Emcoturn 120 lathe retrofit

You almost certainly can use lower-rated breakers. You might even want to. If you are running full power on the spindle and both axes both at the same time it is likely that something has gone badly wrong.

There is a concept in rating for wiring and breakers called "diversity". You don't assume that every part of the system is running full power all of the time.

I don't have any actual fuses in my system, I use MCBs throughout. They are so cheap that it seems pointless not to.

I have a single 20A for the motors and two 3A ones for the switched and unswitched low-voltage supplies.

So, there is a 3-pole isolator switch (with only 2 poles in use) feeding a single 3A breaker and the main contactor. That breaker feeds a 24V supply for GPIO (and the main contactor) and a 12V supply for the PC. The main contactor then powers the 20A breaker for servo and spindle power and a second 3A breaker for the spindle gearbox clutches and brake.

The 20A breaker is C-rated for motor use. The 3A ones are not, but soon will be, as the 3A B-rated type tend to trip on the inrush current of the PSUs.

LutzTD wrote:

replace fuse 1 with 2 pole or 3 pole 20amp/pole MCP, how do I know if it is c rated?.

It says on the front. (well, it does here, things might be different where you are).

www.tlc-direct.co.uk/Main_Index/Consumer...ssories_1/index.html

There are types A and D too, but you rarely see them.
A bit of extra detail: www.tlc-direct.co.uk/Technical/Distribution/Mcb.htm

You can probably take the 5V supply from the PC PSU, it might even be advantageous from the point of view of ground commonality between 7i76 logic and the PC.