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Theory & Plans: Ohm's Law, Definitions & the One-Line
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Theory & Plans: Ohm's Law, Definitions & the One-Line
- Study each rule with its stated conditions.
- Use the linked current sources and check any marked point before relying on it.
How do Ohm's law and the power law tie the four quantities together?
Check this point against current official guidance before relying on it.
- For a DC resistive circuit, voltage E, current I and resistance R obey E = I × R. Power is P = E × I, so it is also I² × R. State the circuit assumptions before applying these formulas to an AC load.
What do ampacity and continuous load mean?
- Ampacity is the maximum current a conductor can carry continuously under its conditions of use without exceeding its temperature rating. A continuous load has a maximum current expected to continue for three hours or more. Select the applicable load calculation and the separate conductor and overcurrent-device sizing checks; one multiplier does not apply to every use of the word continuous.
Source: NEC 2026 Article 100: Ampacity; Continuous Load; 120.5(E); 210.19(A); 210.20(A)
In a 120/240 V 3-wire system, what does the neutral actually carry?
Check this point against current official guidance before relying on it.
- In an ideal single-phase, three-wire 120/240 V circuit, the two ungrounded legs are 240 V apart and each is 120 V to neutral. Equal resistive 120 V loads on opposite legs balance; unequal resistive loads leave their current difference on the neutral. This is a circuit model, not permission to disconnect a shared neutral while energized.
How do you trace a circuit through a plan?
Check this point against current official guidance before relying on it.
- Use the legend printed on the drawing to identify outlets, switches, luminaires and panels. Then trace the circuit designation into the panel schedule and the one-line diagram. A symbol's meaning comes from that drawing's legend, not an assumed universal shape.
Conditions that change the answer
Check this point against current official guidance before relying on it.
- DC resistive example: 120 V × 12 A = 1440 W; 120 V ÷ 12 A = 10 ohms.
- Identify the load first; then perform the applicable load, conductor and breaker checks separately.
- Opposite legs of one single-phase system; balanced resistive loads cancel on the neutral.
- Legend, circuit designation, panel schedule, one-line: follow the same circuit through the drawing.
Carry these checks into practice
Check this point against current official guidance before relying on it.
- DC resistive example: 120 V × 12 A = 1440 W; 120 V ÷ 12 A = 10 ohms.
- Identify the load first; then perform the applicable load, conductor and breaker checks separately.
- Opposite legs of one single-phase system; balanced resistive loads cancel on the neutral.
- Legend, circuit designation, panel schedule, one-line: follow the same circuit through the drawing.
Check your recall
Check this point against current official guidance before relying on it.
- How do Ohm's law and the power law tie the four quantities together?
- What do ampacity and continuous load mean?
- In a 120/240 V 3-wire system, what does the neutral actually carry?
- How do you trace a circuit through a plan?