13.2 Motor Branch-Circuit Conductors

Conductor Sizing After You Have the Right FLC

Section 13.1 locked the current source: Table FLC. This section applies Article 430 Part II conductor rules that produce the wire size Prov expects on Motors and Controls items.

On Arkansas’s open-book form, the arithmetic is simple — FLC × 1.25 — but candidates lose points by (1) using nameplate, (2) forgetting 125%, or (3) picking a Table 310.16 size from the wrong temperature column.

Tab 430.22, 430.24, Table 430.250, Table 310.16, and 110.14(C).

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Single Continuous-Duty Motor — 430.22(A)

430.22(A) general rule: conductors that supply a single motor used in a continuous-duty application shall have an ampacity of not less than 125% of the motor’s full-load current as determined by 430.6(A) — meaning Table FLC for ordinary motors.

Worked example A — 10 hp, 460 V, 3φ

1. Table 430.250 FLC for 10 hp @ 460 V ≈ 14 A (verify).
2. Minimum conductor ampacity = 14 × 1.25 = 17.5 A.
3. From Table 310.16, 12 AWG copper in the 75°C column is typically 25 A — adequate for 17.5 A after termination rules are respected.
4. Do not stop at “14 A needs 14 AWG.” The motor rule is 125%, so 17.5 A is the floor before ampacity table selection.

Worked example B — 25 hp, 230 V, 3φ

1. Table 430.250 FLC for 25 hp @ 230 V ≈ 68 A (verify).
2. Minimum ampacity = 68 × 1.25 = 85 A.
3. Select copper (or aluminum if the stem says so) with allowable ampacity ≥ 85 A at the permitted termination temperature — often 4 AWG Cu or 3 AWG Cu depending on column and conditions (open Table 310.16; do not memorize from a blog).

Worked example C — single-phase shop motor

A 5 hp, 230 V, single-phase motor uses Table 430.248 FLC (commonly 28 A — verify), then 28 × 1.25 = 35 A minimum ampacity. Mixing in the three-phase 15.2 A value is a classic wrong path.

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What “Continuous Duty” Means Here

Most general-purpose motors on Prov stems are treated as continuous duty unless the question specifies intermittent, periodic, or varying duty and points you to other rows in 430.22 / duty-cycle tables. If the stem is silent, apply 430.22(A) 125%.

Other duty types can require different percentages of FLC for conductors — open the specific subsection only when the stem names the duty cycle. Do not invent a reduced percentage for a silent continuous-duty motor just because the machine “only runs eight hours.”

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Multi-Motor and Combination Loads — 430.24

430.24 covers conductors supplying several motors, or motors plus other loads. The core pattern for several motors:

• Ampacity ≥ 125% of the highest-rated motor FLC + sum of the FLC of all other motors + other load currents as required.

Worked example D — three motors on one feeder/circuit conductor set

Motors (all 460 V, 3φ; FLCs from Table 430.250 — verify):

Highest-rated motor FLC = 14 A.

Minimum conductor ampacity = (14 × 1.25) + 7.6 + 4.8 = 17.5 + 12.4 = 29.9 A.

Then select conductors ≥ 29.9 A from Table 310.16 under the correct temperature column.

Trap: Applying 125% to every motor (14×1.25 + 7.6×1.25 + 4.8×1.25) oversizes relative to 430.24 and will not match the keyed answer. Only the largest motor gets the 125% factor in this combination rule.

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Termination Temperature Still Applies

Motor circuits do not escape 110.14(C). If the breaker and starter lugs are 75°C-rated, you generally select ampacity from the 75°C column of Table 310.16 even when the insulation is THHN 90°C. You may still use 90°C ampacity as a starting point for adjustment/correction where the Code allows, but the equipment termination limit controls final sizing to the terminations.

Also apply 310.15 ambient correction and more-than-three current-carrying-conductor adjustment when the stem gives those facts. Compute:

1. Motor minimum from 430.22 / 430.24.
2. Adjusted/corrected ampacity of the chosen size.
3. Confirm adjusted ampacity still ≥ the motor minimum.

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Motor Circuit Equipment Grounding Conductors

The branch-circuit EGC is sized from Table 250.122 based on the rating of the branch-circuit short-circuit and ground-fault protective device, not from 125% of FLC and not from 250.66. Article 430 tells you how to pick the SC/GF device (§13.4); Article 250 tells you the EGC once that device rating is known.

If a stem asks for the EGC of a motor branch circuit protected by a 70 A breaker, open 250.122 for 70 A — do not open Table 430.250 again looking for a grounding column that is not there.

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Conductors on the Load Side of a Drive or Soft Starter

When an adjustable-speed drive or similar equipment is involved, manufacturer markings and Article 430 drive-related sections can change which current is used for load-side conductors. On Prov JW items, if the stem is a plain across-the-line motor starter, stay with 430.22 and Table FLC. If the stem explicitly names a VFD and input/output currents, read the applicable drive conductor rules rather than blindly multiplying motor FLC.

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Exam Workflow — Branch Conductor Stems

1. Identify single motor vs several motors / other loads.
2. Pull Table FLC (correct table and voltage).
3. Apply 430.22(A) (×1.25) or 430.24 (1.25× largest + others).
4. Select from Table 310.16 with 110.14(C).
5. Adjust for ambient/bundling if given.
6. Only then move to overload (§13.3) or SC/GF (§13.4) — those are different percentages and sometimes different currents.

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Timed Open-Book Tips for §13.2

• Scratch-paper line: FLC = ____; ×1.25 = ____ A min.
• Multi-motor: star the largest FLC before multiplying.
• Wire size ≠ breaker size; finish conductors before Table 430.52.
• Nameplate FLA on the page is usually a distractor for this section.
• If ampacity after adjustment falls below 17.5 A (or whatever minimum you calculated), upsize — do not ignore 310.15 facts the stem handed you.