Page review information
Edition covered
NEC Art. 430 / Art. 310 / NFPA 79 / IEC 60204-1 — NEC 2023, NFPA 79:2024, IEC 60204-1:2016
Content status
Reviewed
Coverage
Single-motor branch circuit walkthrough; feeders, taps, and cable-tray ampacity are only pointed at, not worked.
Last reviewed
August 2026
Primary reference
The official published standard — required for final decisions

Confirm the governing edition, jurisdiction, and project-specific requirements before applying anything on this page.

Wiring & Installation

Wire Sizing Walkthrough — Nameplate to Conductor, OCPD, and Overload

One 10 hp motor circuit worked end to end, with every step computed by the cst toolkit and cited to the article that requires it.

Safety. This guide is educational, not a work instruction. Electrical work is performed de-energized and verified by qualified personnel under your site’s LOTO procedures, following the device manufacturer’s manual and the authority having jurisdiction.

Overview

Wire sizing is a decision chain, not a table lookup. Each link has its own rule, its own basis current, and its own failure mode when skipped:

flowchart LR
  A[Load-current basis\ntable FLC or\ncontinuous load] --> B[Min. required\nampacity ×1.25]
  B --> C[Conductor selection\nafter corrections\nand adjustments] --> D[Termination check\n110.14 C concept]
  D --> E[Voltage-drop check\ndoes VD govern?] --> F[OCPD\nTable 430.52 basis\nnext size up 240.6 A]
  F --> G[Overload\nnameplate FLA\n430.32 basis]

This guide works the whole chain once, for a single three-phase motor branch circuit, using the engineering toolkit so every number arrives with its citation. Unlike the other wiring guides, the power and control sections here merge into one worked walkthrough — the chain is the content. Multi-motor feeders, tap rules, and cable-tray ampacity are out of scope (see the Art. 430 workflow page).

Sample-data notice — mandatory reading. Every toolkit output below is computed from clearly marked SAMPLE table data. The rule logic is faithful to the cited articles, but the table values are demonstration stand-ins. Before any design use, transcribe values from your own licensed copy of the NEC into data/standards_tables/ — the tool warns on every run until you do.

Before You Start

Data you need in hand before sizing anything:

What governs where. The NEC (NFPA 70) governs premises wiring up to the machine; NFPA 79 governs conductors inside the industrial machine’s electrical equipment (Ch. 12 aligns with, but is not identical to, Art. 310); IEC 60204-1 (Cl. 7 and Cl. 12) is the international counterpart with its own base tables. This walkthrough uses NEC numbering; the sequence is the same in all three.

The FLA vs FLC rule (Art. 430.6(A) principle). Conductor, OCPD, and disconnect sizing use the NEC full-load current tables (430.247/.248/.250), not the nameplate — deliberately, so the installation doesn’t fail when a replacement motor draws slightly more. Only the overload setting uses nameplate FLA, because it protects the specific motor installed. This distinction drives most of the walkthrough.

Sizing & Protection — the Walkthrough

The circuit: 10 hp, 460 V, three-phase, Design B, service factor 1.15, nameplate FLA 13.2 A. 100 ft one-way run in EMT shared with another motor circuit — 4 current-carrying conductors. Worst-case ambient 40 °C. Inverse-time breaker, 75 °C terminations (verified from the device manuals), THHN copper.

Step 1–2 — Basis current, minimum ampacity, OCPD, overload in one shot

cst motor-branch runs the whole Art. 430 chain:

$ cst motor-branch --hp 10 --volts 460 --nameplate-fla 13.2
Motor branch circuit — 10 hp, 460 V, 3-phase: 17.5 A (min conductor ampacity)
  table_flc_a: 14
  ocpd_rating_a: 35
  overload_max_a: 16.5
  overload_percent: 125
  Warnings:
    ! Loaded SAMPLE table data — demonstration values only; verify ...
  References:
    - NEC 2023 Table 430.250 — table FLC 14 A (SAMPLE data)
    - NEC 2023 430.22 — branch conductors >= 125 % of table FLC
    - NEC 2023 Table 430.52(C)(1) + Exc. 1 — inverse time breaker max 250 %
    - NEC 2023 240.6(A) — standard rating selected: 35 A
    - NEC 2023 430.32(A)(1) — overload at 125 % of nameplate FLA

Reading it back: table FLC is 14 A (not the 13.2 A nameplate); minimum conductor ampacity is 14 × 1.25 = 17.5 A per 430.22 — the same continuous-load 125% concept 210.19(A)/215.2(A) apply to general circuits. The OCPD lands at 14 × 250% = 35 A — already a standard 240.6(A) rating; had the multiplier produced 33 A, the next standard size up would be permitted by 430.52(C)(1) Exception 1. The overload maximum is 13.2 × 1.25 = 16.5 A — nameplate, not table, per 430.32(A)(1).

Notice the OCPD (35 A) is larger than the conductor requirement (17.5 A). For motor circuits this is correct: the breaker handles short-circuit and ground fault only and must ride through starting inrush; the overload relay covers sustained overcurrent. Do not “fix” it.

