear that you could use an American Wire Gauge (AWG) 10 conductor but there is an exception to the rule here. There's an exception to the rule for AWG 14, 12, and 10 wires. Article 240.4(D) limit AWG14, 12, and 10 copper wire for use on branch circuits with an Over-Current Protective Device (OCPD), fuse or circuit breaker (CB) rated at 15, 20, and 30 Amperes respectively therefore you would select an AWG 8 conductor size. Anytime the required ampacity falls between two standard values you select the conductor with the higher rating.

Verify that you can actually use an AWG 8 copper conductor with an ampacity of 60 Amperes for this cable run by computing the voltage drop. Circuit voltage drop is calculated using this formula VD = (2 * L * R * I) / 1000ft where: L = length of the cable run R = resistance of the wire as given in NEC Table 9 I = Load current or 37.5 Amperes in the example given. The Fine Print Notes (FPN) of the NEC allow for a 2 per cent voltage drop (VD) on branch circuits or 2.4 Volts on a 120-Volt circuit and 4.8 Volts on a 240-Volt branch circuit.

For this article's example, assume the cable run is 150 feet Table 9 gives a resistance of 0.7780 Ohms per 1000 feet Therefore VD = (2_L_R_I)/1000 = (2_150_0.778_37.5)/1000 = 8.75 Volts which means that you can't use an AWG 8 cable. An AWG 2 conductor has a resistance of 0.1940 Ohms per 1000 feet which gives us a VD of 2.1825 Volts which is under the allowable 2.4 Volts so you will have to us an AWG 2 cable for this run.

Things You'll Need

  • National Electric Code (NEC), National Fire Protection Association (NFPA), Publication 70, 2008 Revision
  • Pen
  • Paper
  • Pocket calculator
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About the Author

Based in Colorado Springs, Colo., Jerry Walch has been writing articles for the DIY market since 1974. His work has appeared in “Family Handyman” magazine, “Popular Science,” "Popular Mechanics," “Handy” and other publications. Walch spent 40 years working in the electrical trades and holds an Associate of Applied Science in applied electrical engineering technology from Alvin Junior College.