6.1 Branch Circuit Sizing and Conductor Selection
Key Takeaways
- NEC 210.3 fixes branch-circuit ratings at the standard values in 240.6(A): 15, 20, 25, 30, 35, 40, 45, 50 A and larger standard sizes. The practical constraint is 240.4(D), which caps 14/12/10 AWG copper at 15/20/30 A — so 25 A and 35 A OCPDs, though standard, cannot be used on those small-conductor non-motor branch circuits.
- Continuous loads must be sized at 125% of their rated current: conductor ampacity and OCPD rating must both carry 125% of the continuous portion (NEC 210.19(A)(1) and 210.20(A)).
- The small-conductor rules of 240.4(D) cap the overcurrent protection of 14 AWG copper at 15 A, 12 AWG at 20 A, and 10 AWG at 30 A — regardless of the ampacity shown in Table 310.16.
- Conductor ampacity is selected from Table 310.16 using the column that matches the temperature rating of the TERMINATIONS, not the insulation rating of the conductor (110.14(C)).
- A 16 A continuous load requires a 20 A OCPD (16 × 1.25 = 20) and a 12 AWG copper conductor sized to the 60 °C column ampacity of 20 A.
Why Branch-Circuit Sizing Belongs on the Calculations Part
Quick Answer: Branch-circuit sizing is a high-yield topic on the Calculations part of the Texas Journeyman exam (26 questions, 110 minutes). Expect at least one item that hands you a continuous load and asks for both the minimum conductor size and the minimum OCPD rating — and another that tests whether you remember the 240.4(D) small-conductor caps. Get the 125% rule wrong and every downstream number is wrong.
A branch circuit is the portion of the wiring system extending past the final overcurrent device and delivering power to utilization equipment (NEC Article 100). Everything from a kitchen counter receptacle to a water heater is fed by a branch circuit, and Article 210 governs how those circuits are rated, conductored, and protected. Article 240 governs the overcurrent device itself. The Texas exam blueprint lists Branch Circuit Calculations & Conductors as Area 4, and roughly a third of the 26 Calculations items touch this material.
Standard Ratings — NEC 210.3 and 240.6(A)
NEC 210.3 declares that branch circuits rated 1200 A or less shall have ratings from the standard ampere ratings listed in 240.6(A). The standard sizes are:
| Standard OCPD ratings (240.6(A)) |
|---|
| 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200 A (continuing 1600, 2000, 2500, 3000, 4000, 5000, 6000 A) |
25 A and 35 A ARE standard ratings in 240.6(A). If a calculation yields a value between standard sizes, 240.4(B) lets you round up to the next standard size (e.g., 22.5 A → 25 A). The real constraint on small circuits is 240.4(D): a 25 A or 35 A OCPD cannot be used on a 14/12/10 AWG copper non-motor branch circuit because those conductors are capped at 15/20/30 A. Memorize the four smallest values — 15, 20, 30, 50 — because nearly every dwelling branch-circuit question lives in that range.
Identifying Branch Circuits — NEC 210.5
210.5 requires that conductors of different voltage systems be identified by color or marking. On a multiwire branch circuit the ungrounded conductors must be identified by phase: a 120/240 V single-phase system typically uses black for A, red for B, and white for the neutral. Texas exam items often test the identification requirement for circuits of different voltages run in the same raceway.
Conductor vs. OCPD — NEC 210.19 and 210.20
Two companion rules govern the relationship between conductor ampacity and overcurrent protection:
- 210.19(A) — conductors shall have an ampacity not less than the larger of (a) 125% of the continuous load plus 100% of the noncontinuous load, or (b) the maximum load after applying adjustment/correction factors.
- 210.20(A) — the OCPD shall be rated not less than 125% of the continuous load plus 100% of the noncontinuous load.
The two rules run in parallel. Size the conductor to 210.19, then size the breaker to 210.20, then confirm the breaker does not exceed the conductor ampacity (210.20 plus 240.4). For loads that are noncontinuous (operating less than 3 hours), 210.19(A)(1) Exception No. 1 lets you size the conductor to 100% of the load when the OCPD is rated 100% of the load — but only on circuits designed for that purpose.
The Continuous-Load 125% Rule
A continuous load (Article 100) is one expected to draw its maximum current for 3 hours or more. Examples the exam loves: water heaters, electric space heating, commercial refrigeration, data processing equipment, show-window lighting. For these, multiply the load current by 1.25:
Rated current × 1.25 = minimum conductor ampacity and minimum OCPD rating
If the result is not a standard OCPD size, round up to the next standard size per 240.6(A) — but 240.4(B) only lets you round the breaker up to the next size when the conductor ampacity does not equal a standard size. When the conductor ampacity equals a standard size, the breaker cannot exceed that ampacity.
