Quote

[m4tty]

Hi,

Been to look at a job which involves replacing a feed to a workshop which is currently fed in 2.5mm T&E clipped direct .... to fence lol. The workshop is roughly 60m from main CU. Doing the calcs it works out at needing a 10mm SWA. The guy is in his 60/70s and will be using his small tools in the workshop together with a 2kw heater and few florescent lights, so ive worked out it needs 4.5kw supply / 20A. The calcs then work out 4mm is good until voltdrop then reworked comes out to 10mm. This seems excessive but if thats what it works out then that what it has to be.

I know he will say why does it need 10mm when its been fine running on a 2.5mm T&E for years lol.

Other thing is. The 2.5mm is currently split at the middle of the garden to supply a double socket which runs a pond pump and light. If i remove the 2.5mm cable and replace with 10mm im going to have a problem keeping the double sockets going unless i feed them back from the workshop CU.

Any ideas if ive explained clearly enough?

Thanks in advance for any ideas

Cheers

Matty

 

[Andy™]

could always run the SWA to IP box, then spur to socket

or if its buried, branch resin joint, with 2.5 to socket?

 

[Lee321]

Can someone do another calc. Diversity could be applied . Confident you don't need 10mm

 

[m4tty]

Heres the calcs. It would be 6mm if there werent lighting down there (3% VD instead of 5)

________________________________________________________________

Supply voltage = 230 Volts

Ib - design current = 20 Amps

Protective device type = MCB type B

In - protective device rating = 32 Amps

Length of run of cable = 60 metres

Power factor = 1

Cable Type : Thermosetting 90

 

[binky]

Volt drop for lights - lets start a discussion:-

lights are fed from mini CU in shed - ergo it's only that part of circuit that needs 3% , not whole circuit feeding shed.

 

[Sidewinder]

^Sub main volt drop still needs considering, and I will have to think about your suggestion!

 

[Graham Lister]

Heres the calcs. It would be 6mm if there werent lighting down there (3% VD instead of 5)________________________________________________________________

Supply voltage = 230 Volts

Ib - design current = 20 Amps

Protective device type = MCB type B

In - protective device rating = 32 Amps

Length of run of cable = 60 metres

Power factor = 1

Cable Type : Thermosetting 90

 

[SPECIAL LOCATION]

Please help if you can lee but if hes got his 2kw heater going and runs his chopsaw (guess 1.8kw as mine is that) = 3.8kw = 16.52A plus 2 lights and pond pump could be close to 20A

Any help is appreciated as always.

Thanks

Think you need to have a very serious think about what the REAL full loads for any sustained periods may be...

And have a chat with MR old-bod cuz on TLC prices hes going to be talking

 

[m4tty]

Thanks for that reply specs. Tbh my dads got 2.5mm singles in conduit running down garden to his garage under the concrete path which is 200ft away. The volt drop is so bad when the compressor starts up all lights dim down and he welds uses a plasma cutter, lathe, 10t press, big pressure washer plus all other power tools and has no probs. I worked out he needed 10mm to comply with regs but it's been like it since 1970 and still like it now. So the lights dim down when compressor or welder goes.

The guy is in his 60/70s and can't do alot due to health problems but I want to do the job right but 10mm is well overkill.

What do you recon on the 3%/5% as said above. Could I go with 5% as that will bring cable size down. I recon your spot on with the 4mm.

The 2.5 shows no signs of overheating or damage and has coped fine.

Thanks for your help and I'll post more later as gotta go change clutch master cylinder on dads landrover so Ill be back on later.

Thanks

Matty

 

[Lee321]

if he's round the corner nip over wack a clamp on meter on and get him to put all his main stuff on

 

[meerkat]

Hi Matty, I make it

Ib = P/V = 4500/230 = 19.56 Amps

In = MCB (B) 20 Amp

Ref Method = D (buried)

Correction Factors = Cc 0.9 (Buried cable)

It = In/CF's = 20/0.9 = 22.22 Amps

Iz = table 4E4A 6mm = 53 Amps, (6mm cable reqd due to Volt drop)

Fault protection formula = Ib is less or equal to In, In is less or equal to Iz

Ib = 19.56 Amps, In = 20 Amps, Iz = 53 Amps, therefore this complies

VD = mV/A/M x Ib x L/ 1000 = 7.9 x 19.56 x 60/1000 = 9.27 Volts (table 4E4B)

Power Circuit = 5% VD allowed, 5% 0f 230 = 11.5 Volts, therefore our 9.27 Volts complies

I hope this helps, cheers (MEERKAT)

 

[apprentice87]

Hi all,

Well, I agree with Matty and the 10mm. I will tell you why..

