Cable sizing

Cable sizing calculates the minimum conductor size required to safely carry the load current according to NEC 2026 ampacity requirements. The sizing engine accounts for temperature derating, conductor bundling, continuous loads, and parallel conductor configurations.

Ampacity calculation workflow

The cable sizing process follows these steps:

1. Determine load current:

• Use load.ratedCurrent when it is set
• Otherwise calculate from kW, voltage, and power factor

1. Apply continuous load factor (typically 125% per NEC 210.19)
2. Look up ambient temperature correction factor (NEC 310.15(B)(1))
3. Look up conductor adjustment factor for bundled/grouped conductors (NEC 310.15(C)(1))
4. Calculate required base ampacity: Load Current / (Ambient Factor × Adjustment Factor)
5. Select conductor size from NEC 310.16 ampacity tables
6. Verify adjusted ampacity meets or exceeds load current

The selected conductor size is the minimum that meets all NEC requirements.

Load current calculation

For standard loads, load current is calculated as:

Three-phase:

I = (kW × 1000) / (√3 × V × PF)

Single-phase:

I = (kW × 1000) / (V × PF)

Where:

• I = current in amperes
• kW = active power in kilowatts
• V = voltage in volts
• PF = power factor (0.8 to 1.0, typically 0.85 for industrial loads)

For motors, use full-load current (FLC) from NEC tables 430.248 or 430.250 rather than calculating from power. When horsepower and voltage are available, branch-conductor auto-sizing uses the NEC table value ahead of any entered fullLoadAmps. See NEC 2026 motor current filing.

What load.ratedCurrent represents

load.ratedCurrent is a design input current (connected/nameplate-style value), not a solved operating current from power flow.

• If you enter ratedCurrent, cable auto-sizing uses that value directly.
• If you leave ratedCurrent blank, cable auto-sizing derives current from kW, voltage, and power factor.
• Simple branch cable auto-sizing uses the connected/design load current. demandFactor is used for operating/load-flow and aggregate sizing, not to shrink the branch conductor.
• If a branch is intentionally limited below connected load capability, use the branch protection lock/design pin path and keep the cable sized for that branch limit.

Power flow results are used for analysis/verification, not as the primary source for conductor auto-sizing inputs.

Temperature derating (NEC 310.15)

Conductor ampacity from NEC tables assumes 30°C ambient temperature. When ambient temperature differs, apply correction factors:

Higher ambient temperature reduces conductor ampacity. In hot environments (engine rooms, near furnaces, outdoor installations in hot climates), derating significantly reduces conductor capacity.

Example:

A conductor rated 100 A at 30°C in a 40°C ambient:

• 75°C termination: 100 A × 0.91 = 91 A adjusted ampacity
• 90°C termination: 100 A × 0.96 = 96 A adjusted ampacity

Set ambient temperature in component defaults or per-component settings.

Grouping and bundling penalties (NEC 310.15(C)(1))

When multiple current-carrying conductors are installed in the same raceway or cable, heat buildup reduces ampacity. NEC requires adjustment factors:

Count only current-carrying conductors - neutral and ground wires typically do not count.

Example:

A cable tray contains 8 current-carrying conductors:

• Base ampacity: 100 A
• Adjustment factor: 0.70
• Adjusted ampacity: 100 A × 0.70 = 70 A

Large conductor bundles in cable trays require significant upsizing or separation to prevent overheating.

Continuous load factors (NEC 210.19)

Loads operating continuously (3 hours or more) require conductors sized at 125% of the load current:

Required ampacity = Load Current × 1.25

Most industrial and commercial loads are considered continuous. Sizing at 125% provides:

• Thermal margin to prevent conductor degradation
• Compliance with NEC 210.19(A)(1)
• Protection against sustained overheating

Example:

A 100 A continuous load:

• Required ampacity: 100 A × 1.25 = 125 A
• Select conductor with ≥125 A base ampacity
• Then apply temperature and grouping derating

Set continuous load factor in component defaults or specify per load.

Sizing margin

Sizing margin adds a percentage increase to the code-required amperage before conductor selection. This provides headroom for future load growth or design conservatism beyond NEC minimums.

Required ampacity (with margin) = Code-required amps × (1 + margin / 100)

Example:

A 80 A non-continuous load with 20% sizing margin:

• Code-required amps: 80 A
• After 20% margin: 80 A x 1.20 = 96 A
• Conductor selected for 96 A instead of 80 A

Sizing margin is applied after all NEC multipliers (continuous load factor, motor conductor multiplier) but before conductor selection from ampacity tables. A 0% margin produces the exact NEC-required conductor size.

