MC Cable Connector Spacing & Gutter Sizing

Calculate knockout center-to-center spacing for MC cable connectors, locknut clearances, and panel wire-bending gutters per NEC §330.30 & §312.6.

NEC edition

Inputs

cables
conductors

Bundling several MC cables into one enclosure for more than 24 inches triggers the §310.15(C)(1) adjustment factors

inches

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⚡ Test your knowledge
NEC Article 220 was renumbered in the 2026 code cycle. What is the new article number?
  • A. Art. 100
  • B. Art. 120
  • C. Art. 200
  • D. Art. 230
💡 NEC 2026 renumbered Article 220 (Branch-Circuit, Feeder, and Service Calculations) to Article 120.
Energy Management Systems were moved from Article 750 to which article in NEC 2026?
  • A. Art. 100
  • B. Art. 125
  • C. Art. 130
  • D. Art. 150
💡 NEC 2026 moved Energy Management Systems from Article 750 to Article 130.
Which NEC standard size overcurrent device comes after 90A?
  • A. 95A
  • B. 100A
  • C. 110A
  • D. 105A
💡 Per NEC §240.6(A), the standard sizes are …90, 100, 110, 125… There is no 95A standard size.

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How it works & the NEC rules behind it

When terminating multiple metal-clad (Type MC) cables into electrical panelboards, wireways, and junction boxes, electricians must maintain sufficient physical clearance between adjacent knockouts to ensure locknuts can be properly tightened without overlapping.

Core NEC Installation Rules:

  • 1. Knockout Center-to-Center Spacing: Standard 1/2" trade size knockouts require a minimum of 1.25 inches center-to-center to allow the outer flanges and locknut ridges of MC connectors (such as snap-in or set-screw connectors) to seat flat against the enclosure.
  • 2. Wire-Bending Space in Panel Gutters (Table 312.6(A)): The distance from the end of the lug to the opposing wall must satisfy mandatory wire deflection clearance. For one wire per terminal:
    • #14 to #10 AWG: Not specified in the table
    • #8 to #6 AWG: 1½ inches
    • #4 to #3 AWG: 2 inches
    • #2 AWG: 2½ inches
    • #1 AWG: 3 inches
    • #1/0 to #2/0 AWG: 3½ inches
    • #3/0 to #4/0 AWG: 4 inches
  • 3. Cable Bundling Derating (§310.15(C)(1) & §330.80): Where more than three current-carrying conductors are bundled together without maintaining spacing for longer than 24 inches, ampacity derating applies (e.g. 4–6 conductors = 80%, 7–9 conductors = 70%, 10–20 conductors = 50%).

Formulas & equations

Min Knockout Spacing = Connector Flange OD + Minimum Tool Clearance (≈ 1.25" for 1/2" KO) Total Enclosure Width Required = (Number of Cables − 1) × Center Spacing + (2 × Edge Margin) Min Gutter Bending Depth = Sized per NEC Table 312.6(A)

NEC reference table

MC Cable Gauge / Type Knockout Size Min Center-to-Center Spacing Min Wire-Bending Space
14/2, 12/2, 12/3 MC Lite1/2" Trade Size1.25 Inches1.5 Inches (Table 312.6(A))
10/2, 10/3 MC Cable1/2" or 3/4" KO1.375 InchesNot specified (Table 312.6(A))
8/3, 6/3 Feeder MC3/4" Trade Size1.50 Inches1.5 Inches (Table 312.6(A))
4/3 Feeder MC1" Trade Size1.75 Inches2.0 Inches (Table 312.6(A))
2/3 Feeder MC1-1/4" Trade Size2.00 Inches2.5 Inches (Table 312.6(A))
1/0 to 4/0 Feeder MC1-1/2" – 2" KO2.375 – 2.875 Inches3.5 – 4.0 Inches

Worked example, step by step

Example: Laying Out 8 Home-Run 12/2 MC Cables into a 20-Inch Panel Enclosure

Design: 8 × 12/2 MC cables entering the top of an electrical panelboard via 1/2" snap-in connectors.

  • 1. Center-to-Center Spacing: 1.25 inches between adjacent 1/2" knockouts.
  • 2. Total Array Span: (8 − 1) × 1.25" = 8.75 inches from center of first to last knockout.
  • 3. Edge Clearance: Adding 2.0" margin on each side requires an enclosure width of 8.75" + 4.0" = 12.75 inches (easily fits inside standard 14.25" stud cavity panel!).
  • 4. Wire-Bending Space: Requires minimum 1.5" vertical gutter space from KO entry to terminal strip.

Frequently asked questions

Type MC cable must be supported and secured at intervals not exceeding 6 feet (1.8 m), and within 12 inches (300 mm) of every box, cabinet, or fitting.

Under NEC §330.80 Exception, 12/2 and 14/2 solid copper conductor MC cables with no more than 3 current-carrying conductors are permitted to be bundled for up to 24 inches without applying the bundling derating factors of Table 310.15(C)(1).

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