First off the max volt drop has to be 3% because the circuit is supplying lights. It is no good saying that "our circuit DOES comply with the volt drop requirements for lighting unless of course someone does something silly like using power at the same time"....

Secondly,

What about motor starting currents?? Mr man will not be happy if he goes out and buys an air compressor say, [now he has got his nice new power supply] and then finds that it will not start properly...

I have been involved with a few instances where people have had supplies run to sheds and no allowance has been made for motor starting.

I have also seen a motor literally melted into a lump, as, presumably, the cable run was that long that the fault current was insufficient to operate the overcurrent device at the origin of the circuit, and mr motor just got hotter and hotter and hotter as a result of failing to start.

I was also there when the owner of said circuit went and bought a new motor, installed it, and exactly the same thing happened again!!! Would have been a lot cheaper to have got the correct size cable at the outset....

Obviously you could not have a cable so large that we only had a few % volt drop while a motor was starting, but i would try to make an allowance for it all the same..

If you explain to the old chap the reason why you think he should have this large cable i am sure he will understand. I know it will not be cheap, but i think it would be worth it in the end.

Why is he having the original supply upgraded?? Has the cable just deteriorated, or has he got problems with what he is running??

john

 

[davetheglitz]

How about feeding the lights separately from the main CU? Saves the 3% on all of the power - and may work out cheaper.

This link might be useful http://www.talk.electricianforum.co.uk/question-answer-board/4288-why-volt-drop-so-important.html

 

[apprentice87]

At the end of the day i suppose it is up to the customer and what his decision is as to what he requires. All you can do is to give him your honest opinion, and then ensure that whatever you do install complies with regs.

 

[m4tty]

Thanks for the replies so far everyone.

Has he currently got his Chop-saw working of this existing 2.5mm supply? YES

What is the fuse rating on the existing circuit? 20A

Has the existing 2.5mm got any really bad signs of overloading.. NO

melted sheathing, discoloured / excessively stiff conductors etc.. NO

Does the existing 2.5mm supply regularly trip? NO

How many consecutive hours per day is he down the workshop? HIS TOOLS ARE NOT HiGH LOADS

How many of those hours does he actually have everything running? I WOULD SAY ONLY 1 TOOL USED AND HEATER AT SAME TIME. HEATER ON ALOT DURING WINTER THO

Is it only him that works in the workshop? YES AND IT'S MORE OF A SHED THAN A WORKSHOP (bit like admins lol)

As he's getting on in years does he tend to spend less hours down there now in the winter months and dark nights? YES

I'm trying me best to answer your questions but it's quite hard on this poxy phOne lol. Will be better later when I'm on pc

Thanks for helping

Matty

 

[m4tty]

Ill ask the customer a few more questions before i send my quote and give him the options.

The circuit needs to have a single socket outlet near the back of the house and 3 sockets half way down. (2 for pond pump / light & 1 spare for lawn mower.) then onto garage mini CU which is already there.

The run will be in SWA and i was thinking will 2 x 4mm or 6mm cables fit in back of an MK masterseal socket?

Also all socket outlets outdoors need to be RCD'd so if I use an RCD SSO near back of house and use an RCD FCU for 3 sockets near pond? Can you get RCD FCU IP rated for outdoors?

Thanks

 

[SPECIAL LOCATION]

Right before you go off quoting for a national grid size cable to supply a few bits in a pensioners shed...

get a cup of tea ut the kettle on then go back and have a read of Big Red!

page 106. 525 Voltage drop in consumer installations..

12 lines of text in four paragraphs..

525.1

525.2

525.3 7

525.4

Keep your finger in those pages and also read Appendix 12,

page 385..

also called Voltage drop in consumer installations..

Only half a page to read...

Read em both a couple of times, your tea will still be warm and you still got time to go and get a biscuit as well cuz its not much to read...

So should be clear about the key reason for checking volt drop to ensure sufficient voltage is getting to work the fixed current using equipment in accordance with manufactures data..

Also you need to be asking yourself..

Q1/

If my incoming supply is typically around 240v+ (as many are)

Is my 3% volt drop limit ref to the actual incoming 240v+ or the nominal 230v?

i.e. are we looking to exceed 232.8v or 223.1v....????

(cuz thats a 9v difference for starters which is bigger than our 6.9v allowance!?)

Q2/

Is the 3% absolutely set in stone and cannot be altered under any circumstances?

Check out the note about circuits exceeding 100m in length

(0.005% per meter up to max 0.5% I think you will find?)

