Distribution board size calculator

How board sizing actually works

A distribution board is a box with a length of DIN rail in it. Everything that clips onto that rail has a fixed width, measured in modules, and one module is 18 mm. A single pole MCB is one module. A four pole RCCB is four. Add them up and you have the rail length you need.

The part people get wrong is spare capacity. A board sized to exactly fit today has no room for the circuit somebody adds next year, and adding a second board later costs many times what two spare ways would have cost at the outset. Around 20% spare is the usual allowance.

The other common error is forgetting the incomer. An isolator or main switch takes rail space like anything else, as do surge protective devices, contactors, indicators and blanking pieces. They are all in the list above for that reason.

Common questions

What is a module on a distribution board?

A module is the standard width one device occupies on the DIN rail inside the board. One module is 18 mm. A single pole MCB takes one module, a two pole device takes two, a four pole RCCB takes four. Board sizes are quoted in modules for single phase boards.

How much spare capacity should a board have?

Around 20% is normal trade practice, so a board needing 10 modules is built as a 12 module. It is a convention rather than a code requirement. Spare ways cost very little when the board is built and a great deal once the wall is closed up.

Why are three phase boards counted in ways, not modules?

A three phase board is described by the number of three pole outgoing ways it can feed, because each outgoing circuit occupies a set of poles across all three phases. Module counting still tells you the physical rail width you need, which is why this calculator reports both.