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Mastbruch

Power Supply

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Mastbruch

Morning gentlemen.

 

Again just looking for the fundamentals on general installation.

 

Lets say you've got an garden outbuilding which has its own power supply supplied from the main CU in the house, everything has been completed correctly, SWA buried etc. Lets say you want to build another building next door to that building how would the power supply work.

 

Would you take the power supply from the existing outbuilding using SWA buried again and stick another CU in the new building, for arguments sake lets say the new building wouldn't just be powering a lighting circuit but a heavy load.

 

Obviously calculations etc have to be taken into account but the fundamentals I am struggling with.

 

 

 

 

 

 

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Murdoch

that would all depend on the design load of building A, the design load of building B and therefore the supply cable from source to building A....

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Ardet R

As a general rule any given power supply is designed for the intended load and, unless there is an obvious reason to overspec the supply, will be at least reasonably close to the limits of the circuits.

In a domestic situation this may not be the case quite so much but for garden supplies customers are normally aghast at how much the cable costs to meet volt drop limits and opt for the smallest possible cable size, ("oh, there wont be much load just a few battery chargers and the odd power tool, maybe the lawnmower."  " ah, that should be cheap and easy":),  "oh and I might put in the tumbler dryer and a couple of 3kW heaters".:angry:)

Therefore if you were coming to a garden supply it is likely there would not be much further capacity available to manage a large load.

So you would tend to be looking at a new supply.

Also putting two consumer units in sequence will likely mean that there is no division of circuits under short circuit conditions as the supply  mcb from the first board will likely not discriminate from the mcbs in the second board.

As an initial approach the cable size and loads would need to be ascertained and then see if there is any capacity to take the anticipated demand and still comply with volt drop limits.

If there is capacity then possibly just running a circuit from the first board may be the answer rather than running a sub main to the next building.

Then you come to the joyous approach of "is bonding required in the building?" which then messes up all your cable sizes.

It all boils down to calculations in the end though.

(then you sling a 10mm² SWA in and hope!!:BadIdea:)

 

 

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Mastbruch

wont Cheers.

 

Ardet that's the bit I was struggling with which was intended in my first post where you state the possibilities of just running a circuit from the first board if load allow etc.

 

After you have run the SWA to the next shed what would the SWA fix up in to if there wont be a CU to terminate at, this is where I am losing it, what I mean is when you walk into the new shed what would be on the wall to control the lighting circuits etc.

 

I am probably coming across really silly, I know what I am trying to get at but doesn't seem to be working.

 

Basically you walk into one shed you have a CU then walk in to the other shed and don't have a CU what would be on the wall to control ring final, lighting circuit, You wont control all this by coming off a light switch, still don't think I am explaining it right.

 

Edited by Mastbruch

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Murdoch

These jobs, designs and installs are best learnt when doing practical training ...

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Ardet R
10 hours ago, Mastbruch said:

wont Cheers.

 

Ardet that's the bit I was struggling with which was intended in my first post where you state the possibilities of just running a circuit from the first board if load allow etc.

 

After you have run the SWA to the next shed what would the SWA fix up in to if there wont be a CU to terminate at, this is where I am losing it, what I mean is when you walk into the new shed what would be on the wall to control the lighting circuits etc.

 

I am probably coming across really silly, I know what I am trying to get at but doesn't seem to be working.

 

Basically you walk into one shed you have a CU then walk in to the other shed and don't have a CU what would be on the wall to control ring final, lighting circuit, You wont control all this by coming off a light switch, still don't think I am explaining it right.

 

Possibly I am not following but if you walked into your lounge would you think "why is there no CU in here, there is one in the hall?"

If not then why would you think there should be a CU in another room, albeit one that is outside.

It may be necessary for convenience to be able to isolate that circuit locally in which case you could install a DP switch on the wall.

The lighting can be switched normally. 

Possibly you might want to be able to reset an MCB locally once it trips, but this would then require full discrimination from the distant supply which is not practical in most domestic situations.

What other level of control would you need?

The only reasoning where you would really want to install a consumer unit would be where you need a large number of circuits for varying loads, I installed a consumer unit for a shed that had a kiln as well as sockets and lights and to have run the individual cables 50m down the garden would have been much more expensive so local control was best, however I have also installed a radial circuit for sockets to a small garden building and used an FCU to protect the lights that have been branched off the radial.  The supplying SWA was glanded into a junction box and twin and earth run from the junction box to the required sockets.

Edited by Ardet R
more info added

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steptoe

I have a CU in my main shed/workshop, fed from a 40A in the main house CU

with simply a DP switch feeding the light and socket in the second shed via a 16A radial, with the light being switched via a 3A fuse in a switched fused spur, and the socket being directly fed

 

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Mastbruch
54 minutes ago, Ardet R said:

Possibly I am not following but if you walked into your lounge would you think "why is there no CU in here, there is one in the hall?"

If not then why would you think there should be a CU in another room, albeit one that is outside.

It may be necessary for convenience to be able to isolate that circuit locally in which case you could install a DP switch on the wall.

The lighting can be switched normally. 

Possibly you might want to be able to reset an MCB locally once it trips, but this would then require full discrimination from the distant supply which is not practical in most domestic situations.

What other level of control would you need?

The only reasoning where you would really want to install a consumer unit would be where you need a large number of circuits for varying loads, I installed a consumer unit for a shed that had a kiln as well as sockets and lights and to have run the individual cables 50m down the garden would have been much more expensive so local control was best, however I have also installed a radial circuit for sockets to a small garden building and used an FCU to protect the lights that have been branched off the radial.  The supplying SWA was glanded into a junction box and twin and earth run from the junction box to the required sockets.

 

Thanks Ardet you have answered in this.

 

cheers fellas

 

 

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Mastbruch
Ardet R

Yes, I have used those at times, no real problems with them but to gland SWA you then need to buy the additional earth clamp bar (just below on the page) which significantly adds to the cost of an already expensive junction box.

I tend to prefer to use junction boxes that have enough space to fit a piranha nut on the inside.

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