Busbar Sizing Calculator — Copper & Aluminum Ampacity

Short answerAs a first pass, copper bar carries roughly 1,000 A per in² (about 1.55 A/mm²) and aluminum about 700 A per in² at a typical 30 °C rise in open air. The real rating depends on temperature rise, bar spacing, orientation and surface finish — always confirm to the equipment standard.

Want this on your own domain?

We build this calculator as an embeddable tool — engineered, SEO- and AI-ready, delivered in 5 business days for a flat $2,800.

Get yours in 5 days

How it's calculated

Busbar ampacity is set by how much heat the bar can shed at its allowable temperature rise, not by a fixed current density. The rules of thumb below give a starting cross-section; final ratings come from standards such as UL 891 or IEC 61439 and from the enclosure, spacing and plating. Plated (tin/silver) bars and forced ventilation raise the number; tight stacking lowers it.

First-pass area: A (in²) ≈ I ÷ 1000 (copper) or I ÷ 700 (aluminum) at ~30 °C rise, open air Bar size: A = width × thickness → choose a standard bar meeting A

Worked example

1,200 A continuous, copper, open air.
Area ≈ 1200 ÷ 1000 = 1.2 in².
Standard bar 4 × 0.31 in = 1.24 in² ✓ (verify by standard).

Reference tables

Copper bar first-pass ampacity (≈1,000 A/in², 30 °C rise, open air)

Bar size (in)	Area (in²)	≈ Ampacity
1 × 1/8	0.125	~125 A
2 × 1/4	0.50	~500 A
3 × 1/4	0.75	~750 A
4 × 1/4	1.00	~1,000 A
4 × 3/8	1.50	~1,500 A

Rule-of-thumb current density

Material	A / in²	A / mm²
Copper	~1,000	~1.55
Aluminum	~700	~1.08

A number the rules of thumb hide: silver- or tin-plated copper bar in a ventilated enclosure can carry 15–25% more than the bare open-air figure — but stacked, unplated bars carry less.

See a live tool in action

The interactive version of this calculator is on our build queue. Meanwhile, these live example simulators show exactly how yours would look and behave on your own domain:

Conduit Fill · NEC▶ LIVE CALCULATOR Storage Rack quote-simulator▶ OPEN DEMO Packaging / box simulator▶ OPEN DEMO

FAQ

Is 1,000 A/in² a code rule?

No — it is an industry rule of thumb for bare copper at a 30 °C rise in open air. The governing rating comes from UL 891 / IEC 61439 tests for the actual assembly.

What most changes the real ampacity?

Temperature rise limit, bar spacing and orientation, surface finish (plated vs bare), and whether the bar sits in still air or a ventilated enclosure.

Copper vs aluminum busbar?

Aluminum carries roughly 70% of copper for the same section, so bars are larger; it is lighter and cheaper but needs proper plated joints to avoid oxidation.

Does the number change for short-time faults?

Yes — short-circuit withstand is a separate check (I²t and mechanical forces). A bar sized for continuous current must still survive the available fault.

Can I rely on this for a final design?

No. Use it to get in the ballpark, then verify the assembly to the applicable standard and manufacturer data.

This page is a reference aid. Always verify against the current edition of the NEC (NFPA 70) and the authority having jurisdiction. Figures shown are for orientation, not a stamped design.

Build it for your catalog

Put this calculator on your domain so the engineers sizing it land on your site — not a competitor's. See how it works or start the conversation.