Supply to Summerhouse - volt drop

[Midwest]

I've been asked to quote for a supply to a 'Summer House', clients words not mine. Due to the lay out of the property, it's not practical to supply direct from the CU. There is a socket on a RFC, that can provide a supply via FCU.

Although their garden is quite small, the route for the swa will be about 25metres. So I'm a little concerned about Zs and volt drop. When I went to have a look for the quotation, I discovered a fault on the RFC, no end to end continuity on r1. I'm going back this week to rectify that, so I could do Zs test at the said socket. I will then test at the socket to get a Zs reading, than add to R1+R2 for a suitable sized swa.

I'm happy to calculate the volt drop for the new cable, however I'm not too sure about calculating volt drop on the RFC. I have the calculation;

Ib (design current) x (Mv/A/m) x L (length of circuit divided by four)
1000

As it's an existing circuit, I would be guessing at it's length, although I could obtain a rough estimate via end to end resistance. Am I thinking this out correctly?

 

[LeeH]

Measure it, loose the maths?

 

[Richard Burns]

You do not really have a Zs problem, so long as the ring is compliant then the summerhouse supply will be compliant as it has a 13A fuse.
For volt drop as it is a mix of existing and new it is not practical to accurately calculate, however you can hypothesise that a basic domestic ring in an average house will probably have a volt drop of no more than about 3V so a supply to the summerhosue of 4mm² SWA over 25m would keep your volt drop within limits.
Obviously you can better estimate the ring length in your situation and jsut aim for a csa that is over the top.
Alternatively specify that only items capable of operating at 200V are permitted to be used!

 

[Midwest]

Thanks for the replies chaps, perhaps I am over complicating things, just didn't want a bad Zs or VD, having installed too small a cable.

However, it's immaterial now as I had a phone call this morning from my client, who said he has given the job to another electrician, who could not find any fault with the socket I'd tested.

 

[Des 56]

Thanks for the replies chaps, perhaps I am over complicating things, just didn't want a bad Zs or VD, having installed too small a cable.

However, it's immaterial now as I had a phone call this morning from my client, who said he has given the job to another electrician, who could not find any fault with the socket I'd tested.

Yes the old "other" electrician dilemma

In the ideal world of tomorrow,this person who "plays with electrics," sorry I mean this "other electrician," he would actually have the knowledge to understand that he doesn't understand and stop playing around

You then won't need to compete against ignorance, both by the customer and the "other electrician"

 

[Richard Burns]

Thanks for the replies chaps, perhaps I am over complicating things, just didn't want a bad Zs or VD, having installed too small a cable.

However, it's immaterial now as I had a phone call this morning from my client, who said he has given the job to another electrician, who could not find any fault with the socket I'd tested.

Such a pity you had made a mistake and there was ring continuity and that 1.5mm² T&E spurred directly off the socket in a garden house was quite sufficient for the summerhouse supply!
Who needs regulations? They just cost the customer more and it is much cheaper to have a funeral instead.

 

[telectrix]

Such a pity you had made a mistake and there was ring continuity and that 1.5mm² T&E spurred directly off the socket in a garden house was quite sufficient for the summerhouse supply!
Who needs regulations? They just cost the customer more and it is much cheaper to have a funeral instead.

i see you are on sparkling form today. have you changed your medication/beer/smokes?

 

[Midwest]

Bit p'd off. Only went to do a quote.

Tested end to end continuity at selected socket, to ensure it was on the RFC, not spur from spur. r1 = 998oh, rn = 0.60oh, r2 = 1.09oh. Got same readings at another socket. Inspected the original EIC for the property, built 9 years ago, r1 was 0.78 and rn was 0.62, so something wasn't right then. Spent about 3/4 hour, quickly try to find some loose connection behind faceplate, but as it was getting way past my tea time and it been ok for 9 years, I decided to go back this week and do the ol' R1+R2 test. 'Before you go, can you give us ball park figure for it, won't hold you to it? How much!'.

So he rings me says, he's got another electrician, who 'by the way' couldn't find 'your fault'.

 

[haptism]

Im interested in this topic cause i was trying to work out similar, vd on cable from fcu off a ring. I expect you could meet ZS (for 13a) over 25m without ramping up the cable size too much, however to meet a 3% v/d over 25m may require a stupid size of cable. All this depends on the existing vd on the ring, and importantly where exactly on the ring you are spurring from. I might be wrong D) but the formula

Ib (design current) x (Mv/A/m) x L (length of circuit divided by four)
1000

considers max vd at its furthest (mid point). To get the vd where you are spurring from is more complicated and requires Kirchoffs Law calculations; bear in mind that the v/d on a ring fluctuates depending on what is drawing power at the time so even this method wont give a definite figure . If summerhouse had its own supply it would make things alot easier. Only way I could think of would be to fit light fixtures that can run on v/d's much higher, but someone can later change them to standard filaments and so might not be good practice. Would be interesting to see how others tackle this.

 

[Midwest]

Before I posted, I read about Kirchoffs Law calculations, but the grown ups on the IET forum seem to be in dispute over using it for RFC VD calculations, and the formula in my op seems more appropriate, I think?

 

[haptism]

Before I posted, I read about Kirchoffs Law calculations, but the grown ups on the IET forum seem to be in dispute over using it for RFC VD calculations, and the formula in my op seems more appropriate, I think?

Maybe because Kirchofs law is best used on known resistances (fixed loads), for which a RFC is not; its all a bit of an exact science.

 

[HandySparks]

If you measure the line impedance (like Zs, but L-N instead of L-E) at the point on the ring and subtract the line impedance at the board, that'll give you the effective R1+Rn for that point on the circuit. From that, you can calculate the VD for a given load.

 

[Midwest]

If you measure the line impedance (like Zs, but L-N instead of L-E) at the point on the ring and subtract the line impedance at the board, that'll give you the effective R1+Rn for that point on the circuit. From that, you can calculate the VD for a given load.

I'm gonna have to think about that one, being distracted by smell of Sunday dinner?

 

[HandySparks]

If you measure the line impedance (like Zs, but L-N instead of L-E) at the point on the ring and subtract the line impedance at the board, that'll give you the effective R1+Rn for that point on the circuit. From that, you can calculate the VD for a given load.

Or join L to N at the board and measure R1+Rn directly at the particular point on the ring. Of course, the other loads on the ring and their distribution will affect the VD, so maybe not so easy after all.

 

[Toneyz]

You will get a phone call soon asking if you can come back as other electrician not come back to complete

 

[Midwest]

Or join L to N at the board and measure R1+Rn directly at the particular point on the ring. Of course, the other loads on the ring and their distribution will affect the VD, so maybe not so easy after all.

Still not really following you HandySparks, but I'll keep figuring and I'm sure I'll get it eventually, cheers.

 

[Midwest]

You will get a phone call soon asking if you can come back as other electrician not come back to complete

Think I might just say get stuffed, I'm not bitter! Didn't like the last bit in their phone call, implying I was trying to rip them orff. They did name the other spark, so I've no doubt he'll wire up their Summer House; hope he checks for continuity. Otherwise they might be sat in it one day, to see smoke coming from their house.

Time to move on