NEC sizing engine overview

The NEC sizing engine calculates conductor sizes, overcurrent protection device (OCPD) ratings, bus amperage, and transformer kVA according to National Electrical Code 2026 rules. In the SLD editor, sizing is now proposal-driven: the engine shows recommended changes first, and you decide which ones to accept.

What the sizing engine does

The sizing engine:

• Calculates required cable ampacity based on load current
• Applies temperature derating factors (NEC 310.15(B)(1))
• Applies conductor bundling and grouping adjustment factors (NEC 310.15(C)(1))
• Selects appropriate conductor size from NEC ampacity tables (NEC 310.16)
• Sizes overcurrent protection devices (breakers and fuses) and switch ratings
• Accounts for continuous load factors (125% multiplier per NEC 210.19)
• Handles motor circuits with special FLC calculations (NEC 430.22, 430.52)
• Supports parallel conductor runs (NEC 310.4), with automatic parallel sizing for high-current circuits
• Sizes transformer kVA based on downstream load aggregation
• Provides NEC rule citations for all calculations
• Generates pending sizing suggestions on the canvas before writing changes to component data

All sizing follows NEC 2026 edition requirements. Results include warnings when code limits are exceeded.

Current sizing workflow

The current SLD sizing workflow has two layers:

1. Preview proposals

• Electrical edits and topology changes refresh sizing preview suggestions.
• Suggested values appear in green on the canvas, in the canvas header review popover, and in the edit panel.
• Suggestions do not change the saved component values until you accept them.

1. Persisted sizing results

• The Re-size action runs the sizing flow for the selected component path and refreshes stored sizing details.
• Bus amperage suggestions refresh bus sizing details immediately when accepted; other accepted suggestions may need

Re-size for a fresh detailed card.

• Stored sizing cards can lag behind pending suggestions or accepted field changes until a fresh sizing run updates them.

In practice, the common flow is:

1. Edit the circuit.
2. Review the green sizing suggestions.
3. Accept or dismiss the suggestions you want.
4. Use Re-size when you want fresh detailed sizing results for the selected component or when the panel says sizing details are stale.

Sizing vs power flow analysis

Sizing and power flow analysis serve different purposes:

Sizing determines:

• Minimum required conductor size for safe operation
• Appropriate OCPD rating to protect conductors
• Compliance with NEC ampacity requirements
• Based on component electrical properties (not solved power-flow currents)

Power flow calculates:

• Actual currents under operating conditions
• Voltage drops through the system
• Loading percentages on equipment
• Based on solved network equations

You should run sizing before power flow to ensure conductors are properly sized. Then run power flow to verify that actual operating currents and voltages are acceptable.

Current source used for auto-sizing

Auto-sizing does not use the solved "running amps" from power flow. It uses component input fields:

• Cable conductor sizing (standard loads):
• Uses load.ratedCurrent when provided.
• ratedCurrent is treated as a design/connected current input (not solved running amps).
• If ratedCurrent is blank, calculates current from activePower, voltage, and powerFactor.
• Simple branch conductors size from connected branch current without applying demandFactor.
• demandFactor remains an operating/load-flow and aggregate sizing input.
• Applies continuous-load multiplier when enabled.
• OCPD auto-sizing (current behavior):
• Resolves load current from ratedCurrent or from activePower, voltage, and powerFactor.
• Simple branch OCPDs size from connected branch current unless an explicit branch design pin/lock is set.
• Then limits OCPD by effective cable ampacity.
• Switch auto-sizing:
• Uses the same connected-load or aggregate-bus current basis as nearby protection devices.
• Does not apply a cable ampacity cap because switches are not OCPDs.
• Applies the project or per-switch switch sizing margin before choosing the next standard ampere rating.
• Transformer auto-sizing:
• Uses the configured sizing basis: connected or running.
• Running load uses the load factors entered on each load: diversity × demand × utilization.

