Garage expansion / MCB rating

[Magsy]

I've just moved into a virtually new home with detached double garage and has the original install as supplied.


I need to add a lot of light internally in the garage…..something like 12x58w tubes. I also would like to add 2-4x double sockets around the space.


The current setup has 2.5mm T&E running from the utility room / main board on a dedicated 16a MCB where it goes through some conduit out to the garage through the wall. The distance covered is ~8m.


In the garage it directly enters a double socket, where it then runs into a 3a FCU to feed the flood light and interior light switches.


The tubes are going to run right up to the limit of the 3a fuse in the FCU and if the floodlight comes on (400w) it’ll definitely be over. Ideally I’d put a 5a fuse in the FCU.


I do not understand why the MCB is 16a and not 20a. If I could swap to 20a MCB, which the 2.5mm T&E should be rated for in conduit, I could comfortably use a 5a fuse in the FCU feeding the lights?


The test sheet seems to say ‘Garage 20a MCB’ too.


As this is a radial, can I simply add additional sockets by continuing the radial socket to socket?


All pointers appreciated. The likelihood of a full load, exceeding 16a, is not great but plausible in a few circumstances (big jetwash, all the lights on etc)

 

[Marcus John]

Hi Magsy, I'm just coming to the end of my 2nd year EAL Level 2 so am just a novice but from my understanding one reason for down rating the 20A MCB to a 16A could be if the installation conditions of the cable changed like running past a boiler, running through insulation or being grouped with other cables etc... but as you say the tabulated current carrying capacity of the 2.5mm2 cable in conduit (even in an insulated wall) is 20A and for 8M length you meet the voltage drop requirements.
Regarding extending the radial with additional sockets, my understanding is you can extend using 2.5mm2 cable for unlimited sockets but appreciate the longer cable will increase voltage drop and earth fault loop impedance will increase and there will be a maximum resistance (Zs) in order for your MCB to operate within 400 milli seconds.
Regarding the fused spur for lighting, you could put a 13amp fuse in there and come off with 1.5mm2 cable.
This is just my limited opinion and am looking forward to reading more posts from the professionals. Cheers.

 

[westward10]

I see no reason why you cannot up it to 20A and change the fuse in the spur to 5A. As for increasing the circuit Zs well yes you will but being a relatively new house I would assume it has 30ma rcd protection. If in doubt get an electrician in.

 

[telectrix]

first consideration for me would be LED lighting.

 

[Specialist]

Agree with westward10 & telectrix, LED would give you nice bright lighting & cost less to run.

 

[telectrix]

esp. replacing a 400watt monster with a 30/40watt LED flood. no brainer.

 

[Octopus]

12 x 58 w tubes are going to give off a lot of light..... Are you going to be growing plants in there?

 

[Wilko]

Tanning salon ...

 

[telectrix]

blue movie studio?

 

[Specialist]

blue movie studio?

Wouldn't that much light make them Over exposed ?

 

[Spoon]

I'm also curious why you need 700 watts of light in a garage. That's more than I use in my whole house.

 

[Taylortwocities]

I too am suprised that Building Control even allowed a 400watt ouside light on a virtually new home.

Building Regs Part L1&2 sayeth this:
Fixed external lighting should not exceed 150W and be controlled so it automatically switches off or can only accept lamps with efficacy >40 lumens/Watt.
First job: rip it out and put in a 20watt LED floodlight, that sorts your lighting load concerns in one go.

 

[Magsy]

Thanks for the considered responses.

I cannot be certain which method has been used for the run as it exits the house to enable me to exactly determine the (de)rating for the T&E. It passes through the kitchen and exits the wall 4m from the board. The rear of the kitchen is all glass doors and open plan, I doubt they ran under the floor or used a cavity, so I'd assume it is running over the ceiling clipped to the joists. I'm not sure what is between the floors so perhaps a few holes are needed to see if there is insulation. I'd like to know for sure but it will be hard to determine.

The garage will be used for car detailing and some paint work and I need high CRI (93+) lighting. This drops efficiency per watt to something like 0.63, compared to normal tubes you lose nearly 25% of the light output so need more tubes. The garage is 6mx6m, will be painted white and have a white ceiling but you can never have too much light

I also dabble with photography, no naked ladies I may add and full colour reproduction is a major benefit.

They won't all be on all of the time, but obviously need to engineer for worst case.

The flood light is generating 8500 lumens and lights the whole four car square driveway; it is quite impressive. Again I want high CRI and halogen has perfect colour reproduction. I could swap to LED but it would need to be 100w+ for that lumen output and they get expensive if you want a decent, high CRI one. However, I agree that if something has to give, it will have be this. The house runs up the side of the driveway so I could potentially move the flood lights to the house wall and feed.

The flood fitting is marked internally as 400w max bulb. I am unsure what it came with as the bulb was missing when I moved in; I put the monster bulb in.

Can a clarify please how I add the sockets? In short, if I cable around the top of the interor wall with 20mm trunking, can I just T off for each drop, or do i need to take the cable up and down as in the last picture? I could do what the first picture shows but I'd rather have the trunking up high and out of the way.

 

[Magsy]

I too am suprised that Building Control even allowed a 400watt ouside light on a virtually new home.

Building Regs Part L1&2 sayeth this:
Fixed external lighting should not exceed 150W and be controlled so it automatically switches off or can only accept lamps with efficacy >40 lumens/Watt.
First job: rip it out and put in a 20watt LED floodlight, that sorts your lighting load concerns in one go.

Just to add to this the house has a number of PIR activated, external, compact fluorescent lights so they did mostly adhere. I've read this spec as you detail in the handover/planning/reg docs I have. It seems this was missed or didn't apply for some reason on the garage.

 

[Specialist]

Picture 3 would be my preferred method, cable around top then a single drop of 25x16mm trunking down to each socket & not a junction box in sight.