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[greengrass]

There is buried armoured cable to garage
run from 16a mcb in bungalow CU. Running one light 1 13amp socket.
Can a radial of three sockets, one which has its own mcb for my table saw,
three plug in strip 2ft lights appropiately fused in plug. But I also need power to two freezers to run separate from the work sockets so working ( rare occasions) at the bench.
, To separate power for freezers will this require a CU in garage to divide power.

I've been told having freezers in the radial circuit could trip mcb when power tool used. To the bright sparks who respond
" Get electrician" I will run it all and get it tested and certified obviously before use. I know electricity doesn't take prisoners. I never take chances with elec or gas. Thank you in advance.

[Mr White]

You can "split it" anyway you like.
The "problem" is the SWA cable that is buried in the ground, it is too small for your requirements, it must be as it only has a 16A MCB at the beginning.
It does not matter how you "split the supply" in the garage, it still has a 16A MCB at the start, which may or may not activate when you switch things on / they start.

[ericmark]

As said the MCB or RCBO is there to protect the supply cable, I know in North Wales there were a load of homes built, with 2.5 mm cable from distribution board to house wall, and 4 mm house wall to garage, so the weak link is the 2.5 mm in the house, so the MCB rating can't be raised.

The big question with freezers is what is the volt drop. They can use 5 times rated current to start, and they need to get up to speed before the pressure raises to a point when it will stall the motor, for this reason most refrigeration units say on them do not use with an extension lead.

Using a loop impedance meter we can check the prospective volt drop, and if professionally fitted the volt drop should not be enough to cause problems, in the main even down to 200 volt instead of 230 volt, they will still run OK, but some garage supplies were only intended to work a light and a battery charger. The supply was never intended to supply a workshop.

There are today in general only two things where volt drop is important, fluorescent lights and freezers, as long as the overload is correctly selected to match the cable, worse case scenario is the MCB trips, so no harm done.

With fluorescent lights they may fail to strike, but again no harm done, but with freezers the built in overload will trip, these are clearly designed to trip if the power is removed and reconnected before the pressure has dropped, and auto reset, but if the trip on a regular basis, then they soon fail, the problem is a loop impedance tester is not cheap, typical around £250, however there are some cheap (£50) socket testers with loop test, they test at around 1.9 ohm, which really is too high for a ring final, clearly designed for the EU where rings are not used, even if they do have a 13 amp socket connector, but for what you want they are ample. If the loop is less than 1.9 ohms there should be no volt drop problems.

[greengrass]

apologize have taken what’s been said . I should have included following original from the buried cable powered from DP on armoured cable in to a Wickes CU with 80A RCD.
Former In ring One circuit 3 sockets on 32mcb. One loan socket on 32mcb to run two freezers. Lights on 6A and no trips on circuits . Just had new flat roof and have taken out existing cable attached to roof rafters to replace.
As I understand Ring circuits were introduced after wwll in order to save cable and new builds are going back to radial. Having had flat roof replace decided to renew electrics but will reduce
I will be reduce work sockets to two one for work bench/ table saw
Lights down to two radial via external type wall switch with appropriate colour tab both ends up to JB.
All will be tested by certified electrician according to the regulations before use.
Most of my work is hand crafted with occasional table saw use, and charging batteries.

[Mr White]

.............but you still have the same cable you had before you started.

[greengrass]

Able to fight way through rubbish to measure cable up wall inside garage. Round , black measures 12 mm using caliper so guess that's the size and as have had no trip problems using existing cable size is that correct .

[ericmark]

Yes ring final designed end of WW2, been some changes, smaller cable when we went metric, and insulated plug pins which reduce the heat transferred into the socket, and gone from fuse to MCB/RCBO and 30 amp to 32 amp, and dropped from 1.44 ohm to lower figure 5% less, and volt drop changed from 4% to 5%.

The main change however was the 1/3 rule drilling beams for cables, so we ended up with more cable, but the cost of RCBO verse MCB means having fewer circuits means less cost, so although there was a move to radials, this has reversed.

