5.2 Feeder Sizing, Ampacity, and Tap Rules

Feeder Sizing, Ampacity, and Tap Rules

Quick Answer: A feeder is the conductors between the service equipment (or a separately derived system source) and the final branch-circuit overcurrent device. NEC Article 215 sets the minimum conductor ampacity and overcurrent protection for feeders; Table 310.16 supplies the base ampacity; 310.15(B) and 310.15(C)(1) apply ambient and bundling adjustments; and 240.21 plus 215.12 permit short reduced conductor "taps" under specific conditions. This is high-yield on both the Knowledge and Calculations parts of the Texas Journeyman exam.

NEC 215.2 — Minimum Feeder Conductor Ampacity

NEC 215.2(A)(1) requires feeder conductors to have an ampacity not less than the larger of:

1. 125% of the continuous load plus 100% of the noncontinuous load, or
2. The rating of the feeder overcurrent protective device after any applicable 240.4(B) next-size-up.

The 125% factor for continuous loads matches the branch-circuit rule in 210.19(A)(1) and reflects the fact that a continuous load (operating for 3 hours or more at its maximum) drives the conductor toward its steady-state thermal limit. A feeder sized at exactly 100% of a continuous load would overheat.

NEC 215.3 — Feeder Overcurrent Protection

NEC 215.3 requires the feeder OCPD to be rated not greater than the ampacity of the feeder conductors after applying 240.4(B) (next standard size up when the conductor ampacity does not match a standard OCPD rating). Standard OCPD ratings are listed in 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, 1600, 2000.

Base Ampacity — Table 310.16

Table 310.16 lists allowable ampacities for insulated conductors rated up to and including 2000V, in raceway, cable, or earth, based on an ambient temperature of 30°C (86°F) and not more than three current-carrying conductors in a raceway or cable. Selected copper values:

Ambient Temperature Correction — 310.15(B)

Table 310.15(B)(1) provides correction factors when ambient differs from 30°C. For THWN (75°C column) at 40°C ambient, the factor is 0.88; at 35°C it is 0.94; at 45°C it is 0.82. Always select the column matching the conductor's insulation rating and the equipment termination temperature rating (110.14(C) typically limits terminations to 75°C for sizes 1 AWG and larger).

Adjustment for Number of Current-Carrying Conductors — 310.15(C)(1)

When more than three current-carrying conductors are in a raceway or cable, Table 310.15(C)(1) requires derating:

A neutral that carries only the unbalanced current of a 3-wire single-phase circuit (or 4-wire 3-phase wye circuit with balanced harmonic loads) is NOT counted as a current-carrying conductor per 310.15(C)(1) Exception No. 1. On the exam, a 3-wire single-phase feeder with one neutral = 3 current-carrying conductors (neutral not counted), but a 4-wire 3-phase wye feeder with non-linear loads counts the neutral.

Worked Example — Size a Feeder for a 225A Panelboard

Problem: A 225A panelboard is fed by a feeder in a raceway. There are 4 current-carrying conductors (3 phase + neutral counted because of non-linear loads) in one raceway, and the ambient temperature is 40°C. Terminations are rated 75°C. Size the feeder conductor and OCPD.

Step 1 — Required ampacity. Panelboard is rated 225A; treat as the feeder OCPD. Required conductor ampacity = 225A minimum (assuming the 225A load is the basis; if any portion is continuous, apply 125% per 215.2(A)(1) to that portion — here we assume the 225A already accounts for continuous load).

Step 2 — Apply correction and adjustment. Starting from the 75°C column of Table 310.16:

• 4/0 AWG THWN = 230 A base.
• Ambient 40°C correction (75°C column) = × 0.88 → 230 × 0.88 = 202.4 A.
• 4 current-carrying conductors → 80% adjustment → 202.4 × 0.80 = 161.9 A.

161.9 A is less than 225 A — 4/0 AWG is too small.

Try 300 kcmil THWN:

• Base = 285 A.
• × 0.88 = 250.8 A.
• × 0.80 = 200.6 A. Still under 225 A.

Try 400 kcmil THWN:

• Base = 335 A.
• × 0.88 = 294.8 A.
• × 0.80 = 235.8 A. This exceeds 225 A — 400 kcmil THWN is adequate.

Step 3 — Verify the OCPD. 215.3 limits the OCPD to the conductor ampacity after adjustment (235.8 A). The next standard size up per 240.6(A) that does not exceed 235.8 A is 225 A (no next-size-up needed). The 225A OCPD is permitted.

Answer: 400 kcmil THWN copper with a 225A OCPD. The exam may show 350 kcmil (310 A base → 218 A after derating) as a distractor — 218 A is below 225 A, so 350 kcmil fails.

Tap Rules — 240.21 and 215.12(A)(2)

Tap rules permit a short section of conductor smaller than the feeder OCPD rating between the OCPD and the load. There are two feeder-relevant taps:

10-Foot Tap — 240.21(A)(4) (also 215.12(A)(2) for feeders)

Conditions:

• Tap conductor length not more than 10 ft.
• Tap conductor ampacity not less than the load it feeds (and not less than the rating of the OCPD protecting the tap's load).
• The tap is in a raceway or enclosure.
• The tap conductor does not extend beyond the tap enclosure.

25-Foot Tap — 240.21(B)(2)

Conditions:

• Tap conductor length not more than 25 ft.
• Tap conductor ampacity not less than 1/3 of the rating of the OCPD protecting the feeder.
• The tap is protected from physical damage by a raceway or other approved means.
• The tap terminates in a single OCPD (or set) that limits the load to the tap conductor ampacity.

Example: A 400A feeder OCPD feeds a 25 ft tap. Minimum tap ampacity = 400 / 3 = 133 A. A 1/0 AWG THWN copper conductor (150 A at 75°C) satisfies the 1/3 rule.

Feeder OCPD Location — 215.12

NEC 215.12 requires the feeder overcurrent device to be located at the supply side of the feeder — i.e., at the source end, not the load end. This is the mirror of the branch-circuit rule (210.8 GFCI protection is at the source). The tap rules above are the only Code-permitted exceptions that allow a reduced-size conductor downstream of the feeder OCPD.

Common Exam Traps

• Forgetting to apply both the ambient correction and the conductor count adjustment — the corrected ampacity is base × correction × adjustment, applied together, not sequentially to round numbers.
• Using the 90°C column for the final ampacity when terminations are 75°C. The 90°C column is permitted only for derating the starting point; the final ampacity after derating cannot exceed the 75°C column value (110.14(C)).
• Counting a neutral that does not qualify. A 3-wire single-phase wye-neutral circuit with only line-to-neutral loads does NOT count the neutral (Exception No. 1).
• Mis-applying the 25-foot tap rule by sizing the tap at the load instead of 1/3 of the OCPD rating.

Takeaways

• Feeder ampacity = larger of (125% continuous + 100% noncontinuous) or the OCPD rating (215.2(A)(1)).
• Start at Table 310.16; apply 310.15(B) ambient correction and 310.15(C)(1) conductor-count adjustment together.
• 10-foot tap: ampacity ≥ the load served, in a raceway. 25-foot tap: ampacity ≥ 1/3 of the feeder OCPD, protected from damage.
• OCPD is at the supply side (215.12); taps are the only permitted reduced-conductor exceptions.