How to supply a 20A Distribution cct?

It is a cold, wet, miserable windy day here today. I can imagine the customer not being very happy if he can't have a heater or two in use, and a kettle for a brew plus other loads, in his remote cold outbuilding without the lights dimming or flickering.

Well, just meet on site, and what a pain that was - kin'ell. 

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D​​istr cct 72m meter to CU, plus 10m lighting final cct and 8m skts. It's a shed come sun room for someone to sit in when needed. True to form, it was keep it cheap, only want two skts for tv and kettle, and said if had his way wouldn't even have the kettle, they can walk to the flat for a brew. He Glossed over having a heater.


Distr cct needs clipping to a wall approx 15m, of which approx 3m is behind thick bushes. He wants me to beat my way in there to affix cable, and use plastic or galv cond to support the cable, the cond being affixed at each end and possibly once in the middle, over the 3m stretch; it's not his bushes so cant cut them.


He wants the cable affixed to fence, which is actually quite sturdy though, 8 foot posts, hit and miss design.


In the flat, cable needs drilling through head plate of a lathe and plaster wall, without knowing what's in there, and again through a 2 feet thick stone wall (lathe and platter both sides) from kitchen to utility room at ceiling height, right near where the safe zone is for a kitchen light switch. I said, just move along the wall a bit further, but he didn't like that that meant using a bit more cable to go past where needed then back again in the other side. I pointed out that the switch wiring could be damaged, to which he got a bit annoyed at the thought of having to pay for repairs. I said this will all be in my quote, so you're informed in writing, and got a bit annoyed that i was looking for problems, saying well it hasn't happened yet! 


To top it all off, he's only got a 60A sp supply, but had the same it's not been a problem so far attitude, not wanting to hear about possible uprating of supply, or that depending on max load and load profile he could end up popping the main fuse; true, there's no sign of heat damage to tails or CU, but he just shut down from listening to possibilities, at times verbally dismissing it or being somewhat evasive.


Is it just me, or do others find it mentally draining when you're with someone who you can see in there eyes is counting every penny, squeezing every penny, and almost rolling there eyes at the mention of having to have this and that installed?


He then wants the price yesterday and the job finished tomorrow, but if the quotes too high he'll just run a lead, for ......$?#!$$!#?$$!$#?$?$ sake.


I'm thinking just dump this and move on. Sorry for the rant but it is quite helpful to air such things to people who've probably been thru such things themselves. Customers should come with a health warning.


F

I'd walk and move on. You will have nothing but problems there unfortunately. There are more  and nicer fish in the sea. He'll probably argue about the bill when you have completed as well. You can only do your best. You are not alone, we have all been there.


Good luck,


Z.

you should only be prepared to do a proper job, then that means if need be come in  with a quote that is erring on the high side to cover the pain, and be half expecting to be under cut.  If you are under-cut you have not exactly lost your dream job, and if you do get the job, at least you won't make a loss on it if it takes a bit longer.


If the application is temporary, then an extension lead out the window may well be the correct solution for the cost-benefit trade off. It won't work very well of course but that may be where he is happy  to be. Or the other cheapo beloved of garages of a certain era, of twin and earth on a catenary wire - OK so long as inspected regularly for sun damage.

I had a self builder like that once, after about 30minutes I thanked him for asking me to quote and said "Unfortunately on this occasion I am unable to provide you with an estimate". Then I left. 


Sometime you get a bad feeling and if he has already annoyed you, you might resent doing the job.

Farmboy, you already know this job is going to be a pain so if you could do with the work, do as Mike suggests and price it for a proper job and also build in at least a day's contingency. Otherwise chuck in a mad price and if, in the unlikely event, he goes for it you at least have the soothing balm of dollar when he drives you mad. Or walk, who needs the aggro?!

“From the trade assoc., one said from the CU, another said it's got to have its own main sw, another said no problem with the modular encl and fuse carrier. While Hager said its got to be the rotary type, not the fuse carrier.”


Table J1 in the OSG.

BS 88 series and BS 1362 fuses can be used for isolation, but not for emergency or functional switching. Also if it’s a TT installation the isolator needs to be double pole, hence Hager saying you need a rotary isolator upfront of a fuse protected circuit such as this.


Andy

Scanning the thread (sorry it's late)... I'd be saying whoever said separate tails into a block is talking out of the back of their head. The main isolator in the CU provides isolation and is lockable with the correct lockoff kit. It may be more CONVENIENT to have a separate isolator, and that would be a sensible option if total isolation of all live conductors may be required routinely (however slowly the routine rolls round). But REQUIRED, No.


With everyone else on the 'definitely upsize the cable'. Unless you know the load isn't going to grow (single fixed load)... never let volt drop be your guiding rule. It's a last ditch 'this cable's too small' rule.


As to discrimination (sorry, selectivity). Cartridge fuse units for domestic CU's are available on Ebay, from commercial sellers. Just because it's ebay doesn't mean you can't trust it, it just means the level of trust is lower.  Ditto numerous other online marketplaces.


But nobody yet seems to have mentioned (APOLOGIES if someone has, again, scanned it fast) the idea of a C or even D curve breaker to protect the distribution SWA...  and normal B curve in the distant board.


On a short, they'd trip in order of their rating, unless the fault was catastrophic, in which case who cares which one trips provided one of them does, on an overload, the B's will trip faster.  Its possible if you're dedicated enough to feed the curves into software and superimpose them and find out where they cross, but who cares if a BS3036 with 2  strands of 5A wire trips before a C16. I want whatever that is dead <grin>

It is important to limit Voltage drop as required by Table 4Ab "between the ORIGIN of an installation and any load point." (6.4). This will ensure correct operation of equipment and reduce losses in cables.


Z.

It's not a lighting-only circuit, so the larger volt drop figure applies.

But v.d. requirement is from the origin of the installation to the light, not just the final circuit. So to achieve 3% overall and allow say 0.5% for the final lighting circuit, you'd only have 2.5% to play with for the distribution circuit - rather than 5%. (Or specify in your design that only luminaries with a wide voltage tolerance are to be fitted ?)


As for discrimination between MCBs and a 20A fuse - it's the MCB manufacturer's data you'd need for that (and depends on the maximum fault level involved) - but I would suspect 20A would be too low to be useful - e.g. this MEM data (https://www.tradingdepot.co.uk/info/wp-content/uploads/2013/02/the-guide-cat865-circuit-protection-control.pdf) would suggest that a 20A fuse will only discriminate with an MCB up to 6A and then only for fault currents up to 160A. For a 16A MCB and a decent fault level you'd be looking at something closer to a 63A fuse to ensure discrimination.


I don't think there's much mileage in using a C type MCB for the submain rather than a B-type - C types aren't slower as such - they simply don't respond (at all) to smaller fault currents (below 5-10x rather than 3-5x) if the fault level is above the MCB's (magnetic) threshold then it'll practically trip just a quickly as a B type - and if your Zs values are within limits then the fault currents will be high enough to trip it. D types just might work where you can use a 5s disconnection time - as unlike B or C types, they'll trip within 5s on the thermal mechanism for a small range of fault currents - but it would mean some very careful arrangement of circuit impedances (so a ransom to DNO changes for instance).


I guess it's really down to how much discrimination (selectivity) the customer really requires - if none at all, then you could just run a 20A circuit for the sockets and run the lights off that with a 5A fuse in a FCU (or just run a 16A circuit and forget the FCU). If full discrimination is really needed then either a largish fuse on the submain (perhaps relying on the downstream MCBs for overload protection), or simply run two separate final circuits instead of one submain (could even be a single 4-core SWA for a truly minimal solution).


   - Andy.