New “commando” circuit for 3hp compressor and welder

[gasket999]

Hi folks.

my garage consumer unit has a good 50a supply, from which I have a ring of sockets on a dedicated 16a MCB, two vacant MCB ports (future proof as when we get electric cars I’ll need to add chargers) and one that I’m looking to use for my larger amp needs (compressor and welding set).

I have a Clarke 160TM welder (says it needs a 16a supply) and a 3hp belt drive 150L compressor (advice is that it needs a 20A supply).

I’m about to tandem-in a second 3hp compressor - but the pressure switches will be set so they don’t start at the same time. I also won’t be running both a compressor and a welder at the same time.

So I’d like to have a single new ring - with three sockets to connect to the welder and the two compressors.

To that end this is my current thought:

1) fit a 20a MCB
2) from this run 10mm cable to the three 32A wall mounted commando sockets.
3) fit 32a commando sockets to the welder and two compressors.

nb the above would be a radial circuit, not a ring. Like the other socket ring in the garage, it will be surface mounted in round plastic MK conduit and matching joints/elbows/tees.

10mm is obviously bigger than needed for 32A, but means if I ever need 40a I don’t have to change the cable - I just have to swap the sockets for fused switches and change the MCB.

Assuming the above makes sense, could anyone help me with a few queries.

1) is it possible/advised to have both the welder (max 16a) and the compressor (max 20a) on the same circuit? I suppose the nightmare is that the welder goes wrong and it gets the full 20a before melting.

2)is it ok for the sockets to be over engineered for the need (32a socket, 20a requirement) and for the cable to be over engineered for the need (10mm2 on a 20a circuit) provided that the MCB is appropriate to the machines (20a)?

3) would it be advisable to fit a type c (or even d) breaker?

thank you very much. I should point out that, like my last workshop build, I do have a qualified electrician lined up to inspect the install, terminate the cables and test. All I do is buy the components, fit all the conduit and loosely run the cables ready for him.

keeps the cost down and also lets me faff around with positioning and making sure the runs are obsessively level!

thank you.

 

[brightspark]

run the radials in 6.0mm if u intend uprating the mcb to a 42 amp at a later date 4;0mm will carry 32 and 2'5mm 25 amp . no harm in using 32 amp sockets instead of a 16 there just bigger

 

[Westfield-builder]

And use "singles", much easier to get through the conduit than T&E.

The cable code should be 6491B for singles,
6242Y for T&E.

 

[gasket999]

Lads - that’s great, thank you! Doing one big order from the hardware shops to stock up for weekends of lockdown diy.

 

[Westfield-builder]

Ohh, and if you use singles the earth (cpc) may be the next size down
IE; 6.0mm phase & 4.omm earth.
4.0mm phase & 2.5mm earth.

 

[MetalMonkey]

Just to be clear - I think you will find you should run a separate radial to each "commando" ceeform socket from a separate MCB - not one radial to daisy chained sockets.
I do seem to recall there is a way around that for multiple 16A sockets on a 20a MCB but if you are looking at a larger supply in the future they should be on separate MCBs.

 

[Domdom]

they should be on separate MCBs.

Thats how the sparkie did mine. Consumer unit with 63 amp main breaker on the incoming. Then a 32a mcb for the 13a socket ring, 2 more 32a mcbs for the 32a ceeform sockets (one each), a 20a mcb for a 16a ceeform and a 6a mcb for the lights. He was quite vocal that nothing other than 13a sockets should be installed on a ring. They all should have their own spur.

 

[Yamhon]

Thats how the sparkie did mine. Consumer unit with 63 amp main breaker on the incoming. Then a 32a mcb for the 13a socket ring, 2 more 32a mcbs for the 32a ceeform sockets (one each), a 20a mcb for a 16a ceeform and a 6a mcb for the lights. He was quite vocal that nothing other than 13a sockets should be installed on a ring. They all should have their own spur.

I thought the combined total rating of the MCB's had to be lower than that of the incoming breaker... is that not so?

 

[Domdom]

I thought the combined total rating of the MCB's had to be lower than that of the incoming breaker... is that not so?

Not according to my mate, professional sparkie BTW.

Looking at the house CU that has a 100a main and 14 mcb ways, all full, ranging from 32a to 6a. Just the 4 mcbs for the ring mains add up to 128a. That’s all 17th edition, signed off etc.

 

[hotponyshoes]

Sparky has always wired mine individually.
Ie. One socket per breaker.

Personally that's the way I would do it. My big old sip mig welder trips the breaker fairly regularly as do the compressors.
Although the compressors are outside and only trip when starting up still pressurised on a cold morning.

I would not want to trip a breaker with a welder then have to go and switch both compressors off, reset the breaker and wander around switching everything back on.

A few of the more modern welders suggest a 20a breaker so they have had that fitted and use the 32a plugs/sockets.
I dont think for one minute that having a 20a breaker will save a faulty welder as 20a should be plenty to burn all the pcb's but it might stop the workshop catching fire if somebody drives over the welder cable when leaving to go home.

