NEC Quick Reference: Branch Circuits, Continuous Loads, and Motor Sizing

This is a working quick reference for the NEC sections that come up most often in branch-circuit, continuous-load, and motor-sizing work. It is not a replacement for the full code book, but it pulls together the numbers and rules that are easy to mix up (the 240.4(D) small-conductor caps, the 125% continuous-load multiplier, the 3% / 5% voltage-drop Informational Notes, and the four maximum OCP percentages for motor branch circuits under Table 430.52(C)(1)) along with worked examples that show how they interact.

Formulas

Branch-circuit voltage drop: ≤ 3% recommended (NEC 210.19 Informational Note)
Total system drop (feeder + branch): ≤ 5% (NEC 215.2 Informational Note)
Continuous load: conductor and OCP sized for ≥ 125% of continuous load (NEC 210.19 / 210.20)
Motor branch-circuit conductor: ≥ 125% of table FLC (NEC 430.22)
Motor branch-circuit OCP (short-circuit and ground-fault): select device type per NEC 430.52(C)(1), max percentage of table FLC from Table 430.52(C)(1):
  • Non-time-delay fuse: 300% max
  • Dual-element (time-delay) fuse: 175% max
  • Instantaneous-trip circuit breaker: 800% max
  • Inverse-time circuit breaker: 250% max

This page is a quick reference to the NEC sections that come up most often when sizing branch circuits, continuous loads, and motor branch circuits. Scope is residential, commercial, and light industrial work under NFPA 70; it is not a complete code treatment of any one area, and it covers motor circuits under Article 430 because motor loads show up in HVAC, pumps, compressors, and commercial equipment across all three occupancy types. The 3% branch-circuit and 5% total voltage-drop figures are NEC 210.19 and 215.2 Informational Notes, not enforceable code requirements, but exceeding them causes equipment problems and is usually flagged by inspectors. The 125% continuous-load rule and the motor branch-circuit rules in Article 430 are mandatory. Motor branch circuits are sized from the table full-load current in NEC 430.250 (three-phase motors) or NEC 430.248 (single-phase motors), not from the nameplate FLA on the motor itself (nameplate is used for overload protection, not branch-circuit sizing). Short-circuit / ground-fault OCP percentages in NEC Table 430.52(C)(1) are maximum allowances, and the percentage depends on the device type chosen: the 250% figure is the ceiling for an inverse-time breaker specifically, not a blanket motor rule.

Worked Examples

Example 1: 20A continuous load (lighting running ≥ 3 hours)

Conductor and breaker sized at 20 × 1.25 = 25A minimum per NEC 210.19 / 210.20.

Example 2: 10 HP three-phase 230V squirrel-cage motor on an inverse-time breaker

Table FLC from NEC 430.250 = 28A. Branch-circuit conductor per NEC 430.22 = 28 × 1.25 = 35A minimum ampacity → pick a conductor from NEC Table 310.16 at the applicable temperature column whose ampacity is at least 35A (10 AWG Cu at 75°C satisfies this). Motor branch-circuit conductors are sized under Article 430 and are exempted from the small-conductor rule of 240.4(D) via 240.4(G), so the 30A cap on 10 AWG used for general branch circuits does not apply here. Short-circuit / GF OCP maximum from Table 430.52(C)(1) for an inverse-time breaker = 28 × 2.5 = 70A. NEC 430.52(C)(1)(a) allows rounding up to the next standard size if the motor does not start at the calculated value. A dual-element fuse would instead be capped at 28 × 1.75 = 49A → 50A standard size.

Example 3: 10 HP single-phase 230V motor

Table FLC from NEC 430.248 = 50A (very different from the three-phase value at the same voltage). Conductor = 50 × 1.25 = 62.5A. Inverse-time breaker max = 50 × 2.5 = 125A. Always confirm phase before pulling a table value.

Example 4: 120V branch circuit, 100ft, 15A

Max recommended drop = 120 × 0.03 = 3.6V. 14 AWG gives (2 × 100 × 15 × 2.525) ÷ 1000 = 7.58V. Too high. 12 AWG gives 4.76V (3.97%), still over. 10 AWG gives 3.00V (2.5%).

