Bus Bar Size & Bus Bar Ampacity Calculator

Name: Bus Bar Size Calculator
Author: Nadeem Sial

Size copper or aluminium busbars from the load current in metric (IEC) or inch (NEC / UL) bars, estimate the ampacity of a given bar, and check its short-circuit withstand.

By Nadeem Sial, Senior Electrical EngineerUpdated October 28, 2026

Panel Design Engineering Guideline:

Size from current density first, then confirm the rating at the real ambient and enclosure, then check the fault level: rated I_cw to IEC 61439-1, or the SCCR of the listed switchboard or panelboard under NEC 110.10 and 408.6.

How to Use the Bus Bar Size Calculator

1
Choose the bar system: IEC gives metric bars (for example 50 × 10 mm); NEC / UL gives inch bars (for example 1/4 × 4 in).
2
Size from current: enter the load current, material, current density and design factor. Use 1.25 where the NEC 125% continuous-load rule applies to the circuit.
3
Check the rating: switch to "Amps from size", enter the selected bar, the ambient and maximum bar temperature and the mounting. The rating must be at least the design current.
4
Check the fault level: in "Short-circuit check", enter the prospective fault current and duration to get the minimum cross-section, or add the bar area to get its thermal withstand for that duration.

Bus Bar Sizing Formulas

1. Cross-section from current density

A = I × F ÷ J

A = required area (mm²), I = load current (A), F = design factor, J = current density (A/mm²). The calculator then picks the smallest standard bar with at least this area. 1 in² = 645.16 mm², so 1000 A/in² = 1.55 A/mm².

2. Ampacity of a given bar (estimate)

I = W × T × J_30K × (ΔT ÷ 30)^0.61 × k_mount × k_AC × k_thick

J_30K = 1.6 A/mm² for copper and 1.25 A/mm² for aluminium at a 30 K rise; ΔT = maximum bar temperature − ambient. The 0.61 exponent on temperature rise comes from the empirical busbar formula in the Copper Development Association's Publication 22. That formula also depends on the bar perimeter, so an area-based estimate is a check, not a substitute for the bar maker's or DIN 43671 / 43670 tables.

3. Thermal short-circuit withstand (adiabatic)

A_min = I_k × √t ÷ k, k = K × √ln[(θ_f + β) ÷ (θ_i + β)]

Copper: K = 226, β = 234.5 °C. Aluminium: K = 148, β = 228 °C (the same material constants used for conductor withstand in IEC 60364-5-54 and IEC 60949). With θ_i = 90 °C and θ_f = 200 °C, k = 122 for copper and 81 for aluminium.

4. Peak force on the middle bar of a three-phase system

F = 0.2 × 0.866 × i_p² × l ÷ a

F in N, i_p = peak short-circuit current (kA), l = distance between supports, a = centre spacing of the phases (same unit as l). This is the force on the conductor from IEC 60865-1; the load on each support and the bending stress in the bar also depend on how the bar is fixed, which IEC 60865-1 covers with additional factors.

Worked Examples: IEC and NEC

IEC: 800 A copper main busbar, IEC 61439-2 assembly

Size: A = 800 × 1.25 ÷ 1.5 = 666.7 mm². The next standard bar is 70 × 10 mm (700 mm²).
Rating check: 70 × 10 mm bare copper, 35 °C ambient, 65 °C bar (30 K rise), AC: 700 × 1.6 × 1.00 × 0.98 × 0.92 = 1,010 A in free air, or 808 A in a ventilated enclosure (× 0.80), just above 800 A. In a sealed enclosure (× 0.70, 707 A) the next size up is needed.
Short circuit: for a 25 kA, 1 s rating (90 °C → 200 °C), A_min = 25,000 × √1 ÷ 122.1 = 205 mm². The 700 mm² bar withstands about 85 kA for 1 s thermally.
Forces: IEC 61439-1 takes the peak as n × I_cw with n = 2.1 for 20–50 kA, so i_p = 52.5 kA. With supports 500 mm apart and phases 100 mm apart: F = 0.2 × 0.866 × 52.5² × 500 ÷ 100 ≈ 2.39 kN on the middle bar between supports.
Final step: the assembly rating is confirmed by temperature-rise verification to IEC 61439-1 clause 10.10 (test, comparison with a tested reference design, or calculation within the limits the standard allows).

NEC: 800 A copper feeder bus, continuous load

Size: the load is continuous, so apply 125% (NEC 215.2(A)(1) for feeder conductors, 215.3 for the feeder overcurrent device). A = 800 × 1.25 ÷ 1.5 = 666.7 mm² (1.033 in²). The next inch bar is 3/8 × 3 in (1.125 in², 725.8 mm²).
Density check: 1.5 A/mm² is about 968 A/in², below the 1000 A/in² that NEC 366.23(A) allows for bare copper bars in sheet-metal auxiliary gutters.
Rating check: 3/8 × 3 in bare copper at 40 °C ambient and 70 °C bar (30 K rise), AC, free air: about 1,058 A, above the 1,000 A design current.
Equipment: bus inside a switchboard or panelboard is part of the listed equipment (UL 891 / UL 67). Its marked short-circuit current rating must be at least the available fault current (NEC 110.10 and 408.6).

