IMPORTANT
Quick Answer: Under the National Electrical Code (NEC) 40 percent area fill rule (NEC Annex C, Table C.5 for 3 or more conductors), the maximum number of 350 MCM (350 kcmil) THHN / THWN-2 conductors allowed in Liquidtight Flexible Nonmetallic Conduit (LFNC-B / “Carflex”) is:
- 1-1/2″ LFNC Conduit: Maximum 1 wire (1 wire limit = 53% fill)
- 2″ LFNC Conduit: Maximum 2 wires
Key Takeaways
- Commercial 310A & 350A Standard: 350 MCM (350 kcmil) copper wire is specified for 310-amp (at 75°C) and 350-amp (at 90°C) heavy commercial feeds, large industrial chemical mixers, wastewater aeration blowers, and marine dock disconnects.
- 100% Nonmetallic Chemical & Moisture Protection: Liquidtight Flexible Nonmetallic Conduit (NEC Article 356, commonly known as “Carflex” or LFNC-B) provides complete immunity against chemical attacks, sulfuric acid fumes, chlorine gas, and severe coastal salt fog.
- 40 Percent Fill Rule (NEC Annex C Table C.5): When pulling 3 or more conductors into continuous LFNC, total wire area cannot exceed 40 percent of the conduit’s internal cross-sectional area (or 31 percent when pulling exactly 2 conductors).
- 2″ LFNC Fits 2 Conductors: Due to the large diameter of 350 MCM wire (0.817 in. outer diameter), a 2″ LFNC conduit can accommodate a maximum of 2 conductors under standard continuous conduit fill rules.
- Dedicated Grounding Conductor Mandatory: Because LFNC has no metal sheath, a separate insulated green copper equipment grounding conductor is 100 percent mandatory for every circuit under NEC Section 250.118 and Section 356.60.
- Short Nipple Rule: If the LFNC run is a short nipple 24 inches or less between enclosures, NEC Chapter 9 Table 1 Note 4 permits up to 60 percent fill capacity (e.g., up to 3 THHN wires in a 2″ LFNC nipple).
Master 350 MCM in LFNC Fill Capacity Chart (NEC Annex C Table C.5)
The reference table below outlines maximum wire counts for both THHN/THWN-2 and XHHW-2 copper conductors across standard Liquidtight Flexible Nonmetallic Conduit trade sizes, strictly verified against NEC Annex C Table C.5 values.
| LFNC Conduit Trade Size | Conduit Inside Area (Sq. In.) | Max 350 MCM THHN/THWN-2 (40% Fill) | Max 350 MCM XHHW-2 (40% Fill) | Nipple 60% Fill (THHN) | Jam Ratio Status (3 Wires) |
|---|---|---|---|---|---|
| 1-1/2″ LFNC | 1.956 sq. in. | 1 Wire (53% Limit) | 1 Wire (53% Limit) | 2 Wires | N/A |
| 2″ LFNC | 3.326 sq. in. | 2 Wires | 2 Wires | 3 Wires | Safe (Cradle Fit) |
[!PRO TIP] Need Custom Multi-Wire Combinations? If you are combining 350 MCM conductors with smaller ground wires (like 1 AWG or 1/0 AWG), use our live interactive Conduit Fill Calculator Tool to calculate custom multi-gauge combinations in real time.
Typical Applications for 350 MCM Wire in LFNC Conduit
Understanding where 350 MCM wire in Liquidtight Flexible Nonmetallic Conduit is installed helps you plan residential and commercial raceway systems:
- Chemical Processing Motor Feeds: Providing flexible, non-corroding power links to 350A industrial pumps, agitators, and chemical reactors exposed to acidic fumes.
- Municipal Wastewater Treatment Facilities: Supplying large 3-phase aeration blowers and sludge pumps where metallic raceways rapidly corrode.
- Marine Shipyard Heavy Equipment Drops: Wiring heavy marine cranes, dry dock hoists, and shore power transfer switches subject to saltwater spray.
- Outdoor Corrosive Environment Generator Joints: Connecting standby generator outputs to automatic transfer switches in coastal chemical plants.
THHN vs. XHHW-2: Insulation Comparison for 350 MCM in LFNC
Wire insulation thickness and jacket composition directly impact conductor capacity and code compliance in wet locations.