Common Mistakes

  • Treating the 3% / 5% voltage-drop figures as code violations. They are NEC Informational Note recommendations, not enforceable requirements, but they are still the right target for installations.
  • Applying 250% of FLC as a blanket motor breaker rule. 250% is the NEC Table 430.52(C)(1) maximum for an inverse-time circuit breaker. Dual-element fuses cap at 175%, non-time-delay fuses at 300%, and instantaneous-trip breakers at 800%. Always pick the device type first, then apply the matching percentage.
  • Using nameplate FLA instead of the NEC 430.248 / 430.250 table FLC for branch-circuit conductor and OCP sizing. Nameplate current is for overload protection, not branch-circuit sizing.
  • Forgetting that three-phase and single-phase table FLCs at the same HP and voltage are very different (a 10 HP three-phase at 230V is 28A; a 10 HP single-phase at 230V is 50A). Confirm phase before pulling a value.
  • Applying the 125% continuous-load rule to loads that are not continuous. The rule only applies to loads operating 3+ hours without interruption (NEC definition of "continuous load").

Try the Calculator

Use the interactive Wire Size Calculator for instant results with any values. Every result page shows the formula applied with your specific numbers.

All Formulas

functionsWatts to Amps Formula functionsOhm's Law Formula functionsEnergy Cost Formula functionsWire Sizing Formula functionsVoltage Drop Formula functionsThree-Phase Power functionsPower Factor functionsHP to Amps Formula functionsSeries & Parallel functionsAC vs DC functionsElectrical Units
This is a formula reference page for educational use. Always consult a licensed electrician and verify compliance with the National Electrical Code (NEC) and local electrical codes before applying any of these formulas to real installations.

Standards & References

This page cites the following electrical codes and standards. Always consult the current edition of your local adopted standard for authoritative requirements.

  1. NFPA 70 (National Electrical Code). The overarching US standard for safe electrical installation. Adopted (with amendments) by every US state as the legal code for residential, commercial, and industrial electrical work. Revised every three years by the National Fire Protection Association.
    National Fire Protection Association. Reference →
  2. NEC Table 310.16. Allowable ampacities of insulated conductors rated up to 2000V, 60°C through 90°C, not more than three current-carrying conductors in raceway or cable.
    National Electrical Code (NFPA 70), Article 310, Conductors for General Wiring. Reference →
  3. NEC 240.4(D). Small conductor rule: overcurrent protection shall not exceed 15A for 14 AWG, 20A for 12 AWG, and 30A for 10 AWG copper, regardless of ampacity table values.
    National Electrical Code (NFPA 70), Article 240, Overcurrent Protection. Reference →
  4. NEC 210.19(A) Informational Note 4. Branch-circuit conductors sized to prevent a voltage drop exceeding 3% at the farthest outlet. Combined with feeders, total voltage drop should not exceed 5%.
    National Electrical Code (NFPA 70), Article 210, Branch Circuits. Reference →
  5. NEC 215.2(A)(1) Informational Note 2. Feeder conductors sized to prevent a voltage drop exceeding 3%. Total branch + feeder drop should not exceed 5%.
    National Electrical Code (NFPA 70), Article 215, Feeders. Reference →
  6. NEC 430.22. Motor branch-circuit conductors supplying a single continuous-duty motor shall have an ampacity of not less than 125 percent of the motor full-load current rating. The conductor ampacity is taken from NEC Table 310.16 at the applicable temperature column, and motor branch circuits are exempt from the 240.4(D) small-conductor rule via 240.4(G).
    National Electrical Code (NFPA 70), Article 430, Motors, Motor Circuits, and Controllers. Reference →
  7. NEC 430.52 and Table 430.52(C)(1). Motor branch-circuit short-circuit and ground-fault protection. Maximum ratings from Table 430.52(C)(1): non-time-delay fuse 300%, dual-element (time-delay) fuse 175%, instantaneous-trip breaker 800%, and inverse-time breaker 250% of full-load amps. The elevated percentages allow for motor locked-rotor startup current.
    National Electrical Code (NFPA 70), Article 430, Motors, Motor Circuits, and Controllers. Reference →
  8. NEC Table 430.250. Full-load currents in amperes for three-phase alternating-current motors (squirrel-cage, wound-rotor, and synchronous), by horsepower and nominal voltage (115, 200, 208, 230, 460, 575, and 2300V). Used with NEC 430.6(A)(1) as the authoritative FLC source for three-phase motor branch-circuit sizing.
    National Electrical Code (NFPA 70), Article 430, Motors, Motor Circuits, and Controllers. Reference →
  9. NEC Table 430.248. Full-load currents in amperes for single-phase alternating-current motors, by horsepower and nominal voltage (115, 200, 208, and 230V). Used with NEC 430.6(A)(1) as the authoritative FLC source for single-phase motor branch-circuit sizing.
    National Electrical Code (NFPA 70), Article 430, Motors, Motor Circuits, and Controllers. Reference →

Disclaimer: The information on this page is provided for reference. Always consult a licensed electrician and the current edition of your local adopted electrical code before performing electrical work.

person Written by Sean Day verified Reviewed by WireResult update Last updated Apr 11, 2026