What is the max theoretical temperature this heater will heat a 400mm square aluminum build plate to? Ordered this without doing any research (stupid) and the seller refused to cancel the order so now I am stuck with it.
Couldn’t tell you what max it’ll hit, but the general rule of thumb is 0.25-0.5 watts per square centimeter. You’re at the low end of that range so it’s probably ok? May take a good long time to hit ABS temp though.
@Ryan_Carlyle as long as it is able to hit abs temps I’m fine. This printer is fully enclosed and wasn’t made for short prints anyways.
@Carter_Schunk fully enclosed you should be fine. Giving it some time to warm up the enclosure is a good idea anyway.
@Ryan_Carlyle thanks for the help, was freaking out a little when I realized I should have gone AC and the seller refused to cancel. But looks like it should be good enough.
@Carter_Schunk LOT of amperage though. Use an SSR, solder heavy gauge wires directly rather than using crappy 12-15A screw terminals…
@Ryan_Carlyle yes, I have a mks mosfet rated for 30A, bed should draw about 20A, and I’m going to use 8 awg wire.
8ga sounds like overkill but overkill is fineIt’s 450W at 24V. You can calculate how many watts it would be at a higher voltage. Use a TCO in any case, and a MOSFET that supports the voltage you choose. 36 and 48V supplies are available, and your 24V supply might be adjustable to a slightly higher voltage as well. Just make sure you are well within the power limits of your supply and both power and voltage limits of your MOSFET, and set up a TCO correctly to protect against MOSFET failure regardless of the voltage you use. Don’t burn down your house!
Thermally connected to the bed, electrically isolated with kapton tape. Look for reprap forum posts from the_digital_dentist for details.
Be aware that using a higher voltage, you also have to take responsibility for the additional current flowing through the wires to the bed, and a TCO won’t help you there; that will just burn down your house if you get it wrong. I meant to have said that before and just now realized that I hadn’t said it. 
The TCOs I bought are good only to 10A which won’t help you because your heated bed will draw more like 20A even at rated voltage — read and understand https://upload.wikimedia.org/wikipedia/commons/c/c2/Ohms_law_wheel_WVOA.svg before using a different voltage 
I would suggest searching http://mouser.com and http://digikey.com for TCO parts of 30A or higher. If you make sure that the wires to the bed are thick enough to handle higher current, and use a higher voltage to increase power output on the bed, the TCO current rating will need to be correspondingly higher. E.g. if you put a 48V supply on it (eek!) to produce 1800W you would need a power supply able to supply 40A continuous (urk) and a TCO rated enough higher than that for a comfortable engineering margin. I’m not an engineer and am really not qualified to give you advice here on how to size that, only that you should take that into consideration and read datasheets.
When you look at the price of a high current capability power supply and upgrading the wire to try to protect against burning down your house and a high current capability TCO, you might instead say, “Oh, look, I accidentally bought a 400mm x 400mm base for my filament drying and annealing oven that I didn’t know I was going to build some day” and find it to be less expensive overall to go order a new heater rated for your mains voltage and use an SSR to switch it. In that case, the 10A TCOs that I found are likely to be quite sufficient: https://www.amazon.com/gp/product/B0719DHV9P — but remember I’m not an engineer and not qualified to give you advice.
That’s pretty much what I did: I bought a tronxy x5s, ran the bed at 24V so that it was possible to bring it up to temperature before the heat death of the universe, then bought all new parts for the heated bed from cast aluminum tool plate to SSR to mains-voltage-rated heat pad, and plan to build a drying/annealing oven with the old tronxy bed when I get around to it. (It’s so warped it’s pretty much all it is good for… )
A 400w 24v heater will heat it exactly the same as a 400w 240v heater. (I am just talking heat here)
…and you can use Ohm’s law to figure out the resistance of that 400W 240V heater! (You want 800-1600W for a 400x400mm bed.)
@Org_Jansen_van_Rensb correct, but the 24v has 10x the current and 1/1000th the likelihood to kill you