So although your circuit is not a 100m+ circuit, the key point is that 3% is not an absolute end of the world must never be exceeded for fear of death number!

Q3/

Are there any allowances for motor start up inrush currents, such that you could exceed the % figures providing you stay within manufactures limits?

Q4/

Are we advised that diversity is a consideration that can be considered within our circuit design for volt drop compliance?

Guidance note 1 also gives a bit more info on pages 83-84..

and 84 makes comment to the actual acceptable incoming supply tolerance +10% -6% and some consideration on large supply installations where greater percentage volt drops can still be acceptable.

IMHO The bottom line is that in the ideal world if everything is within the 3% & 5% everything should be hunky dory..

But where getting close to these limits there can be exceptions where a few other consideration can be bought into play.

These are where the circuit designer has to satisfy him/herself of what the realistic expected loads are going to be to satisfy themselves that all equipment they connect should have a supply above the manufactures minimum operating voltage.

And that equipment in use will not adversely affect other connected equipment. It does mention the consideration of separate circuits back to the CU rather than a single circuit/sub-main.

More tea Vicarut the kettle on:coffee

consider it another way..

a)

How many lights are you actually putting in this shed?

lets assume you are putting in four of these low energy bulkheads..

ASD Rectangular 2 x 9W Black Bulkhead | Screwfix.com

4x18w = 72watts less than 0.5 amps

or even assume 230watts 1amps worth..

You could run that off a 1.0mm @ 60m and still be well within 3% volt drop!

B)

Now consider your power..

lest say you need 3500watts 15 and a bit amps...

this could be run off a 4mm @ 60m and still be within the 5% volt drop!

So it is actually cheaper to but 1x1.0mm & 1x4.0mm rather than a 10mm

So in essence a 6.0mm should eat it no problem...?????

{this concept is what Davy-Glitzy-Boy was referring to in his previous thread wot he linked to I believe..?}

But if it were me based on the type of installation (pensioners shed) I doubt very much I would put in a 10mm OR even a 6.0mm!!

We are not talking a small self employed mechanic staring up a company in his garage working 10 hrs a day 6 days a week!!!

Actually if this bloke were one of my relatives I would probably think he'd been ripped off and stitched up by a cowboy going over kill to sponge more money out of him if a cable bigger than 4.0mm was fitted!?

:CGuinness

 

[m4tty]

Thanks Specs for that reply. I've had a read of the big red comic and am going to see the price difference between 4mm and 6mm swa then prob go with 4mm as I don't want to cost customer more than required as he's a very old boy with parkinsons.

The shed is already wired up with lights and a few sockets for his power tools off a mini 2 way cu with rcd Protection.

If you were just installing the submain cable would you fully test the install in the shed as I haven't installed it but would want to make sure it's safe to connect?

Thanks for your patience with me on this one just want to get it right. The board was installed by british gas and only the gas has been bonded. No water bonding and it's copper so that's got to be done too.

Cheers

 

[SPECIAL LOCATION]

Yes I would fully test & open up everything in the shed...

Re terminate or tighten every connection...

It wont take too long...

Then fully test it as if you had just installed it..

ESPECIALLY the insulation resistance to double check condition of the existing cables..

IMHO if you are getting all readings (L-N, L-E, N-E) greater then 100Mohm, you can honestly tell the old gent that the installation has been fully tested as far as is reasonably practical will be all OK to see him out the rest of his days.

One other little side note to remember when considering your volt drops...

Most manufactures power ratings are quoted at 240v not 230v..

So take your 2KW + 1.8kW (3800 WATTS)

If calc @ 230v = 16.52A

But if calc @ 240v = 15.83A

approx 0.7amp difference,

0.7A over 60m of 4.0mm could bring approx 0.5Volt error in your volt drop...

Which if you are getting near to your limits another 0.5v dropped could put your expected percentage 0.2% out!

This is a real situation where practical calculations and basic underpinning knowledge has to be balanced with clients budget and realistic expected load levels...

Obviously The only way to 100% belt and braces cover yourself is the bigger cable..

But the real world NEVER works like this..

Costings are always an important consideration.

(don't forget loads of properties still only have a 16mm to the whole property!)

You may never know what the actual typical load is, you just got to make a reasonable judgment call and be confident enough to stand by it!

Actually ANOTHER point you would need to consider is where is the property located?

Is this some remote rural location?

Or just reasonably built up area local to a main town or city?

If its right out in the sticks I may probably err on the side of caution and go up a bit toward the 6.0mm size.

:C ut the kettle on