On 02/01/2014 07:39, Adrian Brentnall wrote:
HI Bill
I'm always a bit nervous of the 'commoned-contacts' approach, as
Murphy's Law suggests that one set of contracts will operate slightly in
advance of the other set, and will end up making or breaking the lion's
share of the current.

I'm deliberately over-engineering this project! (Partly because I want
it to be reliable, and partly for safety reasons - as it's a box of
heat, operating unsupervised, in a timber workshop.
For this reason I'm forgoing the usual 'let's see what's in the junk
box' approach, and speccing / buying new components. Call me paranoid!

Why not look for a solid state power controller..

You can get a 50A rated one for about £50.

They come in mark space control and in phase control.
I think you want mark space for your application.

The power follows a DC input voltage of about 0-5V so you can drive it
with a Pi or similar.

Don't forget to fit a heat rise fire detector to trip the supply.