Setting the margin

Set sizing margin in two places:

• Project-wide default: Settings > Project > Defaults > Cable Sizing Margin
• Per-cable override: Select a cable, then set Sizing Margin in the edit panel (only available in auto sizing mode)

Per-cable overrides take priority over the project default. Clear the override to revert to the project default.

Reading margin in sizing results

When a sizing margin is applied, the Conductor Sizing section in the edit panel shows both values in the expanded details:

Required Amps: 96 A
  80 A (code minimum) x 1.20 = 96 A (20% margin)

The code minimum is the NEC-required value. The final required amps (after margin) is what the conductor is actually sized for.

Parallel conductor runs (NEC 310.4)

For high-current circuits, install multiple conductors per phase in parallel. NEC 310.4 allows parallel conductors if:

• All conductors are the same length
• Same conductor material and size
• Same insulation type
• Same termination method
• Conductors are 1/0 AWG or larger (for copper)

Calculation:

Total current divides equally among parallel conductors:

Current per conductor = Total Current / Number of Parallel Runs

Example:

A 1200 A feeder using 3 parallel conductors per phase:

• Current per conductor: 1200 A / 3 = 400 A
• Size each conductor for 400 A (after derating)
• Total capacity: 3 × conductor ampacity

Parallel runs reduce conductor size and installation costs for high-amperage circuits. Common in service entrance conductors and large feeders.

Automatic parallel sizing

When auto-parallel is enabled (the default), the engine automatically determines the optimal number of parallel runs during auto-sizing. If a single conductor would exceed the configured max conductor size threshold (default 500 kcmil), the engine tries 2, 3, or more parallel runs until the conductor size falls within the threshold.

Auto-parallel updates the cable's parallel runs automatically. A warning appears in sizing results when auto-parallel changes the run count.

See Auto-parallel conductor sizing for configuration options and details.

Manual parallel sizing

To specify parallel runs manually, set the parallel runs value in cable properties. Setting this value manually changes the cable's sizing mode to manual, which prevents auto-parallel from overriding your choice.

Voltage drop considerations

While NEC does not mandate voltage drop limits, recommended practice limits voltage drop to:

• Feeders: 2% maximum
• Branch circuits: 3% maximum
• Total (feeder + branch): 5% maximum

Voltage drop formula:

Three-phase:

VD = (√3 × I × L × Z) / 1000

Single-phase:

VD = (2 × I × L × R) / 1000

Where:

• VD = voltage drop in volts
• I = current in amperes
• L = one-way length in meters
• Z = impedance in ohms/km (R for single-phase)

If power flow analysis shows excessive voltage drop, upsize the conductor even if ampacity is adequate.

Installation type impact

Cable ampacity varies by installation method:

Set installation type in cable properties. The engine uses the appropriate ampacity table and adjustment factors for the installation method.

Reading sizing results

After running cable sizing, the Sizing Results panel shows:

Load current:

• Current the cable must carry (before continuous factor)

Required ampacity:

• Load current × continuous factor × derating factors
• Minimum conductor ampacity needed

Selected conductor:

• Actual conductor size (e.g., "2 AWG", "250 kcmil")
• Chosen from NEC 310.16 tables

Base ampacity:

• Conductor ampacity at 30°C ambient, no bundling
• From NEC 310.16 tables

Adjusted ampacity:

• Base ampacity × ambient factor × adjustment factor
• Actual current capacity under installation conditions

Derating factors:

• Ambient temperature factor (e.g., 0.96 for 35°C)
• Adjustment factor (e.g., 0.80 for 4-6 conductors)
• Combined factor (product of individual factors)

Parallel runs:

• Number of conductors per phase
• Current per individual conductor

NEC references:

• Specific NEC articles applied (310.4, 310.15, 310.16, etc.)

Common warnings

"Conductor undersized for continuous load":

• Selected conductor does not provide 125% capacity
• Increase conductor size or reduce load

"Excessive derating":

• Combined derating factors reduce capacity below load current
• Reduce ambient temperature, separate conductors, or upsize conductor

"Voltage drop exceeds 3%":

• Informational warning (not a code violation)
• Consider upsizing for better voltage regulation

"Parallel conductors too small":

• NEC 310.4 requires conductors ≥1/0 AWG for parallel runs
• Increase individual conductor size

"No conductor size meets required ampacity":

• Required ampacity exceeds available conductor sizes
• Use parallel conductor runs or reduce load

Related topics

• Sizing overview - Introduction to NEC sizing engine
• Auto-parallel sizing - Automatic parallel conductor configuration
• Auto-sizing - How the sizing cascade works
• OCPD sizing - Overcurrent protection device selection
• Sizing warnings and errors - Resolving sizing issues
• Power flow analysis - Verifying voltage drop
• Component defaults - Setting temperature and derating defaults