I bought a double pole switch for my router table. It says it can handle
20a/240v or 35a/120v.Why would each pole be able to handle only 20a on 240v (with 120v on each pole) but 35a on 120v? Or does it assume you are connecting the 120v to both poles to get the greater capacity.
I wrote the company but got a reply that didn't really say anything.
The limitation is on power, not current.
My guess is the reason is if the poles start to open at very slightly different times you will have a 240V arc across the pole that opens earlier. A higher voltage arc is harder to extinguish.
For the same reason you can't reliably parallel the 2 poles for 120V. If one pole opens slightly early the other pole will interrupt the full 35A.
AC only switches (not intended for DC use) intentionally operate slowly so zero crossings of the AC supply voltage help extinguish the arc. Slow operation probably makes it more likely the poles will open at slightly different times.
Okay, that makes sense. Thanks for explaining it simply. My old tablesaw came with a 1hp relay and a 2hp motor. To help it along they wired two poles in parallel, but eventually one fused. I suppose that is the same problem you are describing.
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