workshop design (electrical)

Postby tt » Mon Jan 28, 2008 8:10 pm

Hello this is made up

Only want to learn to understand whats behind the sockets and switches we press everyday

What this question is for is to design the circuits suitable for the consumtion required apon a customers request to an electrician.

This workshop is 40m away from the house and the cable can be buried

Main cu use in workshop

compressor needing 30 amp min
10 double sockets
exterior lighting what would be recomended
welding set/Battery charge both min 30 amp

What I'd mainly like to know is how do you work out what set up to use and the calculations behind your advise and decision

If poss can you tell me where in the 17th or on site guide you get to reference from so i can find my way around them

I'v read to day a post that sparx replied to and he put the table number and the calculation he did very helpful and valuable information thanks sparx something else learnt on this site
thank you
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Postby sparx » Tue Jan 29, 2008 9:58 pm

Hi tt, you knew I would have to answer this one......
Before starting a design such as this you have to know non-electrical stuff ie, how many people will be using workshop at same time? why, cos it means if only 1 then when welding say not able to be using grinder at same time. If several people working then can't allow such diversity.
to add all together not possible from house supply as total load in theory more than 100A, however my workshop has all afore mentioned items plus wood & metal lathes, band /circular/table saws,ect. non as huge as you mention, eg battery charger 30A charging current only draws 2.5A from mains, 140A welder draws 16A from mains, compressor 30A = 8HP motor , unlikely!
in real life my workshop runs on a 4mm2 pyro buried under patio on a 40A MCB, worked fine for 30+years, lights in house dip when welding arc strikes up but hey ho!
give realistic scenario & pleased to help with maths ect,
regards SPARX
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Postby ericmark » Wed Jan 30, 2008 1:27 am

I think you are trying to run before you can walk. The example you give is too large for supply from house so you would need to look at ways to get power else where. Motor driven welding set, or compressor what are you going to do in garage "Air Arc"? Lights can be discharge 70 Watt mercury etc. May be even a generator or with that distance maybe its own supply.

Often you find only one socket for welding set and compressor etc. So only one can be run at a time.

I had a caravan with 10 amp supply. The hot water was electric and so was the heating so I used a relay connected to water heater thermostat so when water heater called for heat the fan heater turned off. Both 1Kw and I could still boil kettle or use micro wave as long as I was careful.

Postby tt » Thu Jan 31, 2008 10:27 pm

Hello sparx & ericmark

Thank you for replying

I was hoping I could ask the right question to get you involved with helping on my way I'v read your replys and you seem to like things to be answered correctly
the answer to your question is there would be one person if it was to be real that would be me I gave you a wish list of what I'd like in my dream workshop what i was aiming for was a break down of how I would go about designing the best way to do it,

For example how to arrive at the correct cable for the instalation which to do that I would have to know what is drawing on it,

I'v unknowingly wanted to much to get the supply for what I'd like.
so I'd like to change the question to, if I wanted to have 2 curcuits 6a lights and 32a for sockets,

Q, so If I wanted to still use the welder,compressor,battery charger. can I use fused spurs that can be my single 32a outlets or isn't there anything that size


Sorry if it looks like Im trying to learn alot to quick but I was hoping to have bits of what was asking broken down into section just like KB did with the lighting he explain the basic calculations then gave me a task to try to see how much I can understand and complete, to me this is the only way I can find my weakness In learning these items.

to keep the lighting draw low
I'd be best to use 4 strip lights would you say
Outside lights 2 sodium 70w
sockets 10 doubles

I hope there's enough to get me started on a challenge.

Thank you for your time,
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Postby ericmark » Tue Feb 05, 2008 11:37 am

OK keep going your doing well. Yes you can get big sockets for 230 volt blue similar to ones used with caravans and 110volt but 32 amp. They jump in stages 16, 32, 63, etc and they don't have to be fused to limit so you can connect to 20 amp MCB.
For a workshop with items having an inrush you may need type c or d MCB's and because these would need a better earth loop impedance you may also need a RCD if not already fitted to comply with earth loop impedance setting this is calculated or selected from table.
Lighting in workshops is not so simple either two main considerations are colour correction and stroboscopic effect and you would need to consider if HF units should be fitted and what tubes should be used.
There are better high pressure discharge lamps than sodium they give a white light are not called mercury and the name sorry escapes me. Sodium with the orange light it seems is the most receptive light for the human eye so lower light levels allow us to still see where same light level with other colours we would not see. They were used for street lighting but they will not reflect on yellow high visibility vests etc so have been replaced in many areas. Mines and Railways use Orange high visibility vests.
As to cable sizes with a max of 100 amp into the house 16mm will take all the power available and then only volt drop to consider. Next step to 25mm jumps to a cable which would take some working and you would need to start and finish in an enclosure able to accommodate such a heavy cable. OK in workshop but not so good in the house. This is where you need to consider other options. For example I have used two parallel 150mm cables from a generator rather than one 240mm cable just so I could physically get the cables connected. I have not worked out you demand again but consider you could put 1 double socket box with lid under your consumer unit and feed two 6mm SWA cables with ease but not enough space for a 16mm in same box. With 16mm you would need an adaptable box a lot bigger and not as acceptable in the house side.
I would always use a copper earth but forgetting regulations if you used for example a 5 core cable two cores for phase and two for return with one for earth plus the steel wire armor you again are using smaller tails worth considering.
I would not expect you to do any of the above but it is as they say food for thought and may help in final design? Sorry must go XYL calls

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