Components that can be sized

The sizing engine works with:

Cables:

• Feeder cables
• Branch circuit conductors
• Service entrance conductors
• Parallel conductor runs

Protection devices:

• Circuit breakers (HV and LV)
• Fuses
• Disconnect and load-break switches
• Motor protection (with special FLC-based sizing)

Motors:

• Conductor sizing based on FLC tables
• Branch circuit protection sizing
• Overload protection coordination

Transformers:

• kVA sizing based on aggregated downstream loads
• Demand factor and nameplate sizing basis options
• Automatic feeder cable sizing on the primary side

Sizing cascade

Sizing still cascades upstream through connected components, but the editor now presents the outcome as proposals instead of silently rewriting the design.

Typical order:

1. Branch cable serving the changed load or motor
2. Upstream feeder cables
3. Upstream protection devices for individual branches
4. Main breakers and bus-protection breakers/fuses/switches sized from aggregate downstream demand
5. Buses
6. Transformers
7. Transformer primary feeder cables when transformer sizing changes

This cascade flows from loads toward power sources and stops at source boundaries such as utility feeds and generators. Each step uses the NEC rules appropriate for that component type.

See Auto-sizing for workflow details.

NEC 2026 code book reference

Sizing calculations reference specific NEC articles:

Each sizing result includes specific NEC rule citations so you can verify compliance.

Sizing modes

Components can be in one of three sizing modes:

Auto:

• The component participates in sizing suggestions and re-size actions.
• This is the default mode.

Manual:

• You changed the value yourself.
• Pending sizing suggestions are suppressed until you resume auto-sizing.

Locked:

• Sizing changes are paused for that component.
• Re-size and preview suggestions do not update it until you unlock it.

For most designs, keep components in auto mode and only switch to manual or locked when you intentionally want to hold a value.

When to run sizing

Use Re-size:

• After building a new branch and you want stored sizing details for it
• When the edit panel says No sizing results available
• When the edit panel says Sizing details are stale. Re-size to refresh.
• After changing installation conditions, margins, cable series, or protection settings and you want refreshed detailed results
• Before exporting or reviewing a final sizing package

Preview suggestions can appear before you run Re-size, but the detailed sizing cards are only as current as the last persisted sizing run.

Integration with defaults

The sizing engine uses your component defaults for:

• Ambient temperature (default 30°C or project-specific)
• Termination temperature rating (60°C, 75°C, or 90°C)
• Conductor material (copper vs aluminum)
• Cable insulation type (THHN, XHHW, etc.)
• Continuous load factor (typically 1.25 or 125%)
• Motor power factor and efficiency
• Auto-parallel settings (enable/disable, max conductor size, max runs)
• Default cable series for auto-sizing

Set appropriate defaults in Settings > Component Defaults to streamline sizing for multiple components. The Sizing Defaults category contains settings that affect the sizing engine globally, including auto-parallel conductor sizing. See Component defaults and Auto-parallel sizing.

Reading sizing results

There are two different sizing views in the editor:

Pending suggestions:

• Shown in green on the canvas and in the edit panel
• Represent proposed changes that have not been accepted yet

Sizing results panels:

• Show the last persisted sizing run for the selected component
• Can be stale after edits or after accepting cable, transformer, or protection-device suggestions until a fresh re-size

updates the stored details

• Stay aligned immediately after accepting bus amperage suggestions

The sizing results panels show:

• Selected conductor size (AWG or kcmil)
• Required ampacity before derating
• Adjusted ampacity after derating
• Derating factors applied (temperature, grouping)
• Selected OCPD size and type
• NEC rule references
• Warnings and errors
• Compliance status (OK, Warning, Error)

Click Details to expand the calculation breakdown. When a sizing margin is applied, the expanded view shows both the code-minimum required amps and the final value after margin (e.g., "80 A (code minimum) x 1.20 = 96 A (20% margin)"). This applies to cable, bus, and switch sizing. Breaker/fuse OCPD sizing follows NEC section-based calculations without an extra user margin.

Green status means the sizing meets all NEC requirements. Yellow warnings indicate marginal conditions. Red errors mean code violations that must be corrected.

Related topics

• Auto-sizing - How the sizing cascade works
• Cable sizing - Detailed cable ampacity calculations
• Auto-parallel sizing - Automatic parallel conductor configuration
• OCPD sizing - Overcurrent protection device selection
• Sizing warnings and errors - Understanding and resolving sizing issues
• Load sizing trust report - Narrative sizing evidence and citations
• Component defaults - Setting sizing parameters