NEC vs IEC: What Governs Busbar Size

Topic	IEC / BS EN	NEC / UL (US, Canada similar via CSA)
Assembly standard	IEC 61439-1 (general rules), IEC 61439-2 (power switchgear and controlgear assemblies); BS EN 61439 in the UK	NEC Article 408 (switchboards, switchgear, panelboards); equipment listed to UL 891 (switchboards) or UL 67 (panelboards); UL 857 for busways
Continuous rating	Verified by temperature rise, IEC 61439-1 clause 10.10; reference ambient 35 °C (24 h average), 40 °C maximum	Set by the listing tests of the equipment; 125% sizing for continuous loads on feeders (215.2, 215.3)
Current density limit	No fixed limit; density follows from the temperature-rise result	366.23(A): 1000 A/in² copper, 700 A/in² aluminium for bare bars in sheet-metal auxiliary gutters
Short circuit	Rated I_cw (usually 1 s) and I_pk, verified to IEC 61439-1 clause 10.11; forces and heating to IEC 60865-1	Short-circuit current rating (SCCR) of the listed equipment not less than the available fault current (110.10, 408.6)
Spacing	Clearances and creepage distances, IEC 61439-1 clause 8.3	Minimum spacings between bare live parts, 408.56 and Table 408.56
Bar dimensions	Metric bars, e.g. copper to EN 13601; ampacity tables in DIN 43671 (Cu) and DIN 43670 (Al)	Inch bars, e.g. copper to ASTM B187, aluminium bus conductor to ASTM B317

Copper vs Aluminium Busbars

Property	Copper (ETP, C11000)	Aluminium (6101 / 1350)
Conductivity	about 100% IACS	about 55–57% IACS (6101), 61% (1350)
Design density used here	1.5 A/mm² (sizing), 1.6 A/mm² at 30 K rise (rating)	1.0 A/mm² (sizing), 1.25 A/mm² at 30 K rise (rating)
Same-size rating	1.00	about 0.78 × copper (1.25 ÷ 1.6)
Joints	Bare or tin/silver plated	Plated contact faces or bimetal (Cu-Al) pieces at copper terminals; joint compound and correct bolt torque

Steel is used for protective earth bars only, not for phase conductors.

Rating Factors Used by the Calculator

Temperature rise ΔT	20 K	25 K	30 K	40 K	50 K	60 K	70 K
(ΔT ÷ 30)^0.61	0.78	0.90	1.00	1.19	1.37	1.53	1.68

Thickness	3 mm	5 mm	6 mm	10 mm	12 mm
k_thick	1.05	1.00	0.98	0.92	0.88
k_AC (50/60 Hz)	1.00	1.00	1.00	0.98	0.95

Inch thicknesses are interpolated (1/4 in = 6.35 mm, 3/8 in = 9.53 mm; 1/2 in uses the 12 mm values). Mounting factors: free air horizontal 1.00, free air vertical 0.85, ventilated enclosure 0.80, sealed IP65 enclosure 0.70.

Not in the calculator: surface finish and parallel bars. A painted or dull bar (high emissivity) radiates more heat and carries more current than a bright one. Two or more bars per phase do not multiply the rating: the inner faces shade each other, so the gain per added bar falls. The Copper Development Association ampacity tables (copper.org) give both effects for inch bars; use them or the bar maker's tables for multi-bar runs.

Bus Bar Size Chart (IEC and NEC)

Calculator results for copper at 1.5 A/mm² and aluminium at 1.0 A/mm², with a 1.25 design factor and one bar per phase. "Parallel bars" means no single standard bar is large enough.

Current	Cu area	Cu metric	Cu inch	Al area	Al metric	Al inch
100 A	83 mm²	30 × 3 mm	1/8 × 1-1/2 in	125 mm²	25 × 5 mm	1/8 × 2 in
200 A	167 mm²	30 × 6 mm	3/8 × 3/4 in	250 mm²	50 × 5 mm	1/8 × 4 in
400 A	333 mm²	60 × 6 mm	3/8 × 1-1/2 in	500 mm²	50 × 10 mm	3/8 × 2-1/2 in
600 A	500 mm²	50 × 10 mm	3/8 × 2-1/2 in	750 mm²	80 × 10 mm	1/4 × 5 in
800 A	667 mm²	70 × 10 mm	3/8 × 3 in	1,000 mm²	100 × 10 mm	3/8 × 5 in
1000 A	833 mm²	80 × 12 mm	1/4 × 6 in	1,250 mm²	120 × 12 mm	1/2 × 4 in
1250 A	1,042 mm²	120 × 10 mm	3/8 × 5 in	1,563 mm²	150 × 12 mm	1/2 × 5 in
1600 A	1,333 mm²	120 × 12 mm	3/8 × 6 in	2,000 mm²	200 × 10 mm	Parallel bars
2000 A	1,667 mm²	150 × 12 mm	1/2 × 6 in	2,500 mm²	250 × 10 mm	Parallel bars

A density-based size is a starting point. Check the rating with "Amps from size" for the real ambient and enclosure.