At some stage I plan to have all the 32a sockets on 32a breakers and mount each bit of kit on it's own trolley with the correct socket and breaker fixed to the back of the trolley but I have not checked this with the sparky yet!

 

[R-D-R]

Sounds like your pretty much heading where I am with my setup I. I have 16amp commando on a radial for the welder then another for the compressor. I then have another radial for the instant Hot water heater and one for the electric car charger on the drive. I then have the usual radial for sockets and lights. Add it all up and it is over the 60amp supply to the garage but nothing ever trips due to the use case diversity. All circuits are run using singles in conduit.

 

[hotponyshoes]

I have 6x 32a sockets (2 with 20a breakers) plus 2x 4hp compressors hard wired.
That's over 200a if added togeather but the incoming trip is 80a (miggt be 100a) .

Never have that much running at once, at most it would be the compressors, maybe 2 people welding and a plasma cutter going. You can hear the compressors slow down and even notice a power drop welding ac tig if the infra-red lights in the paint booth are on so even with everything on it's own circuit you can get a power drop but never tripped the incomer yet.
You can notice a dip in a tig arc when the compressors kick in. Actually 3 dips as they are on staged startup but the other tools (9" grinder etc) dont cause any issues.
If you are in the workshop by yourself you might not have any problems but I would at least think about putting the compressors on a dedicated circuit unless you only ever plan to use one thing at any one time.

 

[mtt.tr]

One word, "diversity"

The 16a mcb doesn't know how many 16a socket's you have nor does it care.

 

[MetalMonkey]

I thought the combined total rating of the MCB's had to be lower than that of the incoming breaker... is that not so?

No, the combined rating of the MCBs can (and usually does) exceed the main incoming switch. The MCB is rated to protect the wiring of the circuit it serves.
The incoming line to your house will probably have a cut out that is only rated for 80A, 100A if you are lucky - but it is not expected that you would run every circuit in the consumer unit at full load 24x7 so it is fine.

 

[mtt.tr]

And another one for yamhon 40% rule, though only sensible to use on domestic

 

[brightspark]

One word, "diversity"

The 16a mcb doesn't know how many 16a socket's you have nor does it care.

correct you may have a 100 amp supply and say you have x number of circuits that may ad up to to 130 amp but at any one point you probably are pulling 90 amp max .an example a shower pulls 10kw constantly so must be wired in 10.0mm cable. a cooker can pull up to 15kw fully loaded and can be wired in 6.0mm cable because of diversity . at any one point not all elements will be running permanently they will be switching on and off through the thermostats .so if you want to wire in 6 x30 amp sockets on a radial with a 32amp breaker you can do so they don't have to be individualy wired the only thing you cant do is take a 30 amp socket of a ring main that supplies 13 amp sockets. heres a clue

id sugest anyone wanting to know more about general wiring buy the on site guide its only about 16 quid and it cuts through all the irrelevant stuff you don't reed to know

 

[gasket999]

Excellent - thanks guys. Have had a good read through the above and have decided to revisit the whole garage setup in one go (not that big an issue as it’s all 20mm round conduit so can be changed to suit).

This pic shows what I’ve finished up on - the black lines are just to show me where the conduit needs to go as I’m running new airlines at the same time and do like things to be neat.

I’ll be letting the spark advise on the cable and the mcbs as I’ll only be running the conduit and fitting the back boxes/lights - he’ll do the terminations.

But it’s nice to have a plan I can talk through with him. Have decided to fit a slightly bigger garage Consumer Unit with more ways while I’m at it.

I’m also going to change the house cu at the same time (needs doing), so might look to see about increasing the supply to the garage above the current 50a (its only a short run so increasing the SWA supply cable isn’t a big job if needed).

 

[R-D-R]

For the future EV's are you planning to just have outdoor sockets and use the Cars 13 amp charger or hard wire EVSE to each one to use the onboard charger? If the latter make sure your incoming cable can cover +64 amps (80 with the other circuits). I built an EVSE and hard wired it into the box on a 32amp MCB, a clamp meter does indicate it pulls the full 32amps for the first 3 hours of charging before it tails off.

 

[gasket999]

Hi - I’m not sure what an EVSE is to be honest - I’m planning on the dedicated charging points you get fitted, I understand these come in 16 and 32a varieties.

good point re: the incoming and leaving enough for the rest. I’ll ask the spark to advise on the max I can send to the garage .

 

[R-D-R]

Sorry it’s me should have been clearer. EVSE Electric Vehicle Supply Equipment it’s the box you have on the wall that you plug your car into to charge using the cars onboard fast charger. It will allow you to fast charge (not rapid) at 16amp or 32amp on single phase. If you use the little charger for 13amp supply’s that comes with the car it will take all day to charge.

EVSE have a contactor in them controlled by a module that talks to the cars onboard charger through a pilot line to turn the supply on and off. Not much to them.