2365 Level 3 Design Project. Volt Drop feed to Summer House and earthing

[Gurnatronics]

Afternoon Gents, just a query regarding volt drop and extending the equipotential zone on a PME in the level 3 design project.

In my design I used a 16mm armoured and the associated volt drop was 5v, this left 1.9 volts for the lighting ring in the summer house. Tutor said not to worry about the feeds volt drop as it was not cumulative and wouldn't effect the lighting circuit. Is this correct, or just leaving it out for sake of simplicity.

Also, if I were to use a 16mm feed, would that also allow the PME to export its earth? The summer house has sinks and toilets so I assume will have copper piping to underground. I can recall seeing a 10mm bond would mitigate the risk but cannot find the appropriate reg with regards to out-buildings. Would a TT earth stake be a better solution?

Thanks for looking this over and look forward to your inputs!

 

[buzzlightyear]

Would a TT earth stake

yes .that would be correct .
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you can not export the earth just in case you lose the neutral has is combined with the earth.
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disagee mr robinson ?it depends if there is anything in the outbuilding which requires main equipotential bonding, if there isn't a water or gas supply, or structural steelwork, then there is no need to TT the outbuilding. .
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https://www.electriciansforums.net/attachments/garage-shed-supplies-pdf.3514/

 

[happysteve]

Quick question: are you sure you need to consider the design of the summer house in the assignment? It's great you're thinking these issues through, and it's possible either (a) the assessment might have changed since I last saw it, or (b) perhaps your tutor set you extra tasks to get you thinking... but just for your own peace of mind, read the assessment questions carefully so you don't end up doing extra work you don't need to.

You need to include the feed to the summer house when determining the maximum demand (with and without diversity), but my recollection is that was the only thing you need consider about the summer house.

Do check, though, I could well be wrong.

 

[Gurnatronics]

No I dont need to do anything to the summer house really, just run a feed to it with a 32amp rcbo. Im just using it as a good exercise to flush out all questions I have in my mind.

So just to wrap my head around the praticalities. 2 core swa (live and neutral), earth the armour at supply end but terminate into whiska or similar without earthing the load side of the armour. Protective bonding pipes to MET in the summer house unit and TT stake.

What are your opinions on the volt drop? Was i correct to size it with consideration to additional at 6.9v total drop or was that over designing it?

 

[happysteve]

In considering your thoughts about PME and outbuildings, consider this:

- Where is the main intake/service head? (In the main hotel building)

- Where is the sub-board from which you were going to take a sub-main for the summer house? (In the cafe/pool block)

These are separate buildings. The cafe/pool block isn't on a separate earthing system (TT), according to the design specifications. Would the summer house be any different?

 

[buzzlightyear]

Tutor said not to worry about the feeds volt drop as it was not cumulative and wouldn't effect the lighting circuit

your tutors is correct .

 

[happysteve]

Fair enough if it's a thought experiment, good on you.

Volt drop needs to be considered from the origin of the installation, so taking that into account on the sub-main is required, as you've been doing.

On a sub-main, although your limit is 3% (unless there are no lights on the sub-board), usually the majority of the load from the sub-board is general power, only a small portion of the sub-board's load will be for lighting. If you design the lighting to be energy efficient, with fixed loads (that users can't replace with higher loads like halogen lamps), you're likely to be drawing less than an amp for a summer house, and so additional voltage drop on the final lighting circuits isn't going to be a big issue.
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is this a job or on paper ?

It's on paper. It's an assignment on the 2365 level 3 qualification.

 

[Gurnatronics]

Fair enough if it's a thought experiment, good on you.

Volt drop needs to be considered from the origin of the installation, so taking that into account on the sub-main is required, as you've been doing.

On a sub-main, although your limit is 3% (unless there are no lights on the sub-board), usually the majority of the load from the sub-board is general power, only a small portion of the sub-board's load will be for lighting. If you design the lighting to be energy efficient, with fixed loads (that users can't replace with higher loads like halogen lamps), you're likely to be drawing less than an amp for a summer house, and so additional voltage drop on the final lighting circuits isn't going to be a big issue.
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It's on paper. It's an assignment on the 2365 level 3 qualification.

So, to clarify. Is it 3% volt drop from the sub-main at the summer-house, or at service head? As originally calculated I had a VD of 10/11v from the cafe block alone which left nothing for the lighting ring and the tutor implied this was ok so I got confused.

In considering your thoughts about PME and outbuildings, consider this:

- Where is the main intake/service head? (In the main hotel building)

- Where is the sub-board from which you were going to take a sub-main for the summer house? (In the cafe/pool block)

These are separate buildings. The cafe/pool block isn't on a separate earthing system (TT), according to the design specifications. Would the summer house be any different?

Agreed so this is where I get confused, In what situation is it ok to increase the pme equipotential zone and when does it require TT. Both the cafe block and summerhouse have water supplies and possible extraneous conductive parts... sorry for being annoying it just seems like there are overlapping answers.

 

[Strima]

Volt drop is from the source of supply, it is culmative.

If you have multiple DBs on long circuits and only accounted for volt drop from each DB you’d soon come unstuck. So if you had four DBs in a row and used the maximum permitted on each then you’d potentially be only getting 202 volts at the end of the final circuit.

 

[Robson689]

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disagee mr robinson ?it depends if there is anything in the outbuilding which requires main equipotential bonding, if there isn't a water or gas supply, or structural steelwork, then there is no need to TT the outbuilding. .
[automerge]1569777615[/automerge]

https://www.electriciansforums.net/attachments/garage-shed-supplies-pdf.3514/

OP mentions using 16mm SWA price difference between 2 core & 3 core is probably what you'd spend on the earthing materials anyway so what's the point in the TT when you can just extend the equipotential zone and lowest earth resistance possible?

 

[buzzlightyear]

mentions using 16mm SWA price difference between 2 core & 3 core is probably what you'd spend on the earthing materials anyway so what's the point in the TT when you can just extend the equipotential zone and lowest earth resistance possible

try reading the guidance note 8 .

 

[Robson689]

try reading the guidance note 8 .

I don't have these books, however the Garage-Shed-Supplies.pdf you posted even says it is possible to do what I said?

 

[Edtwozeronine]

I found I was guilty of over egging the pudding in a lot of my project work, don't make work for yourself innit!

 

[Gurnatronics]

I found I was guilty of over egging the pudding in a lot of my project work, don't make work for yourself innit!

Im trying not to but id rather fail on paper and learn than take the easy route and come undone later.

I don't have these books, however the Garage-Shed-Supplies.pdf you posted even says it is possible to do what I said?

As far as my very limited understanding goes. If no extraneous parts then no TT needed, If extraneous parts, TT. Or, you could run a 10mm main protective bond back to the original MET, or, if the orginal supply cable earth Is greater than 10mm and is sized both to provide main protective bonding and CPC functions concurrently, then the PME can be exported using the supply cable.

Or am I way off base? I only had two coffees today so not thinking clearly

 

[buzzlightyear]

I have put on your post about garages read please.

 

[lee75hunt]

Hi all, Im just starting my design assignment, my query is are we supposed to do all cable calcs for the summerhouse aswell as there is no mention in the question paper about it, only pool and cafe. thx for the help in advance

 

[Strima]

Hi all, Im just starting my design assignment, my query is are we supposed to do all cable calcs for the summerhouse aswell as there is no mention in the question paper about it, only pool and cafe. thx for the help in advance

If there are circuits in the summer house then you will need to design them, so yes, you will need to do the calcs.

TBH once you get into the flow you'll be surprised how quick you can do them.