"Robatoy" Writes:

I have 30 amps of 220 (single phase) available to my shop.
When my electric heater (20 amps) is running, which isn't often because
the shop is very well insulated and my compressor (10 amps) kicks in,
..mmm let's just say.. the lights dim a little.

Toller in another post has pointed you in the right direction; however,
there is even a simpler way.

Install a DPDT non fused, 250V, 30Amp, disconnect switch that you manually
switch.

Leave the switch in the air compressor position.

When you get cold, switch to the heater.

When things warm up, switch back to the air compressor.

Low cost, simple and reliable.

HTH

Lew