- 350 MCM THHN / THWN-2: Uses a PVC insulation jacket covered by a nylon skin. The dual THWN-2 rating is required for outdoor and marine LFNC installations. Individual wire area = 0.5242 square inches (outer diameter = 0.817 in.).
- 350 MCM XHHW-2: Uses cross-linked polyethylene insulation without an outer nylon jacket. Individual wire area = 0.5166 square inches (outer diameter = 0.811 in.).
Because 350 MCM THHN and XHHW-2 have virtually identical dimensions, both wire types support identical wire counts across standard LFNC trade sizes.
How Conduit Fill Is Calculated
Conduit fill is calculated by taking the internal cross-sectional area of the LFNC conduit (from NEC Chapter 9, Table 4), multiplying by the allowable fill percentage (40 percent for 3 or more wires, 31 percent for 2 wires), and dividing by the individual conductor area (from NEC Chapter 9, Table 5).
For a detailed step-by-step mathematical breakdown and formula guide, visit our complete guide on how to calculate conduit fill.
Essential Field Warnings: Pulling Resistance & Trade Size Limits
1. 2″ LFNC Trade Size Capacity
Because 2″ is the standard maximum trade size commonly manufactured for LFNC-B raceways, large 350 MCM conductors are limited to a maximum of 2 wires per conduit run under standard 40% fill limits. For 3-phase 4-wire systems, parallel conduit runs or LFMC/RMC raceways should be evaluated.
2. High Pulling Friction in Nonmetallic Flex
Pulling heavy 350 MCM conductors through corrugated nonmetallic flexible conduit creates substantial friction against the internal PVC convolutions. Always use generous amounts of high-performance wire pulling lubricant rated for nonmetallic raceways.
TIP
Short Nipple Exemption: If your LFNC run between boxes or control enclosures is 24 inches or less, ampacity derating does not apply and fill capacity increases to 60 percent! Read our detailed breakdown of the 24-inch nipple 60% fill rule.
Frequently Asked Questions (FAQs)
Under NEC Annex C Table C.5 rules, you can legally install up to 2 conductors of 350 MCM THHN/THWN-2 in a 2″ Liquidtight Flexible Nonmetallic Conduit (LFNC-B / Carflex).
No. Three 350 MCM wires occupy 1.5726 sq. in., which exceeds the 40 percent fill limit (1.3304 sq. in.) of 2″ LFNC conduit. Three wires are only permitted in short nipples 24 inches or less under the 60% rule.
You can put a maximum of 1 conductor of 350 MCM in 1-1/2″ LFNC conduit under the 53 percent single-conductor limit. If you need 2 wires, you must upsize to 2″ LFNC.
Yes. Because LFNC is completely nonmetallic, installing a dedicated green copper equipment grounding conductor inside the conduit is mandatory under NEC Section 250.118 and Section 356.60.
Related Combination Guides & Tools
Explore 350 MCM in Alternate Conduit Types:
- Compare fill limits for 350 MCM conductors in LFMC conduit for metallic liquidtight applications.
- Review fill limits for 350 MCM conductors in EMT conduit for interior commercial installations.
- View heavy industrial capacity for 350 MCM wire in rigid metal conduit for hazardous locations.
- View intermediate steel capacity for 350 MCM wire in IMC conduit for commercial power risers.
- View underground capacity for 350 MCM wire in PVC conduit for direct burial runs.
- Check dry flexible raceway limits for 350 MCM conductors in FMC conduit for indoor transformer secondary whips.
Compare Adjacent Heavy Wire Sizes in LFNC Conduit:
- Check maximum fill limits for 250 MCM feeder conductors in LFNC conduit on 250A transformer secondary whips.
- Compare capacity for larger 500 MCM conductors in LFNC raceways on 400A/500A main service flexible joints.
- View feeder limits for 4/0 AWG copper THHN in LFNC on 225A switchboard terminations.
- Compare branch feeder limits for 3/0 AWG copper THHN in LFNC on 200A marine service disconnects.
Essential Core Electrical Guides:
- Access our official NEC conduit fill calculator for live multi-gauge calculations.
- Learn how to prevent cable wedging with our conduit jam ratio calculation guide.
- Review short run exemptions using the 24-inch nipple 60 percent fill rule.
- Avoid pulling friction penalties by adhering to NEC 360-degree bend limits.