Laws of Electricity and Electrical Codes
The Laws of Electricity and Electrical Codes domain tests two things at once: whether you can compute circuit quantities correctly, and whether you can apply that result inside the NEC edition adopted by the North Dakota State Electrical Board (NDCC ch. 43-09). Questions mix Ohm's law, series and parallel networks, AC impedance and power factor, conductor resistance and voltage drop with Articles 90, 100, 210, 220, 250, 310, 314 and 430.
Core relationships
- Ohm's law E = I × R; power P = E × I. For AC, E = I × Z, P = E × I × PF, and reactive power Q = E × I × sin θ.
- Series: resistances add, current is common. Parallel: voltage is common, and 1/R total = the sum of the reciprocals.
- Conductor resistance comes from Chapter 9 Table 8 (Ω per 1,000 ft) or from R = K × L / cmil.
- Single-phase voltage drop: VD = 2 × R × I; the factor 2 accounts for the return conductor. Three-phase uses 1.732 instead of 2.
Code anchors
210.19(A) Informational Note limits branch-circuit drop to 3% and feeder plus branch to 5% total; 310.16 sets conductor ampacity; 240.4(D) caps 14, 12 and 10 AWG; 314.16 governs box fill.
Worked example
A 15 A load at 120 V sits 50 ft from the panel on 14 AWG copper (3.07 Ω per 1,000 ft). Conductor length is 100 ft, so R = 0.307 Ω and VD = 15 × 0.307 = 4.6 V, or 3.8% - over the 3% branch recommendation. Switching to 12 AWG (1.93 Ω per 1,000 ft) gives R = 0.193 Ω and VD = 2.9 V, or 2.4%, which passes.
Traps
- Using the one-way distance and halving the drop.
- Mixing 60 °C, 75 °C and 90 °C columns without applying 110.14(C).
- Treating impedance as pure resistance in AC questions.
- Calling the 3% and 5% figures enforceable text - they sit in an Informational Note.
- Confusing power factor with efficiency.