True dat.

Even when they are rated for 25  Amps, if you get a datasheet for that SSR it might have voltage-current curves showing that at a certain point (well before 25 amps) that drop off when reaching the limit. A higher rated SSR is probably what you need to keep out of that drop zone.

Well, tested my current 25A SSR with a lower amp draw device (DC cordless recip saw motor) running full speed for about 5 minutes...the SSR never even got warm; and the voltage measured across the terminals was about 0.44 vs 4.6 when hooked to the PCB Heatbed; So, as you said, this is clearly a matter of overating on the SSR.  Ordered a 60A version and will wait for that.  In the meantime, I switched off PID in the firmware and am using an automotive relay rated at 30A.  Clicks on and off every 30 seconds or so, but it works for now.

Thanks to all for your guidance!

Try heatsinking it. You are supposed to heatsink these relays if your using more than 25% its rated current. It may be overheating and causing it to limit the current.

I do have it on a heatsink; it gets so hot in about 40 seconds that I can barely touch it.  It reads 4.5v over the output connectors on the SSR whenever it cycles on (and connected to heatbed).  I read about 7.5v across the heatbed terminals BTW...

I am told it should read closer to 0.0 across the SSR output terminals while cycled on...

Wiring looks OK . I know from experience those SSR s can only handle 1/2 of their rated current max  Just get one rated for 60 amps or use an automotive relay . The clicking is only 2 or times a minute and I like to hear it because I know the bed is not overheating  just by listening

Will definitely order a larger one...regarding mechanical relay, doesn't that require switching off PID in firmware? And what about arcing and seizing closed?

But also, I truly wonder if I'm just wiring something wrong:

Any thoughts?

Even when they are rated for 25  Amps, if you get a datasheet for that SSR it might have voltage-current curves showing that at a certain point (well before 25 amps) that drop off when reaching the limit. A higher rated SSR is probably what you need to keep out of that drop zone.

Whats the model number of your SSR?

I am going the SSR route with my PCB heater which draws about 20A at startup...I fried the first SSR even with a heat sink in about 10 minutes from startup; and when I tested the replacement I ordered (same unit - rated at 25Amps DC), it started to behave the same way. 

I tested the voltage across the output leads of the SSR whilst powered on, and it read about 4.6...shouldn't that read closer to zero?  4.6 would indicate that there is some internal resistance in the SSR and that's what is causing it to overheat.

I have the signal wires from the RAMPS connected to the Input side (correct polarity) and I have the Output side wired in series into the power loop from a 30A stand-alone power supply (those would be the + and - terminals on the PS).

I never switched off PID in the firmware...the LED on the SSR vibrates on and off so quickly it is flickering.

Any leads (pun intended) on what I am doing wrong?

I have one last question, I did measure the resistance of my custom PCB and its about 0.2 Ohm. I noticed the following lines in Configuration.h

// If your bed has low resistance e.g. .6 ohm and throws the fuse you can duty cycle it to reduce the
// average current. The value should be an integer and the heat bed will be turned on for 1 interval of
// HEATER_BED_DUTY_CYCLE_DIVIDER intervals.
//#define HEATER_BED_DUTY_CYCLE_DIVIDER 4

Have any of you used this for a similar issue?

First off, I am probably not going to start with a SSR, I will only go there if the clicking of a mechanical relay drives me totally bonkers. We will see on that front.

I did have a 12V @ 20A relay sitting around, so I wired it up this morning and tested it out. It works great! The bed heats up to 105C in about 3 minutes, where my standard SD2 took about 30 min to get to 95C. What I didnt like was the wire gage, with the increased current I need to beef up my wire gage so thats #1. The relay also got somewhat warm, but not to the level I couldnt touch it. The one I had lying around was from an Arduino relay unit and has no feature to mount a heat sync of any kind, so its kind of hanging loose (dont worry I insulated it with Kapton). I'm headed out now to get a 12V mechanical car relay, rated for 40AMPS and with a heat sync that I will be mounting to the back of the machine itself. With a heavier gage wire I hope this is a good solution.

I'll post a follow up. But if any one sees something errant in my approach. Please let me know!