Cable sizing

Cable sizing uses your project's electrical-code profile for ampacity, temperature derating, conductor grouping, continuous loads, and parallel runs. The ampacity examples below describe NEC 2026. CEC 2024 also requires the complete voltage-drop check described below before automatic conductor changes can be accepted.

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 recommended conductor size is the smallest available size that meets the modeled requirements for that sizing run. The installed conductor and parallel-run values are checked separately when you run Recheck; a valid installed value can differ from the recommendation.

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 sections 430.6, 430.22, and 430.250.

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. Load factors are used for operating/load-flow and aggregate sizing, not to shrink the branch conductor. Aggregate non-motor sizing uses diversityFactor × demandFactor × utilizationFactor; motor feeders retain the NEC 430.24 code floor regardless of those operating factors.
• 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 310.10(G))

For high-current circuits, install multiple conductors per phase in parallel. NEC 310.10(G) 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. The current conductor and run count remain included in current selection checks.

Voltage drop considerations

CEC 2024 circuits

EKX checks a maximum of 3% across each whole feeder or branch circuit and 5% from the supply to the load, under Rule 8-102. On a radial network EKX starts that path at the grid utility, so model any service conductors between the utility and the service equipment. Splitting a circuit into several cable objects does not give each section a separate 3% allowance. A project or cable design target can make the check stricter, but cannot relax these limits.

Before sizing:

1. For each cable, set Circuit type and Circuit voltage, and enter its length and usable cable impedance data. A zero length is a known value; a blank length is missing information.
2. Confirm Bonding type. A cable whose series has an integral bonding conductor, such as TECK90 or ACWU90, starts as Integral to cable and has no separate bonding conductor to enter. For a field-installed conductor, also confirm Bonding location, material, size and quantity.
3. For a supported motor, enter or confirm its nameplate current. Calculated estimate does not count as nameplate confirmation. Changing the motor's power, voltage or classification can require confirmation again.
4. Click Re-size system, open the review, and correct any named missing inputs. Review the whole circuit, total path, current basis and bonding consequence before accepting the complete group.

The current CEC check supports radial two-wire AC and balanced three-phase AC paths at or below 600 V with one consistent circuit voltage. It supports ordinary loads and continuous-duty direct-on-line induction or squirrel-cage motors. Multiple supplies, unsupported demand methods or missing required information leave the installed conductors unchanged and explain the blocker.

For a two-winding power, distribution, or isolation transformer, the downstream check starts at the secondary and uses its saved secondary voltage. It includes the shared feeder and each load's branch, without adding sibling branch drops. The load or motor's Connected circuit results show its cumulative path check. Transformer regulation and primary-side voltage drop remain separate power-flow considerations. Autotransformers, tertiary windings, and paths connected to the wrong winding remain unsupported by this check.

Voltage drop uses unreduced connected current or power. For an ordinary circuit with unknown load current, EKX can use 80% of an independently established protecting-device rating. It does not use a motor's oversized breaker as a substitute for nameplate current.

If voltage drop requires a larger conductor, review the associated field-installed bonding conductor change under Table 16. Declared integral cable bonding or approved metal raceway does not establish installation approval or continuity. Cable sizing margin changes still go through the complete circuit and path checks.

When no equipment changes are needed, Re-size system records the passing result automatically. The cable panel, sizing API and reports retain the result. Inputs changed; recalculate means a saved result no longer describes the current inputs. An old ampacity PASS without a saved voltage-drop check does not establish CEC voltage-drop compliance.

NEC design targets

EKX's general NEC voltage-drop workflow uses design targets. These checks do not certify special-case NEC voltage-drop requirements or equipment grounding conductor sizing. Common design targets are:

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

When length, voltage, power factor, and cable R/X data are available, ekx checks voltage drop during conductor auto-sizing and may propose the smallest larger conductor in the active cable series that satisfies both ampacity and voltage drop. Power-flow voltage remains a separate solved operating result.

The cable series you select determines the R/X values used for voltage-drop screening. Built-in 600 V and 1 kV cables take NEC Chapter 9, Table 9 values for the cable's raceway, steel conduit or non-magnetic, or the project's Typical raceway when the cable does not name one (see Cables). The check uses whatever R/X the cable data holds, including EKX default values, and records where each value came from. Cable summaries show whether reactance is an exact manufacturer value, a manufacturer substitute, a generic engineering fallback, user-entered data, or an unknown source. For medium-voltage feeders, use exact manufacturer data when the construction matches the project cable. Source-backed MV rows also preserve structured construction basis metadata for comparison, including insulation level, shield or neutral type, conductor arrangement, installation basis, spacing basis, and frequency. Substitute manufacturer values come from separate source-backed manufacturer series and should not be treated as generic defaults.

ekx uses an AC impedance approximation:

Three-phase:

VD = sqrt(3) x I x L x ((R_design / runs) x PF + (X / runs) x sin(theta))

Single-phase:

VD = 2 x I x L x ((R_design / runs) x PF + (X / runs) x sin(theta))

Where:

• VD = voltage drop in volts
• I = current in amperes
• L = one-way length in kilometers
• R_design = conductor resistance corrected to the sizing temperature
• X = cable reactance
• PF = power factor, and sin(theta) is derived from that power factor
• runs = parallel conductor runs

If impedance data is missing, EKX does not invent it. The NEC design-target workflow can still size by ampacity and warns that voltage drop was not evaluated for some or all candidate sizes. The mandatory CEC workflow blocks automatic conductor changes until it can evaluate the complete required group.

Generic medium-voltage system defaults are useful for early sizing review, but they are not a substitute for construction-specific manufacturer or project cable data. Substitute manufacturer values should be reviewed before final design.

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 (310.10(G), 310.15, 310.16, etc.)

The sizing card describes the last stored recommendation and its inputs. Recheck evaluates the currently installed conductor, material, and parallel runs against the requirements established from current project and scenario facts. It is read-only and reports missing or unsupported inputs separately from a failed requirement.

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 310.10(G) 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