B&S Cable Size Chart for 12V

Copper area, amp ratings and lug sizes for every B&S and auto cable size, plus a voltage drop chart that tells you which size your run actually needs.

The smaller the B&S number, the thicker the cable. 0 B&S carries more than 6 times the copper of 8 B&S. And auto cable sold as “6mm” is not 6mm² of copper. That figure is the outside diameter, insulation and all.

Jump to: B&S size chart · Why ratings differ · What size do I need? · Lugs and Anderson plugs · FAQ

B&S cable size chart

B&S stands for Brown & Sharpe, the same gauge system as American Wire Gauge (AWG). In Australia it is the name used for the heavy cable that carries real current: battery links, DC-DC charger feeds, winches and inverters. Copper areas below are calculated from the strand count of the Narva and Tycab cable we stock, every size built from 0.32mm strands.

The AWG nominal column is the theoretical copper area for that gauge number, which is what most online conversion charts quote. Australian automotive B&S cable is built to its own strand pattern and usually carries a little less copper. 0 B&S is 48.9mm² here against 53.5mm² in theory, which is why two charts can disagree on the same size.

Auto cable sizes (3mm to 6mm)

Lighter automotive cable is sold by its outside diameter in millimetres, not its copper area. It is the cable for lights, switches, sockets and small accessories.

Why the amp rating changes between brands

Look at the two rating columns in the chart. At every B&S size where we stock both brands, Narva and Tycab use the same number of 0.32mm strands, so the copper is identical. The ratings are not. 4 B&S is sold as 185A by Narva and 135A by Tycab.

An amp rating is the current a cable can carry before its insulation runs too hot, under the conditions its maker chose to test it in. Ambient temperature, whether the cable is bundled or in free air, and the insulation's own temperature rating all move the number. Different conditions give a different figure for the same copper, so the safe way to read two ratings is as a range. Size on the lower one.

Twin core cable is rated lower again, because two conductors inside one sheath shed heat more slowly. Tycab 6 B&S is rated at 125A as single core and 103A as twin core. Narva 2 B&S is rated at 255A as single core and 225A as twin core.

The rating tells you when the cable overheats, not whether your gear gets enough voltage. On a 12V system, run length usually decides the cable size well before the amp rating does.

What size cable do I need? 12V voltage drop chart

Find the current your gear draws in amps down the side, and the distance from the battery to the gear in metres across the top. The cell shows the smallest cable that keeps voltage drop under 3%, which is 0.36V on a 12V system, and still sits inside the lower of the Narva and Tycab ratings.

Assumes the positive and negative cables both run the full distance, so the cable in the circuit is twice the one-way run. Uses a copper resistance of 0.017 ohm mm² per metre, the figure in REDARC's voltage drop formula. Resistance rises by about 0.4% for every degree above 25°C, so a run through a hot engine bay loses a little more than the chart shows.

Working it out yourself

If your numbers sit between the rows, or you are on 24V, put them into our 12V cable size calculator, which runs this formula and shows its working. Or do it by hand:

Copper area needed (mm²) = 2 × one-way length (m) × current (A) × 0.017 ÷ allowed drop (V)

For a 40A DC-DC charger mounted 5m from the start battery: 2 × 5 × 40 × 0.017 ÷ 0.36 = 18.9mm². 6 B&S at 13.5mm² falls short, so the answer is 4 B&S at 20.3mm².

Charger makers often size tighter than 3%. Put REDARC's recommended cable sizes through the same formula and each works out at about a quarter of a volt of drop. When the manual for your DC-DC charger gives a cable size, use it ahead of any general chart, this one included.

On 24V, the same wattage draws half the current and 3% becomes 0.72V, so you can usually go down a size or two. Rework the formula rather than reading the 12V chart.

Lugs, terminals and Anderson plugs

Cable lugs are sold by metric copper area, not by B&S number. Pick the smallest lug rated for at least the copper area of your cable, which is the Lug to suit column in the chart above. 6 B&S takes a 16mm² lug, 4 B&S a 25mm² lug and 0 B&S a 50mm² lug.

A lug that is too small will not take every strand, and one that is too big will not crimp tight. Use a crimper with a die for that lug size and seal the joint with adhesive-lined heat shrink. Auto cable from 3mm to 6mm uses insulated crimp terminals sized to the cable instead of lugs.

The 50A grey Anderson plug is made for 8 B&S cable, which is why 8 B&S is the usual cable behind a rear fridge or accessory outlet. If the chart says your run needs heavier cable than the plug's contacts accept, move up to a larger connector rather than trimming strands to fit. Our dual battery install guide walks through a full setup.

Common questions

What does B&S mean on cable?

B&S stands for Brown & Sharpe, the same gauge numbering as American Wire Gauge (AWG). The numbers run backwards: the smaller the number, the thicker the cable. 0 B&S carries more than 6 times the copper of 8 B&S. In Australia B&S is used for the heavy cable in battery, charger, winch and inverter circuits.

Is 6mm auto cable the same as 6mm² cable?

No. The mm figure on auto cable is its nominal outside diameter, insulation included. 6mm auto cable has 4.58mm² of copper, and 3mm auto cable has 1.13mm². Always compare cables by copper area in mm², not by the mm size.

Is 2.5mm house wiring the same as 2.5mm auto cable?

No. 2.5mm² house cable is sold by copper area and has 2.5mm² of copper. 2.5mm auto cable is sold by outside diameter and has 0.64mm², about a quarter as much. The auto cable closest to 2.5mm² house wiring is 5mm auto at 2.90mm².

What size B&S is 16mm² cable?

16mm² sits between 6 B&S (13.5mm²) and 4 B&S (20.3mm²). The Tycab 16mm² marine battery cable we stock is listed as 6 B&S and takes standard 6 B&S or 16mm² lugs, so in practice the two names get used for the same job.

What is the amp rating of 4 B&S cable?

4 B&S has 20.3mm² of copper. Narva rate it at 185A and Tycab at 135A. Size on the lower figure, then check your run length, because on a 12V system voltage drop usually limits you before the amp rating does.

What is the amp rating of 3 B&S cable?

3 B&S has 26.5mm² of copper. Narva rate it at 215A and Tycab at 200A.

What is the amp rating of 0 B&S cable?

0 B&S has 48.9mm² of copper. Narva rate it at 330A and Tycab at 315A. It is the size typically used for winch and large inverter feeds.

Can I use the chassis as the negative?

For many accessories, yes, and only the positive cable length then counts towards voltage drop. The table on this page assumes a full negative cable, which is the safer assumption, because a corroded or painted earth point adds resistance no chart can see. For DC-DC chargers and inverters, follow the manufacturer's wiring instructions.

What size fuse should I use?

The fuse protects the cable. Pick one rated above what your gear normally draws but at or below the cable's rating, and mount it as close to the battery as practical. The cable rating is a ceiling, not a fuse size.

Need cable?

We stock Narva and Tycab automotive cable from 3mm auto up to 000 B&S, by the metre and on the roll, plus the lugs and Anderson connectors to finish the job.

6 B&S cable 8 B&S cable 4 B&S cable Cable lugs