Mixing cable sizes

Close enough to a 1/0 and about 30% bigger than a 1.
You mentioned both of those, so I covered them both.

Many thanks Brian.
I should have proof read my post.
I meant 1/0 b&s not 1 b&s
Regards,
Ian

1 B&S has a cross sectional area (CSA) of 42.41 square mm, 2 B&S has a CSA of 33.63 square mm and 4 B&S has a CSA of 21.15 square mm.

Thus a 2 B&S plus a 4 B&S has a combined CSA of 33.63 plus 21.15 or 54.78 square mm, so it has a larger CSA than the 1 B&S. Current carrying capacity is not rated solely on CSA, it depends on the cooling ability (surface area) of the cable and airflow as well.

You need to be careful how you protect the cables by fuses or circuit breakers.
Fuses and circuit breakers are there to protect the cables, to stop them conducting too much current, overheating and then causing a fire.

If for example you have two wires rated at 10A each in parallel, then the combination has a current rating of 20 amps.

If you protect the circuit with a single 20A fuse or circuit breaker and one of the cables has its insulation worn off on the chassis then that cable will draw far more than it is rated for and may overheat and cause a fire.

In your case for chassis wiring 1 B&S is rated at 211A, 2 B&S is rated at 181A and 4 B&S is rated at 135A.

I hope this helps.