What load can my shed cable take?

[KnightWhoSaysNi]

I have an electric cable running from the house to the shed. For most of its length it's buried 6-7" deep in the ground.

The cable is not the flexible / rubber type, it's rigid, most likely copper (?). The external diameter is 7mm.
I wonder what kind of load it can take? I've seen some tables on the web but I think they refer to internal diameters (i.e. diameters of the actual wires inside the insulation)? I need it to be good for up to 5kW or so, otherwise I'd have to lay a new cable.

 

[flameport]

7mm diameter could be one of several sizes, and depending on age it might not be metric.
Do the brass glands at either end have any numbers or other markings on them?

 

[Iggifer]

i have never seen SWA as small as 7mm across - either it's a measurement faux pax, or its a different type of cable.

Sounds like SWA though

 

[securespark]

1.5mm² 23A
2.5mm² 31A

Light duty mineral cable.

But although 1.5 will carry 5kW, you'd need a minimum 25A breaker and because the cable rating is lower than 25A, you cannot do that.

So, if you have 1.5, you'd have to use a 20A breaker, but you'd be limited to 4.6kW.

I guess this is not a hugely long run of cable?

 

[KnightWhoSaysNi]

I can't see any markings.

The breaker it's connected to is 32A. I wouldn't draw any conclusions from that though.

The cable is roughly 50 metres long and most of it is buried in the ground. Can't I use a 23A breaker like this one?

 

[flameport]

Highly unlikely the cable is rated for 32A. That 23A breaker thing cannot be used.

The 50m length is a big problem due to voltage drop, and will restrict the maximum load to a lot less than 23A.

 

[winston1]

What is your 5kW load?

 

[ban-all-sheds]

I wonder what kind of load it can take?

Until you can properly confirm its size nobody can tell you, and guessing is pointless.

I've seen some tables on the web but I think they refer to internal diameters (i.e. diameters of the actual wires inside the insulation)?

No, they refer to the cross-sectional area of the actual conductors.

I need it to be good for up to 5kW or so

What makes up that 5kW load?

 

[KnightWhoSaysNi]

Thanks guys.

To give you a better idea of the project:
I'm planning on replacing the current shed with a larger structure with a floor area of ~30 m2 (5x6 mt). It will be a yoga studio of sorts, with laminate flooring, underfloor heating and a cloakroom (toilet + washbasin).

The shed is not plumbed in, so I'll have to plumb it, and it seems I'm better off replacing the electric cable as well. The cable needs to be a certain distance from the pipes (by regs), so I can't put both in the same trench, but I reckon I don't need to bury the cable in the ground, I can just attach it to the fence.

The underfloor heating would make up most of the load, and to come up with my 5kW estimate I had assumed 100W per sq. m of floor area, which worked out to 3kW for the UFH + 2kW for everything else (lighting, domestic sewage pumping station and the occasional use of a low power electric kettle). But that 3kW might not be enough as the only source of heat (I've seen suggestions of 200W per sq. m), so I think I'll go for something more generous, up to 10kW maybe.

What kind of cable would you recommend please?

 

[flameport]

For that kind of use, you can forget the existing cable. The addition of water & sewage pipes may be relevant, as is the type of earthing system in the house.
As an absolute minimum, the new cable would be 10mm² 2 core SWA.

 

[davelx]

but I reckon I don't need to bury the cable in the ground, I can just attach it to the fence.

No you can't. A fence is not a robust enough structure. Fences often get damaged by high winds. A brick wall might be OK, but not a fence, I'm afraid.

 

[KnightWhoSaysNi]

No you can't. A fence is not a robust enough structure. Fences often get damaged by high winds. A brick wall might be OK, but not a fence, I'm afraid.

I was thinking of attaching it to the top of the gravel boards, as the patio section of the current cable is attached.

 

[davelx]

No you can't. A fence is not a robust enough structure. Fences often get damaged by high winds. A brick wall might be OK, but not a fence, I'm afraid.

I was thinking of attaching it to the top of the gravel boards, as the patio section of the current cable is attached.

Concrete gravel boards slotted into concrete posts? I think that might be OK if you can find a way of securing the clips - hammering in pins is not going to work. Wooden gravel boards and wooden posts I'm not so sure about. Let's see what others have to say.

 

[ban-all-sheds]

I'm planning on replacing the current shed with a larger structure with a floor area of ~30 m2 (5x6 mt). It will be a yoga studio of sorts, with laminate flooring, underfloor heating and a cloakroom (toilet + washbasin).

Forget the idea of UFH - it will not work. If you were to get it to put out enough heat to properly heat the space, the floor would be far too hot for comfort, particularly if doing yoga on it. I'd suggest an air-source heat pump.

Don't forget, BTW, that you're going to need Building Regulations approval.

Can't help thinking that 5x6m is a lot of space for doing yoga - are you going to be running some kind of business?

The cable needs to be a certain distance from the pipes (by regs)

What distance? Which regs?

But that 3kW might not be enough as the only source of heat (I've seen suggestions of 200W per sq. m)

Only 72p/hour to run, assuming 12p/unit.

 

[Iggifer]

Keep the cable as is. Get a small combi boiler, a nice double panel rad, a two loop wet UFH system, and you will be LAUGHING.