Source: https://guthlabs.ai/docs/instruments/nec-iec-conduit-fill-calculator/
instrument / verified-clean
# NEC/IEC Conduit Fill Calculator
The current ledger marks this instrument verified clean within the source-bound verification corpus.
Status: **VERIFIED_CLEAN**Lifecycle: **verified-clean**
## Inspectable records
[01**Overview**Purpose and lifecycle]()[02**Mechanism**Method and assumptions]()[03**Implementation**Binding and validation]()[04**Qualification**Evidence and errata]()
## Ledger state
The current ledger marks this instrument verified clean within the source-bound verification corpus.
**Area**: Electrical & Electronics Engineering — Circuits, Power Systems, Electronics Lab
**Tranche**: tranche-7
**Evidence ID**: t7-106
**Ledger rows**: 1
No ledger note is attached.
## Review findings
The listed formula is directionally correct only after the NEC fill-percentage rule is completed and the correct table areas are selected. This instrument needs explicit NEC versus IEC logic, code edition and jurisdiction, raceway type, conductor construction and insulation, conductor-category counting rules, and a precise definition of max additional conductors before it can be labeled as more than a reference estimate. Always verify against the current locally adopted electrical code.
### Formula and scope findings
- The stated 40% rule is incomplete and is attributed to the wrong table: under NEC Chapter 9 Table 1, maximum fill is 53% for one conductor, 31% for two conductors, and 40% for three or more conductors. Chapter 9 Table 4 supplies raceway dimensions and Table 5 supplies conductor dimensions.
- The formula must use the conductor's overall insulated outside area, not the bare-metal conductor cross-sectional area. The applicable Chapter 9 Table 5 area depends on conductor size, insulation type, voltage rating, and construction.
- The NEC and IEC methods are not interchangeable. An IEC jurisdiction requires its applicable national or IEC-based raceway dimensions, conductor outside diameters, and fill rules; a simple jurisdiction toggle is insufficient.
- Max additional conductors is ambiguous unless the size, insulation type, and construction of every added conductor are specified. The vectors below assume all added conductors match the listed conductor type.
- Equipment grounding conductors, bonding conductors, neutrals, and cable assemblies have special counting and sizing rules that are not represented by the stated plain arithmetic formula.
### Unit assumptions
- All worked vectors use NEC customary units: conduit and conductor areas are in square inches, and conduit trade sizes are US trade sizes.
- Conductor areas are insulated outside areas from the NEC-style table values, not copper-metal areas.
- Fill percentage is calculated as total insulated conductor area divided by the applicable conduit internal area, multiplied by 100.
- The vectors use EMT conduit and individual copper THHN/THWN-2 conductors, with no cable assemblies, fittings, or special local amendments.
- For max additional conductors, every added conductor is assumed to have the same size and insulation area as the listed conductor.
- The 53%, 31%, and 40% fill limits are treated as NEC Chapter 9 Table 1 limits for one, two, and three-or-more conductors, respectively.
## Boundary hardening
**BREAK_CLAIMED** · SELF_FLAGGED
A zero-area or invalid conduit lookup can cause an undefined division instead of a required rejection, and the grounding-conductor counting convention is not specified, producing different max-additional results.
## Errata and corrections
No applied erratum is attached to this unique-name record.
## Inspectable example
4 review vectors are recorded. One source-bound sample is shown below.
````
{
"inputs": {
"jurisdiction": "NEC",
"conduit_type": "EMT",
"trade_size": "3/4 in",
"conductors": [
{
"material": "copper",
"size": "12 AWG",
"insulation": "THHN/THWN-2",
"count": 3
}
]
},
"expected_outputs": {
"conduit_internal_area_in2": 0.533,
"allowed_fill_percent": 40,
"allowed_area_in2": 0.2132,
"total_conductor_area_in2": 0.0399,
"fill_percent": 7.4859,
"passes": true,
"max_additional_matching_conductors": 13
},
"reference_data": {
"conduit_row": {
"trade_size": "3/4 in",
"internal_area_in2": 0.533,
"source_convention": "NEC Chapter 9 Table 4, EMT; widely tabulated standard NEC value"
},
"conductor_row": {
"material": "copper",
"size": "12 AWG",
"insulation": "THHN/THWN-2",
"outside_area_in2": 0.0133,
"source_convention": "NEC Chapter 9 Table 5, insulated copper conductor; widely tabulated standard NEC value"
},
"fill_rule": {
"conductor_count": "3 or more",
"allowed_fill_percent": 40,
"source_convention": "NEC Chapter 9 Table 1; widely tabulated standard NEC value"
}
},
"working": "1. Conduit internal area = 0.533 in².\\n2. Three conductors use the 40% limit. Allowed area = 0.40 × 0.533 = 0.2132 in².\\n3. Existing conductor area = 3 × 0.0133 = 0.0399 in².\\n4. Fill = (0.0399 ÷ 0.533) × 100 = 7.4859%.\\n5. Remaining allowed area = 0.2132 − 0.0399 = 0.1733 in².\\n6. Additional conductors = floor(0.1733 ÷ 0.0133) = floor(13.0301) = 13.\\n7. Check: 16 × 0.0133 = 0.2128 in² ≤ 0.2132 in²; 17 × 0.0133 = 0.2261 in² > 0.2132 in²."
}
````
## Related workflows
No source-defined multi-instrument workflow membership is recorded.
## Sources and evidence
These pointers bind the page to evidence. They do not grant authority or collapse verification into deployment.
- `VERIFIED_LEDGER.json · VERIFIED_CLEAN`
- `evidence-tranche7/tranche7-results.json`
- `BOUNDARY_HAZARD_CATALOG.json`