Traditionally we were allowed 88 meters of cable on a ring final, the change in volt drop resulted in this being extended to 106 meters, the formula does not seem to have been published, but was 20 amp centre and 12 amp even spread, so worked on 26 amp load when calculating volt drop.

The problem with the radial is it still has 20 amp at final socket, so you need three radials to do same area as one ring final.

I was worried about volt drop, and some one coming back to me and saying the volt drop has been exceeded, so I wrote a program to work out volt drop using the loop impedance, however once written it was clear the accuracy of the loop impedance meter was a major problem, and one could have 150 meter of cable before anyone could prove you had made an error.

And in the real world I have only once found a problem with volt drop, a shrink wrap machine failed to work due to volt drop.

In the main the switch mode power supply has resulted in volt drop not being a problem, old freezers and fluorescent lamps with magnetic ballasts have traditionally been a problem, but the inverter drive used on modern freezers and the electronic ballast used with fluorescent lights have removed this problem.

However the 5 year EICR required with rental property has raised a lot of issues, as electricians the last thing we want is for some one to say "I have had an EICR done, and your work has failed".

However the forms suggested by the IET do not show volt drop, or any reading which can be used to show volt drop, seems wrong, and likely they will realise and alter the standard forms.

However the meters required to show the loop impedance is within limits are darn expensive, so unlikely any DIY person will have them, even a plug in tester with loop and RCD test costs over £50, and only tests to 1.7 ohm and 200 mS when it is required to be under 1.4 ohm and 40 mS, although personally I would not be worried.

[Mr White]

Able to fight way through rubbish to measure cable up wall inside garage. Round , black measures 12 mm using caliper so guess that's the size and as have had no trip problems using existing cable size is that correct .

No.


It does not matter how you "dress it up" (Saying what you like, leaving out parts of the facts to make it suit your requirements) the existing cable is too small for your proposal.

I will leave you with this thought:

Sometimes people ignore advice given with the comment of "Well it works, so you must be wrong" To that I say "The Titanic was a ship built that could never sink"

[ericmark]

Looking at Batt cables, a two core SWA at 1.5 mm sq is 12.3 mm outside diameter. And it depends if thermal setting or thermal plastic, and how installed, but seems unlikely to be rated over 25 amps could be lower.

Personally to be on safe side I would protect with a 20 amp MCB/RCBO, and no larger, but even then there can be problems.

It is common to find a garage supplied from the ring final, one would expect to find a fused connection unit (FCU) so a 13 amp fuse, however I have seen where an unfused spur has been taken to the garage, should not be done, but that does not mean it's not done.

And to power a light and a battery charger it is unlikely to cause a problem, even if not to regulations, but when some one adds freezers, and machine tools then very easy to exceed the limit.

Outside diameter of the cable it not really the way to measure its cross sectional area, I will normally either read it on the cable, or see what size lug will fit on the wire with a snug fit. I may be able to see if 1.5 or 2.5 mm sq, but with 4 or 6 mm sq, very hard to work it out.

Bring back 7/0.012 I was raised as an imperialist.

[greengrass]

ericmark have taken in all said and et-al( others ) Basically consult a registered electrician.
Yes wilk contact my daughter to introduce or chase up 'er indoors' as she too knows her but not as well.to have words and visit site. That way It'll be safe installation.
I was not being flippant when I said had been using the existing circuit of 4 plugs three strip lights and never had it trip. After accident working for GPO/ BT excavating for damage BT Duct put road drill through 45v local cable. hoke went bang ,black smoke ,sparks like Firework. from the hole caught me in face. Boy did it go. Nurses at hospital said i look like panda black face and two white circles where glasses saved my eyes. Fortunately well insulated boots, gloves and drill so able to write this. I have never taken chances, why this happened electric plan T.Q. was wrong did not show new local cable directly below abandoned old local and cutting out concrete went straight through new joint. I C.A.T area tracing cables . A close call. Thanks for your advice.