Is there any drawback to use oversize AC SSR for heated bed?
I plant to use on 220V 950W heated bed and looking at Fotek SSR-10DA, SSR-25DA and SSR-40DA. All of them have 3.0mA leakage current.
Should I go for 10A or 40A since price seem to be very close.
Check the on-state “resistance” of both. The 10A will do but the 40A may be designed to dissipate a similar amount of heat for its larger current rating i.e. it might not get as hot during use as the 10A one.
A downside to using the 40A model might be a higher holding current. That is the minimum current required to keep the relay in an on state once triggered.
If the load cannot guarantee this minimum current (over as much of each mains half-cycle as possible) it can lead to early turn-off of the relay and hence failure to deliver full power to the load.
Aside from the excellent points Neil mentioned, you DO want a decent safety factor on the rating to account for non-optimal heatsinking, bed resistance tolerances, etc. You shouldn’t ever put say 8-10A on a 10A circuit unless you’re capable of doing the electrical engineering to apply proper de-rating factors. But 10A should be adequate for your <5A bed.
Or go for the middle one, 25A?
yup, a lot of clone floating around on ebay and aliexpress.