7.2 Small-Conductor and Continuous-Load Rules

Why This Matters for the TX Journeyman Exam

The Calculations part of the Texas Journeyman exam almost always includes one or two questions that hinge on 240.4(D) small-conductor limits and one or two on the 240.4(B) next-size-up rule. The Knowledge part tests the exceptions to 240.4(D). Miss the distinction between (B) and (D) and you will mis-size feeders and branch circuits across the entire exam.

NEC 240.4(A) — The General Rule

NEC 240.4(A) states the general rule: conductors shall be protected against overcurrent in accordance with their ampacities specified in 310.14. That sets the baseline — the OCPD rating cannot exceed the conductor ampacity — and 240.4(B), (D), and the tap rules in 240.21 are all exceptions and refinements to that general rule.

NEC 240.4(D) — Small-Conductor Limits

240.4(D) sets hard maximum OCPD sizes for small conductors, regardless of the ampacity shown in Table 310.16. The rule applies to copper and aluminum at the sizes listed.

Even though Table 310.16 lists 14 AWG copper THWN at 20 A (75 °C column) and 12 AWG at 30 A, 240.4(D) caps the OCPD at 15 A and 20 A respectively for these small conductors. The ampacity is not lowered — the OCPD is.

Exceptions to 240.4(D)

The small-conductor cap does not apply to specific equipment whose own article governs overcurrent. The exceptions the exam expects you to know:

• Motors — governed by 430 Part III and Part XIII (branch-circuit short-circuit/ground-fault per 430.52, overload per 430.32). A 12 AWG copper conductor feeding a motor can be protected at more than 20 A under 430.52.
• Air-conditioning and refrigeration equipment — governed by Article 440. The nameplate MCA and MOCP values control, not 240.4(D).
• Electric welders — governed by Article 630. The conductor and OCPD are sized from the welder's rated primary current and duty cycle.
• Tap conductors under 240.21.

If a question names a motor, an A/C condenser, or a welder, do not apply 240.4(D). Apply the equipment article.

NEC 240.4(B) — Next-Size-Up Rule

240.4(B) allows the OCPD to be raised to the next higher standard rating when the conductor ampacity does not equal a standard OCPD rating. Two conditions must both be met.

1. The conductor ampacity does not equal a standard rating listed in 240.6(A).
2. The next lower standard OCPD rating is the limiting value — i.e., the conductor ampacity falls between two standard ratings, so the next higher standard size is used.

The rule lets you round up one step to the next standard size only. You may never round down, and you may never go more than one step up. Above 800 A, 240.4(B) does not apply — the OCPD rating cannot exceed the conductor ampacity.

Worked Example — 240.4(B)

A feeder is run with 4 AWG copper THWN in a 75 °C raceway. Table 310.16 lists 4 AWG copper at 85 A in the 75 °C column. What is the maximum standard OCPD permitted?

Step 1 — Find the conductor ampacity. 4 AWG Cu THWN, 75 °C column = 85 A.

Step 2 — Check 240.6(A) standard ratings around 85 A. Standard ratings are ... 70, 80, 90, 100 ... 85 A is not a standard rating.

Step 3 — Apply 240.4(B). The conductor ampacity (85 A) falls between 80 A and 90 A. The next higher standard rating above 85 A is 90 A.

Result: The maximum OCPD is 90 A. The 80 A standard breaker is allowed (next lower), but 240.4(B) lets you use a 90 A breaker because 85 A is not standard. You may not go to 100 A — only one step.

When 240.4(B) Does NOT Apply

• When the conductor ampacity equals a standard rating — use that rating, no bump.
• For equipment rated over 800 A — the OCPD cannot exceed the conductor ampacity.
• For small conductors governed by 240.4(D) unless an exception (motor/A/C/welder) applies.

NEC 240.4(A), 240.21 — Tap Rules (Brief)

The general rule in 240.4(A) is that conductors are protected at ampacity. Tap rules in 240.21 allow shorter conductor runs to be sized smaller than the upstream OCPD when physical conditions limit fault duration. The common taps:

• 10 ft tap (240.21(B)(1)) — tap not less than 1/10 the rating of the upstream OCPD, in a raceway or cable, length not more than 10 ft.
• 25 ft tap (240.21(B)(2)) — tap not less than 1/3 the rating of the upstream OCPD, length not more than 25 ft, and the tap must terminate in a single set of fuses or breaker sized at not more than the tap ampacity.
• Outdoor tap (240.21(B)(5)) — for outdoor line tap, with the OCPD at the building or structure per 225.
• For 800 A+ feeders, the high-fault tap rule (240.21(B)(3)) applies, with specific minimum sizes and clearing-time requirements.

Tap rules are exam-relevant in the Calculations part for feeder and service-entrance questions. The 1/10 and 1/3 multipliers are the values to memorize.

Continuous Loads — 125% and the 210.19(A)/210.20(A) Link

A continuous load is defined in Article 100 as a load where the maximum current is expected to continue for 3 hours or more. When a load is continuous, two related rules apply.

• 210.19(A) — branch-circuit conductors are sized at 125% of the continuous load plus 100% of the noncontinuous load.
• 210.20(A) — the branch-circuit OCPD is sized at 125% of the continuous load plus 100% of the noncontinuous load.

The 125% factor and the next-size-up rule interact: you compute the OCPD at 125% of the continuous portion, then apply 240.4(B) if that value is not a standard rating. Chapter 6 covers full branch-circuit sizing; the key here is that overcurrent protection for continuous loads is always bumped by the 125% factor before any 240.4(B) adjustment.

Worked Example — Continuous Load + 240.4(B)

A 208 V single-phase branch circuit feeds a continuous load of 56 A. Size the OCPD.

Step 1 — Apply 210.20(A). OCPD = 125% × 56 A = 70 A.

Step 2 — Check 240.6(A). 70 A is a standard rating.

Result: Use a 70 A OCPD. No 240.4(B) bump is needed because 70 A is already standard. If the continuous load had been 60 A, then 125% × 60 = 75 A, which is not standard, and 240.4(B) would let you round up to 80 A.