Tool · Switchboard and Switchgear Builders

Busbar Calculator: size, ampacity, weight and cost for a switchboard lineup

Enter the current. Get the bars per phase at a published current density, the single-bar ampacity from the Copper Development Association table, and the weight and metal cost of the bus for the whole lineup.

1 · Size the bus

Current (A)

Metal

Current density (A per sq in)

Bar thickness (in)

Widest bar (in)

Bar size

Bars per phase

Carries it at 1,000 A per sq in, copper

2 × 1/4 × 4 in per phase

2.000 sq in (1,290 sq mm) per phase · good for 2,000 A at this density · running at 1,000 A per sq in

7.73 lb per foot per phase (11.50 kg per metre) · each bar 6.35 × 101.6 mm

One 1/4 × 4 copper bar on its own, CDA Table 1: 1,250 A at 30°C rise, 1,700 A at 50°C, 1,950 A at 65°C · 8.23 microhms per ft DC at 20°C. These are single-bar values; they do not multiply when bars are stacked.

2 · Take off the bar for the whole lineup

Sections

Section width (in)

Neutral bus

Ground bus, 1/4 × 2

Vertical bus (A)

Sections with vertical bus

Vertical bus height (in)

Run-backs to devices

Run-back current (A)

Run-back length (in)

Your copper bar price ($ per lb)

Scrap and fabrication allowance (%)

Bus bar for the lineup, copper

788 lb

358 kg · 240.0 ft of bar in 6 runs · lineup 12.0 ft wide

Enter your price per lb to turn the weight into a cost.

Vertical bus, run-backs and neutrals are sized at the same density, thickness and width limit as step 1. Plating, joints, hardware, supports and labour are not in this number.

Run	Bars	Each	Bar total	Weight
Main horizontal bus, phases	3 × (2 × 1/4 × 4 in)	144.0 in	72.0 ft	278.2 lb
Main horizontal bus, full neutral	1 × (2 × 1/4 × 4 in)	144.0 in	24.0 ft	92.7 lb
Vertical section bus, phases	12 × (1 × 1/4 × 3 1/2 in)	72.0 in	72.0 ft	243.4 lb
Vertical section bus, full neutral	4 × (1 × 1/4 × 3 1/2 in)	72.0 in	24.0 ft	81.1 lb
Run-backs to devices, phases	24 × (1 × 1/4 × 2 in)	18.0 in	36.0 ft	69.6 lb
Ground bus	1 × (1 × 1/4 × 2 in)	144.0 in	12.0 ft	23.2 lb
Total			240.0 ft	788.3 lb

How to calculate busbar size for a panel or switchboard

The busbar calculation formula is one division: area = current ÷ density, where density is the current each square inch of bar is allowed to carry. Makers of UL 891 switchboards offer two kinds of bus. One is sized by a heat-rise test on the finished design. The other is density rated: no more than 1,000 A for every square inch of copper, or 750 A for every square inch of aluminum. The density rule needs no test, which is why it is the one a calculator can do honestly.

Worked example, the one the tool opens with: a 2,000 A copper main bus needs 2,000 ÷ 1,000 = 2.0 sq in per phase. A 1/4 by 4 inch bar is exactly one square inch, so that is two 1/4 × 4 bars per phase. If your enclosure only takes a 3 inch bar, set the widest bar to 3 and the tool moves to three bars. Leave bar size on Auto and it picks the narrowest standard bar that carries the current. Change the density field if your own tested design runs at a different figure.

Busbar ampacity: the table and the density rule give different numbers

Look up that same 1/4 × 4 bar in the Copper Development Association table and it says 1,250 A at a 30°C rise, 1,700 A at 50°C and 1,950 A at 65°C. The density rule says 1,000 A. Both are right, for different situations. The table is for one bar, on edge, in open air indoors at 40°C, free to shed heat from both faces. A switchboard bus is several bars stacked inside a steel box next to breakers that make their own heat.

So the tool shows both, labelled. Use the table to compare bar shapes and to see how much a wide thin bar gains over a thick narrow one. Use the density rating, or your own tested design, for the bus you are actually quoting. The table values do not multiply when you stack bars, and the tool will not multiply them for you.

Copper busbar weight calculation

Weight is cross-section times length times the metal's density. Copper is 0.322 lb per cubic inch, which works out to 3.86 lb per foot for every square inch of bar. Aluminum bus conductor is about 0.0975 lb per cubic inch, or 1.17 lb per foot per square inch.

