Comprehensive Guide to NEC Requirements for Metal and Nonmetallic Wireways: Cross-Sectional Area, Fill Limits, and Ampacity Adjustments
Comprehensive Guide to NEC Requirements for Metal and Nonmetallic Wireways: Cross-Sectional Area, Fill Limits, and Ampacity Adjustments
When working with metal (376) and nonmetallic (378) wireways, it’s essential to follow the guidelines set by the National Electrical Code (NEC) for cross-sectional area, conductor fill limits, and ampacity adjustments. These rules ensure safety by managing heat dissipation, conductor spacing, and proper overcurrent protection. While the rules for metal and nonmetallic wireways are similar in some respects, key differences—such as how ampacity adjustments are applied—must be clearly understood.
What is Cross-Sectional Area?
To understand cross-sectional area, imagine a wireway as a loaf of bread. Now, think about cutting a single slice of that bread and looking at the flat surface of the slice. That flat surface represents the cross-sectional area of the wireway, which is the internal space where all conductors pass through.
The cross-sectional area of a wireway is measured in square inches and is calculated using the wireway's internal width and height.
Example:
- For a 4-inch x 4-inch wireway:
- Internal dimensions = 4 inches x 4 inches.
- Cross-sectional area calculation:
Cross-sectional area = Width × Height
Cross-sectional area = 4 in × 4 in = 16 in²
NEC Fill Limits for Wireways
Metal Wireways (NEC 376.22(A))
If the metal wireway contains only conductors (without splices, taps, or devices), the maximum allowable fill is 20% of the cross-sectional area. This ensures adequate airflow and prevents overheating.
Example:
- For a 4-inch x 4-inch metal wireway with a cross-sectional area of 16 in²:
Allowable fill = Cross-sectional area × 20%
Allowable fill = 16 in² × 0.20 = 3.2 in²
Nonmetallic Wireways (NEC 378.22)
The same 20% fill limit applies to nonmetallic wireways. However, the material difference between metal and nonmetallic wireways impacts when ampacity adjustments begin, as explained later.
Splices, Taps, or Devices (NEC 376.56 and 378.56)
For both metal and nonmetallic wireways, if splices, taps, or devices are present, the allowable fill increases to 75% of the cross-sectional area.
Example:
- For a 4-inch x 4-inch wireway (metal or nonmetallic) with a cross-sectional area of 16 in²:
Allowable fill = Cross-sectional area × 75%
Allowable fill = 16 in² × 0.75 = 12 in²
Ampacity Adjustments in Wireways (NEC 376.22(B) and 378.22)
Metal Wireways (NEC 376.22(B))
For metal wireways, ampacity adjustments under 310.15(C)(1) only apply if there are more than 30 current-carrying conductors at any cross-sectional area. Conductors beyond the 30-count threshold require adjustment factors to account for reduced heat dissipation.
Nonmetallic Wireways (NEC 378.22)
In nonmetallic wireways, 310.15(C)(1) adjustments must be applied as soon as there are 3 or more current-carrying conductors in the wireway. Unlike metal wireways, nonmetallic wireways lack provisions that delay adjustment factors until the conductor count exceeds 30.
Ampacity Adjustment Factors (310.15(C)(1))
When there are multiple current-carrying conductors in a wireway, the ampacity of each conductor must be adjusted based on the following table:
Number of Current-Carrying Conductors | Adjustment Factor |
---|---|
1–3 | No adjustment |
4–6 | 80% |
7–9 | 70% |
10–20 | 50% |
21–30 | 45% |
31–40 | 40% |
41+ | 35% |
Example Calculation: 35 Current-Carrying Conductors in a Metal Wireway
Scenario:
- Wireway: 6-inch x 6-inch metal wireway
- Conductors: 35 current-carrying conductors (CCC)
- Conductor type: 12 AWG THHN, initial ampacity = 30 amps (from 90°C column)
Step 1: Cross-Sectional Area
Cross-sectional area calculation:
Width × Height = 6 in × 6 in = 36 in²
Step 2: Check Allowable Fill
Allowable fill calculation:
Allowable fill = Cross-sectional area × 20%
Allowable fill = 36 in² × 0.20 = 7.2 in²
Total conductor area calculation:
Total conductor area = 35 × 0.0133 in² = 0.4655 in²
Step 3: Apply Adjustment Factors
Adjustment factor for 35 conductors = 40%:
Adjusted ampacity = Initial ampacity × Adjustment factor
Adjusted ampacity = 30 amps × 0.40 = 12 amps
Step 4: Ensure Overcurrent Protection
Conductors must be protected at 12 amps. The next size up rule may apply if the adjusted ampacity complies with 240.4(B). However, if the conductor is being used for circuits that require a 20-amp minimum, such as those outlined in 210.11(C) (e.g., small appliance, laundry, or bathroom branch circuits), this rule will not apply. While the conductor could theoretically be protected at 15 amps, it would not meet the 20-amp circuit requirement.
Key Takeaways
- 376.22(A) and 378.22: Both have a 20% fill limit.
- Ampacity Adjustments:
- Metal wireways: Start adjustments at 31 conductors.
- Nonmetallic wireways: Start adjustments at 3 conductors.
- Example of 35 CCCs: Adjusted ampacity = 12 amps.
- Circuits requiring a 20-amp minimum (e.g., NEC 210.11(C)) cannot use a conductor with an adjusted ampacity of 12 amps, even if it is protected by a 15-amp breaker.
CEO and Founder of Electrical Code Academy, Inc. A Virginia Corporation located in Mineral, Virginia
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