8-100 Current Calculations

When calculating currents that will result from loads, expressed in watts or volt amperes, to be supplied by a low-voltage ac system, the voltage divisors to be used shall be 120, 208, 240, 277, 347, 416, 480, or 600 as applicable.

Watts Law states that power (VA) = voltage × amperage. Voltages in residential systems are sometimes expressed as 110V, 115V or 120V or 220V, 230V and 240V, depending on the manufacturer. For consistency, the Code specifies the use of 120V and 240V when expressing voltages in residential scenarios.

8-102 Voltage Drop (See Appendices B and D)

1. The voltage drop in an installation shall be based on the connected load of the feeder or branch circuit if known; otherwise it shall be based on 80% of the rating of the overload or overcurrent device protecting the branch circuit or feeder, and not exceed
   1. 3% in a feeder or branch circuit; and
   2. 5% from the supply side of the consumer’s service (or equivalent) to the point of utilization.
2. ……..
3. Notwithstanding Subrule 1), wiring for general-use branch circuits rated at not more than 120 V or 20 A in dwelling units, with the insulated conductor length measured from the supply side of the consumer’s service to the furthest point of utilization in accordance with the values in Table 68, shall be acceptable.
4. ………..

Voltage drop occurs when the voltage at the end of a run of cable is lower than at the beginning. This is affected by conductor type, size, and length as well as by the amperage (current) being carried. Subrule 3) above references Table 68 which lists 38 m (124 feet) as the maximum length of a No. 14 AWG circuit protected by a 15 A fuse or circuit breaker. Therefore most electricians use 30 m (100 feet) as a rule of thumb maximum length for those circuits before having to upsize to the next larger wire diameter. The notes to 8-102 in Appendix D explain the math behind calculating a maximum conductor length, however, for the purposes of this learning guide, such a description and calculation is unwarranted.

Now complete Self-Test 8 and check your answers, quoting applicable Code rules.

Self-Test 8