That gives the copper against aluminum trade in two numbers. Aluminum needs a third more cross-section to carry the same current at its 750 A density, so the bars are bigger and the enclosure has to hold them. But the aluminum bus weighs only about 40 percent of the copper one. Which is cheaper depends on your bar prices, which is why the price field is yours to fill in and the tool does not guess at it.

Where the bar in a lineup actually goes

Sizing one bus is the easy half. The quote needs the total, and the total is a count:

Main horizontal bus: the full width of the lineup, three phases, plus the neutral if there is one.
Vertical bus: one set in each section that feeds devices, usually at a lower rating than the main bus.
Run-backs: the short bars from the vertical bus to each breaker or switch.
Ground bus: one bar along the bottom, full width.

The opening example is a four section, 12 foot lineup: 2,000 A main bus with a full neutral, 800 A vertical bus in every section, eight 400 A run-backs and a ground bus. It comes to 788 lb of copper in 240 feet of bar. The main bus is 371 lb of that. The vertical bus is 325 lb, nearly as much, which is easy to miss if you size only the main bus. We worked this example by hand before publishing the tool, and it agrees with the weights printed in the Copper Development Association table to within a tenth of one percent.

What this busbar calculator does not check

A bus that carries the current still has to survive a fault. Short-circuit bracing, support spacing and the withstand rating come from the tested design, not from cross-section. Neither does this tool cover temperature rise inside your enclosure, joints and plating, or what your listing allows. Treat the result as the takeoff for a quote and a check on a design, not as the design.

And the bus is only as right as the rating you start from. On one public switchgear specification we read, page 9 gave two fault ratings that could not both be true, and the cheaper reading and the safer reading were different lineups. That kind of problem is in the document, not in the arithmetic. We wrote up what twelve real specifications ask for in how to quote switchgear.

Sources. Bar sizes, weights, resistance and single-bar ampacities: Copper Development Association Inc., Table 1, Ampacities of Copper No. 110 Busbars. Density ratings of 1,000 A per square inch for copper and 750 for aluminum: as published by switchboard makers for UL 891 gear, for example this product sheet. Found a mistake? Write to atishay@mavlon.co and we will fix it.

The bus is one line of a switchgear quote. The specification is the rest.

Mavlon reads the customer's specification and drawings, ties every requirement that changes the price to its page, flags the ones that disagree, and drafts the quote in your own pricing rules.

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Shown on a public switchgear specification

Frequently asked questions

How do you calculate busbar size?

Divide the current by the current density you are designing to. That gives the cross-section each phase needs. Density rated switchboard bus uses 1,000 A per square inch for copper and 750 for aluminum, so 2,000 A in copper needs 2.0 square inches, which is two 1/4 by 4 inch bars per phase. Then check that the bars fit the enclosure and that the design is braced for the short-circuit rating.

How many amps can a 1/4 x 4 copper busbar carry?

On its own in free air, the Copper Development Association table gives 1,250 A at a 30 degree C rise, 1,700 A at 50 degrees and 1,950 A at 65 degrees, for one bar on edge indoors at 40 degrees C ambient. Sized by the 1,000 A per square inch density rating, the same bar is a 1,000 A bus. Inside an enclosure, next to other bars, use the density rating or your own tested design, not the free-air number.

What is the weight of copper busbar per foot?

3.86 lb per foot for every square inch of cross-section, from copper's density of 0.322 lb per cubic inch. A 1/4 by 4 inch bar is one square inch, so it weighs 3.86 lb per foot. Aluminum bus weighs 1.17 lb per foot per square inch.

Does stacking two bars double the ampacity?

Not in the ampacity table. Table values are for a single bar that can shed heat from both faces. Bars stacked face to face heat each other, so two bars carry less than twice one. The current density method avoids the question by counting total cross-section and holding it to a conservative density.

Is aluminum busbar bigger than copper for the same current?

Yes. At 750 A per square inch against copper's 1,000, aluminum needs one third more cross-section. It is much lighter, though: about 40 percent of the weight of the copper bus that carries the same current.

What is a density rated bus?

A bus sized by cross-section rather than by a heat-rise test: no more than 1,000 A per square inch of copper or 750 A per square inch of aluminum. Makers of UL 891 switchboards publish it as their fully rated option. Specifications often ask for it by name, and it uses more metal than a bus sized by test, so it belongs in the quote as a priced line.