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hmartin

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I want to supply power from a let property to a nearby Barn (30 metres away). I will have a meter and separate fuse box in the house to supply the barn, but how can I ensure that the fuse box I fit in the barn will trip before the one in the house, so I don't need to disturb the tenants in case of a problem in the barn? This is a DIY query.
 
How much power?

If you need less than 13A you can use a 13A plug in to the house and run a length of suitable cable to the barn, if it is protected from physical damage, rodents chewing it, etc, then something like:


If it is not protected well or if you plan on burying it then you would need to use SWA (steel wire armour) cable and that needs proper termination with glands at both ends, and from that termination box to flexible cable to the 13A plug in the house.

Achieving selectivity is difficult though, with a 13A fuse at one end you could only use a couple of 5A fuses (at most) at the far end for any ciruits you are planning on to have a good chance of only the end fault taking out its own fuse and not further back to the 13A plug. Even then it is a bit harder to be sure it would not trip a MCB as frequently they trip faster than fuses under certain fault currents.

Ultimately your best plan is to have a dedicated circuit from your CU (consumer unit = fusebox) but that would be notifiable work so you would need to get a qualified electrician to do that and complete the Part P stuff.
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Reading your question again it is definitely not a DIY thing to do if you are looking at a separate meter, etc.

The proper approach would be a fused switch from the incoming mains, suitably rated SWA cable, and CU in the barn for whatever circuits you are planning. Depending on the current you need, and type of earthing, etc, there are various aspects to calculating the cable size, main fuse size, and the choice of protection in the barn.
 
Last edited:
How much power?

If you need less than 13A you can use a 13A plug in to the house and run a length of suitable cable to the barn, if it is protected from physical damage, rodents chewing it, etc, then something like:


If it is not protected well or if you plan on burying it then you would need to use SWA (steel wire armour) cable and that needs proper termination with glands at both ends, and from that termination box to flexible cable to the 13A plug in the house.

Achieving selectivity is difficult though, with a 13A fuse at one end you could only use a couple of 5A fuses (at most) at the far end for any ciruits you are planning on to have a good chance of only the end fault taking out its own fuse and not further back to the 13A plug. Even then it is a bit harder to be sure it would not trip a MCB as frequently they trip faster than fuses under certain fault currents.

Ultimately your best plan is to have a dedicated circuit from your CU (consumer unit = fusebox) but that would be notifiable work so you would need to get a qualified electrician to do that and complete the Part P stuff.
[automerge]1588286503[/automerge]
Reading your question again it is definitely not a DIY thing to do if you are looking at a separate meter, etc.

The proper approach would be a fused switch from the incoming mains, suitably rated SWA cable, and CU in the barn for whatever circuits you are planning. Depending on the current you need, and type of earthing, etc, there are various aspects to calculating the cable size, main fuse size, and the choice of protection in the barn.
I do intend having a permanent dedicated wired-in supply with a separate CU and meter in the house, and connecting with SWA cable, but can I fit an MCB in the barn that will trip before the one in the house? I would of course get it checked by a Part P qualified professional electrician before use.
 
I do intend having a permanent dedicated wired-in supply with a separate CU and meter in the house, and connecting with SWA cable, but can I fit an MCB in the barn that will trip before the one in the house? I would of course get it checked by a Part P qualified professional electrician before use.
I would need to run dehumidifiers and electric saws etc so would need a 13amp supply in the barn
 
Have you made a list of what you will be using and the power for each to work it out for certain?

If you can run a cable from a RCD protected 13A socket (or FCU) in the house then it is within DIY operations and no building control, etc, but you would be very much limited to 13A total.
 
I would need to run dehumidifiers and electric saws etc so would need a 13amp supply in the barn
Yes, but each of those may require 13A each.
electric saws have a high start up current that has to be catered for. You haven’t mentioned heating, lighting, etc. Or how far the barn is from the house. Or the supply type to the house.
as well as discrimination of fuses, there is the same issue with RCDs.
As above, this is not a DIY project and requires the services of a competent electrician.
 
Have you made a list of what you will be using and the power for each to work it out for certain?

If you can run a cable from a RCD protected 13A socket (or FCU) in the house then it is within DIY operations and no building control, etc, but you would be very much limited to 13A total.
The most at any one time would be a dehumidifier and an electric saw or electric drill, not both at once! I am currently running this off a temporary cable plugged into the house and have not had any problems so far, but would prefer a permanent solution.
 
If you need anything beyond a glorified 13A extension cable that means a dedicated circuit and at that point it is a professional electrician's job.

But no matter what you decide to do you should go through the steps of working out what you need and what you might possibly want in a year or so's time. As already said you need to think about:
  • Lights (possibly the type with emergency backup so you don't end up in a dark place with power saws running down if the supply fails for any reason)
  • Heating in winter
  • Running more than one machine at a time (as you will want to if you have a few sockets around the building)
While it depends on what they find on inspecting your property's electrics, almost certainly an electrician would recommend a fused switch, SWA cable to the barn, and a "garage" style of CU so all your RCD and over-current protection is in the barn.

