How many amps can I draw from an add on fuse circuit connected to the fuse box?

I want to install an invertor on my 1996 Honda using a fuse add in circuit like this on:

http://m.aliexpress.com/item-desc/702894978.html

I do not want to install a relay off the battery so is this a safe option to draw 20amps? Though the fuse box.

I only want to know if its safe to draw that much current though this method. I know all about the alternators limitions etc.

If you connect to the battery with the 20 amp fuse you can draw up to 20 amps before the fuse will melt and the circuit opens. If you do know all about alternators then you should be able to calculate how much the invertor will draw from the alternator when you are using the inverter. If you draw too much you have a dead battery.

If depends on the current draw by the device you intend to power up by the inverter to run your ac electric device. I suggest you do a temporary hook up to the battery and measure the current taken by the inverter and the ac load then if less than 20 Amps power the inverter by the method you intend ie a fuse connected into a leg off the wiring loom- making sure the wiring loom cable you intend to piggy back off will take the additional load. If in doubt go back to the battery direct and make a direct connection, always remembering as Jim has said don't overload the vehicle alternator or the battery won't charge correctly.