what size armoured cable

[leeweimaraner]

i need to run a supply to a garage 90metres from the fuse board the garage will have electric door ,sockets and lights ,may want to run a hot tub from the garage?? what size armoured cable should i use and why???

 

[Sintra]

i need to run a supply to a garage 90metres from the fuse board the garage will have electric door ,sockets and lights ,may want to run a hot tub from the garage?? what size armoured cable should i use and why???

You first of all need to know how many amps the cable is drawing and then work out the voltage drop to come up with the correct size.

 

[wattsup]

The hot tub...5kw?
Garage door 2kw?
Sockets & lighting 4kw +?
90M!
25mm 3-core swa on a 63 amp breaker
(assuming single phase)

 

[leeweimaraner]

ok you are saying 11kw how do yuo get to 25mm cable please

 

[Dic Penderyn]

Maximum Demand with applied diversity (my calculations)
Garage supply

Lighting: ? type plus watts

Hot tub:
100% 5kW/230 = 21A


Power
1 x ring main @ 100% = 30A
1 x radial (garage door) 2kW/230 = 9A

Estimated total amount of power of 50A

Phase: Single Phase
Power:11.5 kilowatts / 50 amps
Length:90m
Core Size: 25 mm²
Voltage Drop:8.55 volts


As diversity isn't and exact science I'd ask yourself if you'd use the hot tub whilst using the sockets and garage door.

That might lower the design current with regards to power hence small cable size.

 

[MarcT]

ok you are saying 11kw how do yuo get to 25mm cable please

Ib = 11000/230 = 48A

select In as : Ib < In (say 50A mcb allowing some diversity)

calculate max mV/A/m

max volt drop x 1000 = 2.66
Ib x length

select suitable cable for volt drop from table 4E4A (25mm closest)

determine It value (116A)

calculate Iz (It x Ca x Cg x Ct)

in this case i assume) Iz = It = 116A

Iz must be bigger than In = true

therefore 25mm is the smallest size of cable that satisfies both volt drop and current carrying requirements

 

[ezzzekiel]

purrrrrfect markt

 

[Guest123]

Phase: Single Phase
Power:11.5 kilowatts / 50 amps
Length:90m
Core Size: 25 mm²
Voltage Drop:8.55 volts

The VD here is too high as the sub-main will be supplying lighting circuits the max allowed will be 3% = 6.9V.

 

[Dic Penderyn]

The VD is Irrelevant here! The cable is still 25mm. If you look back to my original reply you'll see a question mark with regards to lighting which when calculating diversity is relevant.


ezzkiel says "Whoopee do Marc your amazing! let me suck your lolly pop"!

 

[MarcT]

The VD here is too high as the sub-main will be supplying lighting circuits the max allowed will be 3% = 6.9V.

If it was a lighting only distribution circuit i'd be inclined to agree with you

 

[Guest123]

The VD is Irrelevant here! The cable is still 25mm. If you look back to my original reply you'll see a question mark with regards to lighting which when calculating diversity is relevant.

I was merely pointing out that the value was above the max permitted which to me IS relavent.

 

[Dic Penderyn]

Well thank you Marc!

Is it lighting or power? Well has the power and the lighting are a complete factor in terms of design current, I'll choose the lowest VD percent in Lenny's case lighting. That's odd the cable size is still 25mm.

 

[MarcT]

duly noted, have a lollypop

 

[Guest123]

My VD for a 70degreeC 25mm PVC SWA with a 50A Ib at 90M comes out to 7.87V.

Worst case scenario obviously, not likely to pull 50A all the time.

 

[MarcT]

I was factoring in diversity there, but if the 11kw is a constant load then yes it's the lower limit