SSR size

I'm putting together a 110v dual element ebiab system on 20a circuits. I've read a lot of the threads on similar systems and I've noticed that most of them are using 40a ssr's. What is the reason for using 40a ssr's on a 20a system wouldn't the 25a ssr (ssr) be enough?

Thanks for the help

 

The reasoning is the heat generated. SSRs produce a lot of heat as a biproduct of switching the power output. A 40A SSR will have larger components capable of handling heavier loads. Running it at 50% capacity will generate significantly less heat than running a 25A SSR at 80% load.
-Kevin

 

Not true - a SSR will dissipate about 1 watt of heat per amp passed through. And the heat generated by a 40 Amp SSR will be the same as the heat generated by a 25 Amp SSR. You will need the same size heat sink for 20 Amps regardless of the SSR you use.

The difference is the 40 Amp SSR contains a TRIAC capable of handling 40 Amps while a 25 Amp has a TRIAC capable of handling 25 Amps. A 25 Amp SSR will work fine.

 

Partially true, but as previously posted a 40A SSR will be designed to dissipate more heat (as it was designed to run up to 40A), which means it will run cooler at 20A then a 25A SSR will, and hence will provide longer life/more reliable usage (W/degC rating). A 20A ssr will have a higher junction temperature at 20A then a 40A SSR @ 20A (the heatsink point is right though). I almost always see SSR's over sized in commercial designs for this same reason. If your going to use a high quality ($$$) SSR then you could get away with a 20A one, if your planning to use the cheaper ones most homebrewer's do then I would defiantly oversize.

 

The 40a will probably last you longer, and there's very little price difference.

 

will 2 120v elements work on 1 40a SSR?

 

I use this one!

 

It depends on the wattage of the elements, not the voltage.

 

Hmmm. So, if you run 40A through the 25A triac, what happens?

 

True, but the highest wattage in a 120V element is 2000W. Two of those draw 33.3A. That's well below the SSR's rating. To my knowledge, that's the highest power you can get at 120V (even two 6000W 240V element at 120V won't draw that much current).

So to answer your question, that should work fine.

 

Perfect. I was thinking (2) 2000w 120v powered by 1 paid and 1 40a SSR.

 

Well make sure you have a 40A circuit in your control box, or be ready to pay a couple hundred for an electrician to install it. If you threw in (2) 1500W elements you could probably run this on a 30A circuit-something to consider. If you're brewing 10gal you'll probably need the two elements, but I don't think it's necessary for a 5 gal system (overkill).

 

It shorts out & stays on all the time.

 

If you go with 2 elements, you need the 40A SSR FYI