Step 3 — Conductor selection with corrections and adjustments

Table ampacities assume 30 °C ambient and ≤ 3 current-carrying conductors. This run violates both, so the table value gets corrected (ambient, via the 310.15(B) equation) and adjusted (Table 310.15(C)(1)). First try 12 AWG:

$ cst ampacity --awg 12 --ambient 40 --ccc 4
Allowable ampacity (12 AWG CU, 75 degC col.): 17.6383 A
  base_ampacity_a: 25
  correction_factor: 0.881917
  adjustment_factor: 0.8

12 AWG survives at 17.64 A vs 17.5 A required — a 0.14 A margin. Legal, but a fifth conductor pulled into that EMT next year, or a 45 °C summer, sinks it. Generally accepted practice is to step up on margins this thin:

$ cst ampacity --awg 10 --ambient 40 --ccc 4
Allowable ampacity (10 AWG CU, 75 degC col.): 24.6937 A
  base_ampacity_a: 35
  correction_factor: 0.881917
  adjustment_factor: 0.8
  References:
    - NEC (NFPA 70) Table 310.16 — base ampacity 35 A (SAMPLE data)
    - NEC 2023 310.15(B) Equation — ambient correction 0.882 at 40 degC
    - NEC 2023 Table 310.15(C)(1) — adjustment 80% for 4 CCCs

10 AWG Cu: 24.7 A ≥ 17.5 A with real margin. Both runs used the 75 °C column because of the terminations — the 110.14(C) concept: THHN’s 90 °C rating may serve as headroom for corrections, but the ampacity delivered to a 75 °C lug may never exceed the 75 °C column value.

Step 4 — Voltage drop: does it govern?

The NEC treats voltage drop as informational (210.19(A) Info. Note 4: 3% branch/feeder, 5% combined); NFPA 79 and IEC 60204-1 reach it through the requirement that equipment work across the supply-tolerance band. Check the selected conductor at load current over the 100 ft run:

$ cst voltage-drop --amps 14 --feet 100 --volts 460 --phases 3 --awg 10
Voltage drop (10 AWG CU, 3-phase): 3.01269 V
  percent_drop: 0.654933
  voltage_at_load: 456.987
  Assumptions:
    - K = 12.9 ohm-cmil/ft; reactance neglected (<= ~4/0, ~unity PF)

0.65% — ampacity governs this run, as is typical at 460 V in-plant. The balance flips on runs of several hundred feet, and almost immediately at 120 V AC or 24 V DC, where voltage drop usually selects the conductor and the ampacity table merely confirms it. Whichever check demands the larger conductor wins; run both every time.

Step 5 — Result

Item Value Basis
Table FLC 14 A Table 430.250 (sample data — verify)
Min. conductor ampacity 17.5 A 430.22 (125% of table FLC)
Conductor 10 AWG Cu THHN Table 310.16 at 75 °C, corrected 0.882, adjusted 0.80 → 24.7 A
Voltage drop 0.65% at 100 ft Ch. 9 Table 8 K-factor method; does not govern
OCPD 35 A inverse-time breaker Table 430.52(C)(1) 250%; 240.6(A) standard rating
Overload ≤ 16.5 A 430.32(A)(1), 125% of nameplate 13.2 A

The equipment grounding conductor follows the 35 A OCPD (Table 250.122 basis); if the breaker is later upsized under the starting exception, repeat the EGC check. Termination torque values are vendor-specific — consult the device manual and record actual torques.

Common Mistakes

  1. Everything sized from nameplate FLA. Conductor and OCPD come out slightly small, pass commissioning, then a replacement motor with a higher-draw nameplate makes the installation non-compliant. Shows up years later as an inspection finding or trips after a motor swap.
  2. Termination ratings ignored. The 90 °C THHN column gets used directly against 75 °C lugs. Shows up as discolored lug insulation and high-resistance joints found by thermography — the conductor was fine, the termination was cooking.
  3. Bundle adjustment forgotten in loaded wireways. Each circuit added was “fine” calculated alone at ≤ 3 conductors. Shows up as a warm wireway and embrittled insulation on the oldest runs in it.
  4. 90 °C ampacity used with 75 °C terminations even after corrections. Correcting from the 90 °C column is legitimate; delivering more than the 75 °C column value to the lug is not. The subtle version of mistake 2 — it survives review because the correction math looks diligent.
  5. Voltage drop skipped on long runs, then blamed on the drive. The 460 V habit (“VD never governs”) gets carried to a 350 ft run or a 24 V DC valve bank. Shows up as undervoltage faults at start, chattering relays, and analog readings that sag with load — after weeks of drive-parameter tinkering that could never have fixed it.
  6. OCPD maximum confused with conductor protection. Someone “corrects” the 35 A breaker over 17.5 A conductors down to 20 A and the motor can’t start; or, inverted, sizes the conductor up to the breaker and pulls 8 AWG everywhere. The Table 430.52 OCPD is short-circuit and ground-fault protection only; the overload relay covers overcurrent.
  7. Next-standard-size-up applied where it isn’t permitted. The 240.6(A) round-up habit from motor branch OCPDs gets applied to overload settings or other cases with their own rounding rules — verify which permission you’re actually using.

Verification Checks

Standards References

Trust Boundary — Engineering Judgment Required

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