Short-Circuit Withstand of Bus Bars

Minimum cross-section for a 1 s fault, from the adiabatic formula with 90 °C initial and 200 °C final temperature. For other durations, multiply by √t (0.5 s: × 0.71; 3 s: × 1.73).

I_cw (1 s)	Copper, k = 122	Aluminium, k = 81	Peak factor n (IEC 61439-1)
10 kA	82 mm²	124 mm²	1.7
25 kA	205 mm²	310 mm²	2.1
50 kA	409 mm²	620 mm²	2.1
65 kA	532 mm²	806 mm²	2.2

At high fault levels the mechanical force on supports, not heating, usually decides the design.

Common Mistakes in Busbar Selection

Using current density as the rating. Density only gives a starting size. The rating depends on temperature rise, enclosure and the bars around it.
Using a free-air rating inside a closed enclosure. The calculator uses 0.80 for a ventilated and 0.70 for a sealed enclosure; IEC 61439 assemblies are verified as built.
Ignoring the fault level. A bar can pass the thermal check at 50 kA and still bend or break supports if they are too far apart. Check the peak force (IEC 60865-1) and use a tested support arrangement.
Treating density as a code limit. The NEC 1000 A/in² figure applies to bars in auxiliary gutters (366.23(A)); IEC 61439 sets no density limit, only temperature rise.
Making single bars very thick on AC. At 50/60 Hz, skin effect makes bars much over about 10 mm thick inefficient; two thinner bars with a gap about equal to their thickness work better.
Poor joints. Unclean or under-torqued joints and copper-to-aluminium contacts without plating or bimetal pieces cause hot spots.

Bus Bar Size Calculator Frequently Asked Questions

What size busbar is needed for 1000 A?

At 1.5 A/mm² with a 1.25 design factor, 1000 A copper needs 833 mm², which gives 80 × 12 mm (metric) or 1/4 × 6 in (inch). The calculator rates 80 × 12 mm bare copper at about 1,284 A in free air at a 30 K rise (AC), or 1,027 A in a ventilated enclosure. Aluminium at 1.0 A/mm² needs 1,250 mm², such as 120 × 12 mm or 1/2 × 4 in.

How many amps can a 40 × 10 mm copper busbar carry?

The calculator gives about 577 A for a single bare 40 × 10 mm copper bar on AC in free air at 35 °C ambient and 65 °C bar temperature (30 K rise), and about 788 A at a 50 K rise. Painted bars and manufacturer tables often give higher values; aluminium of the same size gives about 450 A at 30 K.

What is the NEC busbar current density limit?

NEC 366.23(A) limits bare copper bars in sheet-metal auxiliary gutters to 1000 A/in² (1.55 A/mm²) and aluminium bars to 700 A/in² (1.09 A/mm²). Bus inside switchboards and panelboards is not sized by that rule; it is part of the listed equipment and tested to UL 891 or UL 67.

What temperature rise is allowed for busbars under IEC 61439?

IEC 61439-1 does not set one number for bare busbars. The limit is set by the bar's mechanical strength, the effect on nearby equipment and the temperature limit of insulation it touches. Terminals for external insulated conductors are limited to a 70 K rise. Ratings are verified at a 35 °C average ambient.

Is aluminium busbar as good as copper?

For the same rating, an aluminium bar needs roughly 1.3 times the copper cross-section (1.6 ÷ 1.25 in this calculator) and weighs about 40% as much (density 2.7 vs 8.9 g/cm³). It is widely used in switchboards and busways. Joints need plated contact faces or bimetal pieces where they meet copper.

What size copper busbar is needed for 100 A?

About 83 mm² at 1.5 A/mm² with a 1.25 design factor, which gives 30 × 3 mm or 1/8 × 1-1/2 in. In small distribution boards, 100 A is usually carried by the manufacturer's comb or pin busbar, which is rated with the breakers it connects.

Standards and References

The formulas and reference values on this page follow these standards. For design work, check results against the current edition that applies in your country and the equipment manufacturer's data.

IEC 61439-1 and -2 – Low-voltage switchgear and controlgear assemblies: service conditions, clearances (8.3), temperature-rise verification (10.10), short-circuit withstand and peak factor n (10.11). Published by IEC.
IEC 60865-1 – Short-circuit currents, calculation of effects: electromagnetic forces on rigid conductors. Published by IEC.
IEC 60949 – Thermally permissible short-circuit currents, adiabatic method and material constants K and β. Published by IEC.
NFPA 70 (NEC) – 110.10 circuit impedance and short-circuit ratings, 215.2 and 215.3 feeders, 366.23(A) bus bars in auxiliary gutters, Article 408 switchboards and panelboards (408.6, Table 408.56). Published by NFPA.
UL 891, UL 67, UL 857 – Switchboards, panelboards and busways. Published by UL.
DIN 43671 and DIN 43670 – Current ratings of copper and aluminium busbars; EN 13601, ASTM B187 and ASTM B317 for bar material.
Copper Development Association – Publication 22, Copper for Busbars, and the copper.org busbar ampacity tables.