Garage wiring

What size fuse or breaker is at the supply end of that cable ? when you say 6m, I assume that is a typo and you mean 6mm squared copper cross-section  - if so buried is's max load is a touch under 50 A but will depend on cable type, and if it runs in thermal insulation or is well cooled.

you could do worse than play with the cable calculator on-line here https://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html

If the supply end fuse or breaker is rated to protect the cable (so probably anything up to  about 40 to 45 amps) then there is no risk of anything nasty happening as you increase load, except that fuse or breaker may operate. Given the loads you describe I'd be tempted to verify the protection on the cable is close enough and then see if in practice you get problems with it tripping - I'd suggest you probably will not, and you will do yourself a far greater favour, especially with current price trends, if you use the money saved to draft proof and insulate the snooker room, rather than try to over-heat it. It is remarkable what 6 inches of polyurethane foam slab in the right places can do for an electricity bill ;-)

Mike.

Hi Mike, much appreciated for your reply. The supply fuse breaker is a old metal casing box with a 30A wired fuse, (30A 250V 30FR 22SWG MAX) it just runs under floor crawl space (waist height) then leaves the bungalow, then buried 12ft length underground bringing the cable into the garage.

My garage has loads and loads of insulation, 50mm kingspan in between the studs then 30mm on top of that, So 80mm in total on the walls. 100mm kingspan in the ceiling and also same amount in floating floor. It was a small fortune. 
It’s never broke the wired fuse or tripped from the new CU in the garage. just was wondering if I should upgrade the wire or put in new supply isolator. 
Thanks Mark 

Sorry I meant the cable runs from the metal casing down into the crawl space

Well if it is 6mm2 cable, and not showing signs of damage or age deterioration, then a 30A fuse will certainly be be sensitive enough not to allow it to cook. (In the sense the fuse will always let go long before the cable gets overheated) You may at some point feel the urge to change that fuse to something more modern, but to be honest there is little technical need to do so unless there is something else wrong with it.

In your shoes I'd leave the cable and fuse alone, and use the room as you intend for a year, and see if there is really any problem in practice. Unless there is, there is little point in wasting your money.
The number of sockets and so on is a red herring, you won't really have very many kilowatts of load plugged in for very long, or it will get unbearably hot and after a short time,  either you, or the thermostat, will remove the load, and the cable will not have had time to get hot by then.

Mike

The maximum supply available at the garage will be in the order of 30 Amps, say 7kW total loading approx.  Just add up the individual appliances to find the total loading in Amps or kW. I can't imagine that you will overload the underground armoured cable. 1kW = approx. 4.0 Amps.

Z.

It is probably an M.E.M. switch fuse, so is of very good quality and will last for years if not rusted through or physically damaged.

Do not install fuse wire rated at above 30 Amps, paper clips or nails etc.

https://www.ebay.co.uk/itm/224420972825?hash=item3440884919:g:EPQAAOSwsJtgc05z

It is worth getting en electrician to check the earthing at the garage end. Or as a minimum, buy a socket tester, plug it into a garage socket and see that the correct lights illuminate.

Z

Thank you all guys, especially zoomup. You are 100% correct, yes it’s M E M and I already have the plug tester and all three lights working perfect. Very much appreciated for all the  advice. 
cheers 

Mark