Sizing electrical cable for garage - is this possible?

[julianf]

I need to put in a cable going to the garage, and im not sure about sizing.


Id like to run (all at the same time) -

My CNC machine (say 1.5kw, but probably a bit less)
My biodiesel reactor (about 3.5kw, but currently runs off a 13amp extension lead, so im not sure of the numbers)
...and have my air compressor (3hp) start up all at the same time.

This seems do-able.

If i add a domestic electric oven into the mix, i suspect this gets difficult, right?

The whole lot is about 30m from the consumer unit.


Note, the compressor blows 13amp fuses on startup if its run on an extension lead.

Is this lot even possible to run at the same time? Houses have limited amp supplies, right?

 

[Wendelspanswick]

House supplies vary from about 63amps up to 100amps, have a look at the main cut out by your meter (it will be in the suppliers side of the meter).
It's worth knowing that fuses and circuit breakers are not all the same, you can draw more than the maximum current from a fuse/CB for a duration that varies according to the type.

 

[Wendelspanswick]

You also get diversity, it would be highly unlikely that all the electrical items would be at peak draw at exactly the same time.

 

[julianf]

The cnc machine is fairly constant. Engraving is less draw than milling, but i need to go for worst case really.
Bio reactor is PID controlled, so the heater pulses on and off every half second or something. I will be converting to water heating, but ive not done that yet.
Compressor will certainly use less when running, and even just when warm, but im trying to avoid having to think about these things.

At present (as in as i type this) the cnc mill is running, but id like to be heating up a batch of oil too, but ill have to wait to do that.


Why im asking right now, is that the cable will have to go through the room that im working on (the one im doing the oak windows in) and i need to have it in place before i plasterboard one of the walls.


Also -

Do joins in cables reduce their carrying capacity? Im asking as, say, if i bring the cable to a consumer unit in the "oak frames" room, and then on from there, does the join at the unit reduce the capacity?

 

[Wendelspanswick]

As a guide use
Volts X Amps = W
Which is the same as
W ÷ Volts = Amps

So if you have a 10.5KW shower and your supply is 230V
10500W ÷ 230V = 46 Amps
Your supply cable to the shower must be rated higher than this and the fuse/CB must be rated at higher than the current drawn by the shower BUT lower than the the amount of current that would damage the cable in a overload or short circuit fault.
The fuse/MCB is there to protect the supply wiring.

So with the above example if the cable run for your shower was short you could use 6mm cable and a 50amp CB but not a 60amp CB as this would no longer protect the cable.

Just to add this is a simplified example not to be used to install your shower!
You need to consult the appropriate tables in the latest edition of the regulations.

 

[Wendelspanswick]

The fuse is ONLY to protect the wiring, it doesn't care if the current is flowing through an appliance or through you just so long as it's under it's rating. Seeing as well under one amp will satisfactorily kill you, how would a 13A plugtop fuse protect a person?

An RCD is for people protection...

Sorry, meant as in stops your cabling from overheating and burning your house down, I will edit my post.

 

[julianf]

right,

compressor 3.5kw
reactor 3.5kw
oven 4kw
cnc 1.5kw

total 12.5kw (aprox)

distance 30m (aprox)

this calculator -

https://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html

gives me -

Required Cable Size 16 mm
Voltage Drop 4.57 Volts.
Percentage Drop 2.0%
Current Load 54.3 Amps
Max Cable Load
Max Cable Load refers to the maximum allowable current for the cable used in the configuration given above and will vary considerably under different configurations. 57.0 Amps




So, i think im looking at -

CA 16G/50
6242Y 16.0mm² Grey Twin & Earth Cable 50m Reels
per 50M

£162.00



Hmm... £200 (with the tax) for a bit of wire. Makes the £60 i spent on a tin of wood oil the other day seem like pocket change.
The next size down is 10mm sq, which, surprisingly, is pretty much spot on half the cost. I would imagine with the greater ability to loose heat, a twin run of 10mm sq would carry more current than a single of 16mm?

Im still unsure about the joining question...

 

[sasquar]

Ive just done my workshop with 16mm t+e, and im sure it wasnt that much! If i remeber, ill have a look at the receipt when i get home.....
Be aware, that you may well have to run a seperate earth lead as well...... im sure it was explained why, probably something to do with meter reading for testing, but to be honest, i trust the guy who spec'd it completely, so I just did what i was told!!

