Discuss Another Pair of eyes on an outbuilding job please in the Electrical Forum area at ElectriciansForums.net

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claret73

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

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

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

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

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.
 

needasparks

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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.
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:
 

ackbarthestar

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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.
A 1.5kW fan should provide a reasonable throughput, but the positioning of the fan and vents will be critical.
Sticking all the switchgear and protective devices externally is also something to think about.
I once installed a10 way dist. board on the outside of a summerhouse in a surface mounted meter cabinet.
 
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claret73

Are you ending up with two RCDs in series? If so, and both 30mA non-delayed, you won't have any discrimination between them
I realise that & have seen that before. I was thinking of an RCBO at the house & just MCB's at the Outbuilding contrary to the above.

- - - Updated - - -

I'm glad Tel could help as I was slipping into a coma by the 6th paragraph of the opening post lol.
I know you're having a laugh, Sorry to bore you! But I wanted to run it by somebody
 
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claret73

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.
He doesn't spray with aerosols. It's mixed & sprayed from an air gun. It's a fairly large extract Fan not a Bathroom 'Manrose'...hence the Wattage. I would think if it's flammable & combustive, he wouldn't be brazing with a welding torch in the same space!...
 
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ackbarthestar

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He doesn't spray with aerosols. It's mixed & sprayed from an air gun. It's a fairly large extract Fan not a Bathroom 'Manrose'...hence the Wattage. I would think if it's flammable & combustive, he wouldn't be brazing with a welding torch in the same space!...
If the paints are suspended in acetone then spaying it from a compressed air cylinder electrically powered will require some adherence to BS EN 60079. Ventilation in this case will be very important.
 

Guitarist

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I have to say, it's nice to see a poster actually giving us all the info we need, along with his own opinions and calculations.
Are you planning to use 2-core swa, with the armour as your cpc?
Also, if the swa enters the building close to the CU, it is much easier to take it straight to it, thus eliminating the T&E at the house end. If you are using 3-core, then your cpc will be 10mm anyway so even tidier.
 
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claret73

I have to say, it's nice to see a poster actually giving us all the info we need, along with his own opinions and calculations.
Thanks! I know it's a long one & if I was 200% I wouldn't have gone to all the typing! But I only sub occasionally as a Mate to another Guy & I don't always get what I'm after. That's more down to more ways to skin a cat & how it's always been done. I was asked to take a look & have done. If I felt completely out of my depth I'd have declined it. Just wanted to run it by & gain some thoughts ensuring I was on the right path...
 
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claret73

I don't think I can get around the discrimination with the RCD at the Distribution CU in the house. 16th Board, all circuits protected. He won't go for a Board change to allow me to put on the unprotected via an RCBO. If I was to use an MCB for the supply SWA to the CU in the Garage, if that CU was RCD'd (all circuits surface in Conduit, so no real need) would there be a time delay in the event of an Earth Fault here due to the 50m run? IE, the Garage would go first? Or not?
 
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claret73

Are you planning to use 2-core swa, with the armour as your cpc?
Also, if the swa enters the building close to the CU, it is much easier to take it straight to it, thus eliminating the T&E at the house end. If you are using 3-core, then your cpc will be 10mm anyway so even tidier.
I had planned on 3 core & still fly leading the Armoured. The Dis Board in the house is at ceiling height, about a 2m drop & 1m round to the outside wall. So felt T&E in Trunking was a neater option (or rather best option).
 
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Guest123

Max Zs for a 63A type B is 0.73.

Max CCC for 10mm T&E is 64 A clipped direct, enclosed in trunking is 52A.
 

sparks1973

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go for a 3% VD here...if you havn`t already...theres lighting for a start....plus what Tel said about that compressor....
 

BroadReach

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One thought, have you asked about how the spray booth is used?

1.5kW is a HUGE fan and will move a lot of air. Certainly more than a 2.5kW heater will be capable of heating up by any meaningful amount. Is the heater being used when the fan is running? If not then the main load will be either:
1. Extract and compressor when spraying
2. Heater alone when baking?

Not sure if this helps but worth checking.
 

sparks1973

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I don't think I can get around the discrimination with the RCD at the Distribution CU in the house. 16th Board, all circuits protected. He won't go for a Board change to allow me to put on the unprotected via an RCBO. If I was to use an MCB for the supply SWA to the CU in the Garage, if that CU was RCD'd (all circuits surface in Conduit, so no real need) would there be a time delay in the event of an Earth Fault here due to the 50m run? IE, the Garage would go first? Or not?
if its a TN system...then why put an RCD on the feed upstream?...use an armoured and RCD it downstream....or you could go for a 100mA s type feed....30mA downstream end.....
 

telectrix

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his problem is that the house has a 30mA RCD front end. unless he can split the tails at the main CU and feed from there, he's stuffed.
 

sparks1973

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his problem is that the house has a 30mA RCD front end. unless he can split the tails at the main CU and feed from there, he's stuffed.
yes sorry...just re-read the post...i think he`s on about earth leakage......if you have 2 RCDs in series....say both 30mA..with similar individual traits.....the one upstream SHOULD let go first under fault as its also being subjected to any earth leakage from other finals and/or subs it serves....
 
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claret73

Max CCC for 10mm T&E is 64 A clipped direct, enclosed in trunking is 52A.
Looks like I'll be taking the Armoured into the house. Thanks for pointing out Lenny...

Rather than have the SWA Cleated up to the board at ceiling level, could I terminate into the adaptable box & strip back to the insulated conductors making them long enough to then have those in trunking?? No method for installing like that, so assume a NO? Going to look offensive as the board is ceiling height, 1m in from the external wall with the spare way access on the left. Was going to come down vertically to skirting & round to existing opening...
 
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claret73

One thought, have you asked about how the spray booth is used?

1.5kW is a HUGE fan and will move a lot of air. Certainly more than a 2.5kW heater will be capable of heating up by any meaningful amount. Is the heater being used when the fan is running? If not then the main load will be either:
1. Extract and compressor when spraying
2. Heater alone when baking?

Not sure if this helps but worth checking.
Yeah I think he said the Fan & compressor together. The Heater would I imagine be used stand alone. I assume you are asking due to the ventilation query & not with regards to the demand of the install?
Cheers!
 

BroadReach

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Yeah I think he said the Fan & compressor together. The Heater would I imagine be used stand alone. I assume you are asking due to the ventilation query & not with regards to the demand of the install?
Cheers!
So is it possible to put the heater and fan on a changeover switch. This will reduce possible maximum loading? And still allow compressor to run at all times for air tools etc.

I admit I am not an electrician but a HVAC engineer that does a lot of design work on systems involving heaters and fans...............
 

telectrix

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could I terminate into the adaptable box & strip back to the insulated conductors making them long enough to then have those in trunking?

absolutely nothing wrong with that method. just have to ensure you get a good connection to the armour in the ad. box. with a fly leadand banjo or earth nut.
 
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