Subpanel Sizing Calculator

Background.

Use Subpanel Sizing Calculator when you need to apply entered demand factor and margin to connected branch loads as a planning check. A feeder or subpanel must be sized from the applicable calculated load, conductor ampacity, overcurrent protection and neutral treatment. Here the arithmetic follows “design load = connected load × demand factor × (1 + design margin),” rather than silently mixing alternatives.

The editable entries are connected or calculated load, applicable demand factor, design margin, system voltage or design divisor. Use values from the document or measurement that governs this subpanel sizing question; the defaults are only the worked fixture below. The most consequential input mistake would be to ignore that the generic factor is not an NEC load calculation and does not select breaker, wire, grounding or panel spaces.

NFPA 70, National Electrical Code 2023; load calculations, feeders and continuous loads documents the convention or governing rule used here. The subpanel sizing output is a transparent scenario under those facts: it does not manufacture an unentered market price, professional determination, carrier quote, legal eligibility finding or locally adopted code value.

What is subpanel sizing calculator?

Subpanel Sizing is the relationship behind this decision: a feeder or subpanel must be sized from the applicable calculated load, conductor ampacity, overcurrent protection and neutral treatment. On this page it means design load = connected load × demand factor × (1 + design margin). The generic factor is not an NEC load calculation and does not select breaker, wire, grounding or panel spaces; that is the line between the reported quantity and a broader electrical analysis.

How to use this calculator.

1. 01Confirm that “design load = connected load × demand factor × (1 + design margin)” matches the subpanel sizing convention you need.
2. 02Replace the fixture values for connected or calculated load, applicable demand factor, design margin, system voltage or design divisor with dated values from the governing record.
3. 03Keep all currencies, measurement units and time periods on the same basis before calculating.
4. 04Read design load together with this boundary: The generic factor is not an NEC load calculation and does not select breaker, wire, grounding or panel spaces.

The formula.

The calculation uses design load = connected load × demand factor × (1 + design margin). In this subpanel sizing model, the entered terms are connected or calculated load, applicable demand factor, design margin, system voltage or design divisor. A feeder or subpanel must be sized from the applicable calculated load, conductor ampacity, overcurrent protection and neutral treatment, which is why the relationship is presented under this name rather than as a universal alternative. The generic factor is not an NEC load calculation and does not select breaker, wire, grounding or panel spaces. Calculations keep full decimal precision through the relationship and round only the returned display values.

A worked example.

Frequently asked questions.

References& sources.

1. [1]NFPA 70, National Electrical Code 2023; load calculations, feeders and continuous loads. Retrieved 2026-08-07. access: open unless marked otherwise.
2. [2]International Code Council. 2024 International Residential Code (IRC). Retrieved 2026-08-07. independence: primary; access: open.
3. [3]U.S. Occupational Safety and Health Administration. Electrical — standards and hazards. Retrieved 2026-08-07. independence: secondary-check; access: open.