 

[colnerov]

Yes you could earth it separately.

 

[angellonewolf]

wait till you add the glands fuse board clips and then its p certed so building controll will have to be instructed and paid better part of £600 at a guess

and plaster the walls first and cilp to the plastered wall or you will have to have a bigger cable as in part of the run its buried that reduces the heat dissipation and reduces the current carrying of the cable

there are more sparkys on here but @brightspark could add more to this

 

[dobbslc]

And remember if the cables buried or in a duct it down rates it quite a bit.
Also if you do draw 57 amps in the garage and the missus turns on the oven/electric shower/big f off hair dryer and your house cut out fuse is only a 60 amp one, then pop!
Most houses have only a 60 amp supply...

 

[Kayos]

And remember if the cables buried or in a duct it down rates it quite a bit.
Also if you do draw 57 amps in the garage and the missus turns on the oven/electric shower/big f off hair dryer and your house cut out fuse is only a 60 amp one, then pop!
Most houses have only a 60 amp supply...

I'd say that's too general, I've seen a broad mix of 60/80/100 in the 2 years I was installing meters.

 

[daedalusminos]

I've recently wired my garage you need to consider:

Cable run- underground steel wire armoured might be favourable, use the armour as earth and you can get away with two core. 16mm2 two core SWA is 3.47/m at www.electriccable.co.uk

At 30m you'll probably need to sink an earth stake and use that as your garage earth. Use the SWA screen up to the garage and isolate.

Protect garage circuits with RCBO in the garage CU.

You need to calculate earth loop impedance etc for your own application.

This wiki is VERY useful:

http://wiki.diyfaq.org.uk/index.php?title=Taking_electricity_outside

 

[daedalusminos]

Connections: minimise any connections as these will be the weakest point, however if using SWA you'll almost certainly want to terminate this in a junction box in the house (e.g. Adaptable Box) to continue to the house CU in T&E.

 

[Wendelspanswick]

right,

compressor 3.5kw
reactor 3.5kw
oven 4kw
cnc 1.5kw

total 12.5kw (aprox)

distance 30m (aprox)

this calculator -

https://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html

gives me -

Required Cable Size 16 mm
Voltage Drop 4.57 Volts.
Percentage Drop 2.0%
Current Load 54.3 Amps
Max Cable Load
Max Cable Load refers to the maximum allowable current for the cable used in the configuration given above and will vary considerably under different configurations. 57.0 Amps




So, i think im looking at -

CA 16G/50
6242Y 16.0mm² Grey Twin & Earth Cable 50m Reels
per 50M

£162.00



Hmm... £200 (with the tax) for a bit of wire. Makes the £60 i spent on a tin of wood oil the other day seem like pocket change.
The next size down is 10mm sq, which, surprisingly, is pretty much spot on half the cost. I would imagine with the greater ability to loose heat, a twin run of 10mm sq would carry more current than a single of 16mm?

Im still unsure about the joining question...

Your forgetting about diversity though, what are the chances that the compressor will start up at the same time as the thermostat on the cooker and the reactor kicks in as well as you using the CNC?
If it's possible then go for the full whack but if not you can downrate the total load by 10% at least.

 

[daedalusminos]

Twin run: that's an odd one! Redundancy is great, if one cable fails you've got the other one...but both must be rated at individual capacity. Doubling up at lower ratings is not redundancy, it's spreading capacity over two circuits. Problem is, if one fails you won't know until you exceed the single rating....and that could be dangerous

 

[Wendelspanswick]

I prefer the Doncaster Cables Calculator.
http://www.doncastercables.com/technical-help/

 

[Wendelspanswick]

If the garage is attached to the house use 10mm twin and earth.

 

[angellonewolf]

If the garage is attached to the house use 10mm twin and earth.

must be a big house if the run is 30 meters

 

[julianf]

Garage is separate, but there will be an above ground utilities duct, which, as i understand it, removes the need for armour.

You should see what i have at present. In fact, you can -

See that bit of white cable, taped to what looks like a washing line, up on the top left of the photo?


I would really like to state - not my work! That wire currently does all the power to the outbuildings, inc lighting and 13amp sockets. At points its nailed to a wall behind ivy. In other areas theres more washing line....