Achieving good selectivity usually means having as high a fuse rating as is safe for feeding the cable together with as small an MCB as is realistic for your loads. Maybe splitting off the heaters (if used) from your sockets, or even having two separate 13A socket circuit so they can all have fairly low MCB trip levels (16A/20A), etc, rather than blindly going with the standard 32A rating used for house ring final circuits, etc.

Discuss what you need with the electrician and if you want to do something to save money then digging the trench for the armoured cable is always going to help!
 
I want to supply power from a let property to a nearby Barn (30 metres away). I will have a meter and separate fuse box in the house to supply the barn, but how can I ensure that the fuse box I fit in the barn will trip before the one in the house, so I don't need to disturb the tenants in case of a problem in the barn? This is a DIY query.
Get advice from a local electrician who could then make an assessment and decide what is required. They could even give you a price to do the work or, if they consider you competent to carry out the work, may even 'overlook' to some degree, with a view to inspection and registration.
Not being a qualified electrician, you would be more suited by the DIY forum.
 
I'm not sure we've actually answered the original question re selectivity.
Two types of protective device are likely to be needed in the barn, RCD(s) and MCB(s).

To avoid an RCD at the house tripping first, the best solution is not to have one (other than a time-delated device protecting a TT installation, which does not provide additional protection against shock). That in turn requires that a suitable cable such as SWA (in which the conductors are surrounded by the earthed armour and which will trip an MCB if penetrated) is fed from a non-RCD protected source in the house.

To avoid OCPD (MCB or fuse) tripping at the house first in the event of a fault, its rating must provide full selectivity with the largest device at the barn under short-circuit conditions. To achieve the same for overload, then either the total rating of the final circuit OCPDs in the barn or one overall device in the barn must be considered for selectivity likewise. If the final circuits in the destination are correctly sized, then specifying a normal MCB (e.g. 60898) at the source tends to result in an oversize device being needed, hence an oversize cable to achieve the required lower loop impedance to the far end (although sometimes it is voltage drop that ultimately governs the cable size). Better results, i.e. an In value more closely matching the load Ib, can often be obtained with a cartridge fuse at the source.

Therefore, a good solution is often to split the meter tails to feed a switch-fuse, as per PC1966's description. This is definitely out of DIY scope.
 
If you need anything beyond a glorified 13A extension cable that means a dedicated circuit and at that point it is a professional electrician's job.

But no matter what you decide to do you should go through the steps of working out what you need and what you might possibly want in a year or so's time. As already said you need to think about:
  • Lights (possibly the type with emergency backup so you don't end up in a dark place with power saws running down if the supply fails for any reason)
  • Heating in winter
  • Running more than one machine at a time (as you will want to if you have a few sockets around the building)
While it depends on what they find on inspecting your property's electrics, almost certainly an electrician would recommend a fused switch, SWA cable to the barn, and a "garage" style of CU so all your RCD and over-current protection is in the barn.

Achieving good selectivity usually means having as high a fuse rating as is safe for feeding the cable together with as small an MCB as is realistic for your loads. Maybe splitting off the heaters (if used) from your sockets, or even having two separate 13A socket circuit so they can all have fairly low MCB trip levels (16A/20A), etc, rather than blindly going with the standard 32A rating used for house ring final circuits, etc.

Discuss what you need with the electrician and if you want to do something to save money then digging the trench for the armoured cable is always going to help!
Thank you for that very detailed reply. No heating or backup lighting required but separate circuits for each socket is sound.
I'm not sure we've actually answered the original question re selectivity.
Two types of protective device are likely to be needed in the barn, RCD(s) and MCB(s).

To avoid an RCD at the house tripping first, the best solution is not to have one (other than a time-delated device protecting a TT installation, which does not provide additional protection against shock). That in turn requires that a suitable cable such as SWA (in which the conductors are surrounded by the earthed armour and which will trip an MCB if penetrated) is fed from a non-RCD protected source in the house.

To avoid OCPD (MCB or fuse) tripping at the house first in the event of a fault, its rating must provide full selectivity with the largest device at the barn under short-circuit conditions. To achieve the same for overload, then either the total rating of the final circuit OCPDs in the barn or one overall device in the barn must be considered for selectivity likewise. If the final circuits in the destination are correctly sized, then specifying a normal MCB (e.g. 60898) at the source tends to result in an oversize device being needed, hence an oversize cable to achieve the required lower loop impedance to the far end (although sometimes it is voltage drop that ultimately governs the cable size). Better results, i.e. an In value more closely matching the load Ib, can often be obtained with a cartridge fuse at the source.

Therefore, a good solution is often to split the meter tails to feed a switch-fuse, as per PC1966's description. This is definitely out of DIY scope.
Thank you, I think that the issue of selectivity is the crucial one; It looks like I need to settle for doing the donkey work for a professional who is willing to let me do so!
 

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