Worked Example — 16 A Continuous Load
A 240 V electric heater draws 16 A continuously. Size the conductor and OCPD.
Step 1 — Apply the 125% rule:
16 A × 1.25 = 20 A
Step 2 — Minimum OCPD (210.20(A)): 20 A is a standard size in 240.6(A). Use a 20 A breaker.
Step 3 — Minimum conductor ampacity (210.19(A)(1)): must be ≥ 20 A.
Step 4 — Termination temperature (110.14(C)): for equipment rated 100 A or less, use the 60 °C column of Table 310.16 unless the equipment is marked for 75 °C. The 60 °C column gives 12 AWG copper = 20 A.
Step 5 — Small-conductor check (240.4(D)): 12 AWG copper is capped at 20 A by 240.4(D)(5). Our 20 A breaker matches the cap. Compliant.
| Result | Value |
|---|---|
| Minimum OCPD | 20 A (240.6(A)) |
| Minimum conductor | 12 AWG Cu, 60 °C column (Table 310.16) |
| 240.4(D) cap | 20 A — OK |
If the heater had been 18 A continuous: 18 × 1.25 = 22.5 A → next standard 240.6(A) rating is 25 A (25 A IS standard). A 25 A breaker on 12 AWG violates 240.4(D) (12 AWG cap = 20 A), so bump the conductor to 10 AWG Cu. 240.4(D) caps 10 AWG at 30 A, so a 25 A OCPD on 10 AWG Cu is compliant (25 A ≤ 30 A cap). Result: 10 AWG Cu / 25 A OCPD.
The Small-Conductor Rules — NEC 240.4(D)
Even when Table 310.16 shows a higher ampacity, 240.4(D) limits OCPD on the three smallest copper sizes. This is one of the most-tested rules on the exam.
| Copper AWG | Table 310.16 ampacity (60 °C) | 240.4(D) OCPD cap | Notes |
|---|---|---|---|
| 14 AWG | 15 A | 15 A | Practically equal — no headroom |
| 12 AWG | 20 A | 20 A | Matches the 60 °C column exactly |
| 10 AWG | 30 A | 30 A | Matches the 60 °C column exactly |
Exceptions in 240.4(D) waive the cap for motor circuits (Part XIII), air-conditioning/refrigeration (Part IV), and certain resistive welders. When the question involves a motor, use 430.22 and Table 430.250, not 240.4(D).
Termination Temperature — NEC 110.14(C)
The single most common sizing mistake is reading the wrong column. 110.14(C)(1)(a) says: for equipment rated 100 A or less (or marked for 14 AWG through 1 AWG conductors), conductor selection is based on the 60 °C column. 110.14(C)(1)(b) lets you use the 75 °C column only when the equipment is marked 75 °C. The 90 °C column is reserved for derating — you start at 90 °C, apply temperature correction (Table 310.15(B)(1)) and ampacity adjustment (Table 310.15(C)(1)), and the resulting ampacity must still meet the termination-temperature limit.
For a 12 AWG copper THHN conductor: 90 °C column shows 30 A, but if the breaker is rated 20 A (≤ 100 A equipment), you select from the 60 °C column = 20 A. The 90 °C number (30 A) is used only as the starting point for derating.
Putting It Together — Sizing Sequence
- Determine the load current (nameplate, or calculated per Article 220).
- Identify continuous vs. noncontinuous. Apply 125% to the continuous portion.
- Add noncontinuous portion at 100%.
- Select OCPD from 240.6(A) — round up to next standard size if needed.
- Select conductor ampacity ≥ the same number from Table 310.16 using the correct column (110.14(C)).
- Apply 240.4(D) small-conductor cap.
- Apply temperature correction and ampacity adjustment if more than 3 current-carrying conductors or ambient > 30 °C.
Memorize this sequence. The Calculations items are built around it, and a single skipped step (forgetting 240.4(D) or using the 90 °C column) flips a right answer into a wrong one.
A 240 V commercial water heater draws 14 A continuously. What is the minimum OCPD rating per NEC 210.20(A) and 240.6(A)?
Using the 60 °C column of Table 310.16 and the 240.4(D) small-conductor rules, what is the maximum permitted OCPD for a 10 AWG copper conductor feeding a non-motor load?
A 12 AWG copper THHN conductor is landed on a 20 A circuit breaker that is not marked 75 °C. Which ampacity column of Table 310.16 governs the conductor's selection under NEC 110.14(C)(1)(a)?