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claret73

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Afternoon. As I don't do any work for myself at the moment, just occasionally a few small simple bits outside of sub contract, wondering if I could have a few pairs of eyes on this? I got asked to look at improving a Domestic install to an outbuilding at the top of the garden. It's basically a small workshop where he brazes the odd bicycle frame & spray booth for painting. Also works on a car in there.

Have taken a look today & have come back, done some calcs & thought about improving the install for what he needs. Now as I'm thinking far too hard, confusion is creeping in & I may well pass this off if I'm way off. But wondered if I could have some feedback on what I've come up with.

Main Incoming TNS 60A 1361 (Marked up but not ascertained) to a 3871 Push Fuse Board in the hallway. This is spilt to a Fused Switch (Fuse not known?) feeding a distribution board further down the hall for the Kitchen extension. This board has RCD Protection. Currently there is a 2.5mm Armoured supplying the outbuilding into what looked like a Plug in RCD from a Socket off the Distribution board. Runs from the house & is connected to the outbuilding at another Board, also RCD Protected which supplies sockets, lights, a fan in the spray booth & a compressor. Too much overload, tripping the sub board in the house.

These are the circuits I'd like to split up:
1/ 2.2kw Compressor Single Phase
2/ 2.5kw Heater (located in the spray Booth)
3/ 6x 70w Fluorescent light fittings
4/ 4x 13 Socket outlets (currently has lights, heater & fan plugged into them)
5/ 1.5kw Fan Extract in the spray booth (Class 2)

Calculating the Demand on these comes out near enough 50A, Using diversity only on the lighting. Just wondered if anybody would cast an eye over my ideas & tell me if I'm way off, overkilling it or missing something?

Max demand of 50A

It's a 50m Run from house to Outbuilding approx, so looks like 10mm SWA will be needed for the Volt Drop.

Here's my thinking...
63A MCB at Distribution Board in extension in 10mm T&E to outside Wall Adaptable Box.
SWA from here, clipped direct 50m run into Outbuilding CU or adaptable box next to CU.
CU in Outbuilding 63A 30mA RCD with sized MCB's as above. (House CU has RCD Main Switch, perhaps need to check it's at 80A?)

Now, everything is surface Mounted. Am going to tidy up & run singles in Conduit PVC. Flexihose off the Compressor. Sockets just below the Board pending what he needs to plug in after I take every lead out of the sockets & connect into CU correctly.

Final Circuits:
Compressor - if 20A 2.5mm
Heater - 1mm
Lights - 1mm (T&E Clipped direct) Replacing first leg to light fitting as he's refurbing the ceiling at some point, so could do the rest then.
Sockets - 2.5mm
Extract Fan - 1mm

Final MCB Selection:
16A Compressor (20A?)
16A Heater
6A Lighting
20A Radial for Sockets (nothing major plugged in)
10A for the Fan

Bit confused on the Compressor as it calculates just under 10A, yet he has been advised to run a 20A Breaker. Would 16A be OK or is it to do with the Inrush current upon start up of the Motor that I need a 20A? (Probably off here, as not that up on Motors from Practical experience). He does have a Manual so should find requirements for this.

I haven't tested Ze Yet, but calculating Zs from 0.8Ze TNS & R1+R2 for 10mm T&E & SWA Combined gives me 1.05 ohms with Fault current of 219A. Log Log disconnects at that & Max ZS OK for 63A MCB.
Adiabatic confirms CPC of 4mm in 10mm T&E is OK.

Any Earth/Overcurrent Faults in the outbuilding would then be protected at the CU & will not disconnect the 63A MCB in the house or trip its RCD Protected circuits causing a nuisance at the house.
Only thing to do is connect a bond to the incoming Gas as he hasn't got one. Incoming water is plastic, so right to leave that I believe?

Right! I've overcooked it as to thinking & calcs I know, but I needed to see if I have this down right. If not, I shan't be going in blind to it & will get somebody else to do it & me be helping out. Any suggestions, omitted or varying opinion really welcomed.
Cheers!
 

telectrix

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you should consider the small size of the cpc in that 10mm T/E. maybe install a separate 10mm conductor alongside. i think also, you could apply a bit more diversity and use a design current nearer to 40A than 50A.
 
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claret73

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  • #3
Telectrix
Thanks for the reply. I did consider the 4mm in the T&E as the weakest point & calculated with the Adiabatic at 0.6mm. Have 4mm so saw this as OK? I didn't think anything other than the lighting circuit I could reduce down? I've treated them as final power circuits.
 
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claret73

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  • #5
Thanks! For the Heads up on that. I'll ask to look at the Manual for it. He did show me briefly...so does that all look to be OK in your eyes? I'm going to run it by the Spark I work with occasionally, but TBH I'd like a few points of view.
Cheers, much appreciated!
 

telectrix

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looks reasonable. but you don't want an RCD feeding from the house. a fault in the outbuilding can then take the whole house out if you're coming from that upfront RCD. can you re-configure the main CU to get out of this? for the compressor, use a 20A Type C MCB, 2.5mm cable. these compressors are very susceptible to volt drop. i've seen the motors burn out due to low voltage.
 

HandySparks

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Are you ending up with two RCDs in series? If so, and both 30mA non-delayed, you won't have any discrimination between them.

Edit: Tel's too quick for me!
 
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Guest55

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I'm glad Tel could help as I was slipping into a coma by the 6th paragraph of the opening post lol.
 

telectrix

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i've been in a coma for 4 weeks. back working tomorrow. sod the doctors. i've packed in the fags. what more do they want? they've had an armfull of blood already.had to recharge with 5 pints of old peculier.
 

ackbarthestar

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Just to add a little more.

Ventilation is going to be an important factor if spraying using acetone based aerosols. So perhaps a look at BS EN 60079 series of standards may be something to consider.
 
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Guest55

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  • #11
Acetone ? is that flammable ?
oops , looks like a full pyro install is needed.
I can see all the Electrical Trainee breaking out in a nervous sweat lol.
 
Just to add a little more.

Ventilation is going to be an important factor if spraying using acetone based aerosols. So perhaps a look at BS EN 60079 series of standards may be something to consider.
Thats a good point, staying with the subject of fans, I thought the 1.5kw was a bit ambitious for a fan but could be wrong.
 

ackbarthestar

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Acetone ? is that flammable ?
oops , looks like a full pyro install is needed.
I can see all the Electrical Trainee breaking out in a nervous sweat lol.
I once considered wiring my bathroom lighting circuit in Pyro, on the advice of my first wife :biggrinjester:
 

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