UK Electric supply to new shed.. The main property is TT earth system.

[Steve1968cymru]

I propose to use a redundant shower supply that terminates in the attic. This will be connected to swa 4mm 3 core cable and run 45m to a new shed with a separate consumer unit with rcd and 1 6a and 1 20a breaker. I'm not proposing to have any high consumption equipment in the shed.

My questions are:
1. As the house has a TT earth system, will I need an earth rod at the shed?

2. Can I use 2 core swa and use the outer as the earth?

Thanks in advance

 

[davesparks]

What condition is the earth electrode system in at the moment, what is its Ra?
How many rods have been used to make up the earth electrode system?

There is no need to provide a seperate earth electrode for the outbuilding, though if the protective conductor is suitable all the way back to the MET it may assist in lowering the overall Ra to put additional rods in at that location.

4mm over that distance sounds quite small, what size is the OCPD of the submain and what VD have you calculated?

You need to calculate whether or not the armour is suitable for use as the sole CPC of the circuit, it probably is, but i don't know the details of the installation to give a definate answer.

 

[Vortigern]

Notionally if you used a 60898 20a for a 50m run you would need 6mm² given that the route of the cable was clipped direct. If travelling through thermal insulation the answer will be different. It is quite likely the cable going to the shower has travelled through insulation. I assume lighting will be used at the garage hence the need for a larger cable due to v.d.

 

[Steve1968cymru]

I'm not sure of the Ra value, not had it tested. From what I can see there is one rod outside the property with a cable to the MET. I'm not sure of the rating for the circuit breaker on the shower supply as I'm not in the property at the mo but I believe it is 50a.

As for voltage drop, I have assumed non lighting circuit in the calc, as I will be using LED lights. Using TLC 's calculator they suggest 2.5mm but I will prefer 4mm to cover myself.

I will get a sparky to quote for the work, but I just wanted advice on here as the prices I have had so far don't make sense. The price quoted varies too much for me to have confidence between an insufficient job, and someone 'having my pants down'

 

[davesparks]

I'm not sure of the Ra value, not had it tested. From what I can see there is one rod outside the property with a cable to the MET. I'm not sure of the rating for the circuit breaker on the shower supply as I'm not in the property at the mo but I believe it is 50a.

As for voltage drop, I have assumed non lighting circuit in the calc, as I will be using LED lights. Using TLC 's calculator they suggest 2.5mm but I will prefer 4mm to cover myself.

If the ocpd is 50A then 4mm is woefully undersized.

LED lights are still lights, so how on earth do you think that counts as non-lighting?
Anyway, I asked what voltage drop you had calculated, not what limit you are working to, although that is also useful information.

 

[Steve1968cymru]

If the ocpd is 50A then 4mm is woefully undersized.

LED lights are still lights, so how on earth do you think that counts as non-lighting?
Anyway, I asked what voltage drop you had calculated, not what limit you are working to, although that is also useful information.

I only considered the shower circuit as it was redundant and available in the attic. The OCPD can be changed to 32A of course.
My assumption with LED lighting is that they are not affected by VD to the same extent as incandescent lamps.

Perhaps I need to get the quotes broken down in detail to see what I'm getting

 

[davesparks]

I only considered the shower circuit as it was redundant and available in the attic. The OCPD can be changed to 32A of course.
My assumption with LED lighting is that they are not affected by VD to the same extent as incandescent lamps.

Perhaps I need to get the quotes broken down in detail to see what I'm getting

The OCPD can be changed to any value you like.
If your intend for it to be changed to 32A then why the CU in the shed?

The regulations do allow you to assess volt drop based on the specific load not just the percentages in the book, but this needs to be done with technical knowledge and calculation, not just assumptions

 

[Steve1968cymru]

The OCPD can be changed to any value you like.
If your intend for it to be changed to 32A then why the CU in the shed?

The regulations do allow you to assess volt drop based on the specific load not just the percentages in the book, but this needs to be done with technical knowledge and calculation, not just assumptions

I wanted to split the circuits for 13a sockets and lighting, and I thought it was best practice to do so. Also to isolate supply when the shed isn't used in the winter months.

 

[pc1966]

If it is only for occasional use (i.e. really a shed/store, and not a workshop or hobby room conversion, etc) then your best bet is to have the feed from the unused circuit in the house changed to a 20A-32A breaker with RCD (it might already be from an RCD protected circuit in which case just a new MCB, otherwise use an RCBO) and at the shed just use a FCU for the lights with the sockets directly off the feed. The FCU doubles up as the light switch and you can put in a 3A fuse to protect that circuit.

Generally lighting parts are not rated for above 10A so they should not be put directly off a 13A socket circuit.

If it shares the same RCD as other circuits in your house, then you might want a means of completely isolating it so any faults can be contained until repairs are possible. The MCB/RCBO will usually only isolate the line conductor so a N-E fault can still trip your house RCD (recent Wylex/Crabtree ones also isolate N but that is unusual).

If you go with a 20A MCB then you can easily get a double-pole switch in the same style as your household lights/sockets/etc for that aspect.

Generally there is no down-side to adding another earth rod, except obviously if it is driven in without care and hits a buried pipe or cable!

 

[davesparks]

Generally there is no down-side to adding another earth rod, except obviously if it is driven in without care and hits a buried pipe or cable!

As long as the conductor connecting it to the MET is suitable, otherwise you may end up with a lot of current flowing down an undersized conductor.
[automerge]1592661487[/automerge]

I wanted to split the circuits for 13a sockets and lighting, and I thought it was best practice to do so. Also to isolate supply when the shed isn't used in the winter months.

If you are running a 32A radial to the shed then that in itself can be a 'standard circuit' for socket outlets as detailed in the regulations. An SFCU connected to this can supply the lights from this circuit.

Isolating the supply can be achieved at the beginning of the circuit.