Auto-parallel conductor sizing

When a single conductor cannot carry the required load current without exceeding your configured maximum conductor size, the auto-parallel feature automatically splits the circuit into multiple parallel runs. This eliminates the need to manually calculate parallel conductor configurations for high-current circuits.

How auto-parallel works

Auto-parallel activates during the sizing cascade whenever a cable is in auto sizing mode. The engine:

Calculates the total required amperage for the circuit.
Attempts to size a single conductor. If the selected size is at or below the maximum conductor size threshold, no parallel runs are needed.
If the single conductor exceeds the threshold, the engine tries 2 parallel runs, then 3, and so on up to the configured maximum.
For each run count, the engine sizes a conductor for Total Amps / Number of Runs and applies NEC 310.15(C)(1) adjustment factors for the increased conductor count.
The first configuration where the conductor size falls at or below the threshold is selected.

The result is the lowest number of parallel runs that keeps conductor size manageable while meeting ampacity requirements.

NEC 310.4 requirements

Auto-parallel enforces NEC 310.4 rules:

Parallel conductors must be 1/0 AWG or larger. The engine will not select parallel runs with conductors smaller than 1/0.
All parallel conductors are the same size, material, length, and insulation type.
The practical limit is 6 parallel runs per phase.

Configuring auto-parallel

Three settings control auto-parallel behavior. You can set them at any defaults scope (user, organization, or project) under the Sizing Defaults category.

Setting	Default	Description
Auto Parallel Sizing	Enabled	Turn auto-parallel on or off
Max Single Conductor Size	500 kcmil	Largest conductor size before the engine tries parallel runs
Max Parallel Runs	4 runs	Upper limit on the number of parallel runs

To change these settings:

Go to Settings > Component Defaults.
Select the scope (User, Organization, or Project).
Select the Sizing Defaults category.
Adjust the three auto-parallel settings.

Choosing a max conductor size

The max conductor size threshold determines when the engine switches from a single large conductor to multiple smaller ones. Common choices:

Threshold	Use case
350 kcmil	Prefer smaller, easier-to-handle conductors
500 kcmil	Balanced approach (default)
750 kcmil	Minimize parallel runs, accept larger conductors

Lower thresholds result in more frequent parallel runs with smaller conductors. Higher thresholds allow larger single conductors before paralleling.

Choosing max parallel runs

More parallel runs allow the engine to handle higher currents with smaller conductors, but increase installation complexity and cost. The engine always prefers fewer runs when possible.

Max Runs	Trade-off
2 runs	Simplest installation, may require large conductors
4 runs	Good balance (default)
6 runs	Maximum flexibility, NEC practical limit

Example

A 480 V three-phase continuous load draws 800 A:

Sizing current: 800 A x 1.25 (continuous) = 1000 A
With max conductor size set to 500 kcmil, a single 500 kcmil conductor at 75°C provides only 380 A — insufficient.
The engine tries 2 runs: 1000 A / 2 = 500 A per conductor. A 500 kcmil conductor at 380 A per run is still short after derating.
The engine tries 3 runs: 1000 A / 3 = 333 A per conductor. A 350 kcmil conductor may satisfy the requirement depending on derating.
The engine selects the first valid configuration and updates the cable's parallel runs automatically.

Reading auto-parallel results

When auto-parallel changes the number of parallel runs on a cable, the sizing results panel shows:

The selected conductor size per run
The number of parallel runs applied
Total adjusted ampacity across all runs

A warning message appears:

> "Auto-parallel: X runs of [size] selected (max single conductor: [threshold] kcmil). Per NEC 310.4."

This warning is informational — it confirms the engine made an automatic change. Review the result to verify it meets your installation requirements.

Auto-parallel with feeder cables

Auto-parallel applies to both branch circuit cables (cables connected directly to loads) and feeder cables (cables serving multiple downstream loads through buses and protection devices). Feeder cables aggregate demand from all downstream loads before determining parallel runs.

When auto-parallel does not apply

Auto-parallel is skipped when:

The cable's sizing mode is manual or locked
Auto Parallel Sizing is set to Disabled in sizing defaults
The cable has no cable series selected (a warning appears instead: "Cable has no series selected. Select a cable series to enable auto-sizing.")
A single conductor already meets the requirement without exceeding the threshold

Overriding auto-parallel results

If you need a specific parallel run configuration:

Select the cable on the canvas.
In the edit panel, manually set the Parallel Runs value.
The cable's sizing mode changes to manual, preventing auto-parallel from overriding your choice.

To return to automatic sizing, unlock the cable by clicking the lock icon.

Cable sizing - Conductor ampacity calculations and parallel runs
Auto-sizing - How the sizing cascade works
Sizing warnings and errors - Understanding auto-parallel warnings
Setting defaults - Configuring sizing defaults
Component defaults - How the defaults hierarchy works