16amp Question?

[graduate_owner]

Hi Jinxy,
Can I query your response? I'm not an electrician and you are so I'm not arguing here, just asking because I don't understand, but-

if you put a 16A socket fed by the ring main, wouldn't the 16A socket and cable be protected by only the 40A MCB, which would be far too large to protect the cable in the event of over current?

As I said, just asking and not trying to tell you your job.

K

 

[n0legs]

Hi Jinxy,
Can I query your response? I'm not an electrician and you are so I'm not arguing here, just asking because I don't understand, but-

if you put a 16A socket fed by the ring main, wouldn't the 16A socket and cable be protected by only the 40A MCB, which would be far too large to protect the cable in the event of over current?

As I said, just asking and not trying to tell you your job.

K

Correct.

 

[Jinxy]

It is complicated but in short no. It comes down to something called ohms law. For instance take a common example; a lamp on your bedside table. That lamp is supplied with a 1mm flex which is only rated at 10a and the socket is rated at 13a. But the breaker is rated at 32 amp in a standard UK ring circuit so by that logic you would assume the 32 a that is permitted by the circuit would burn the 10a flex cable on the lamp out, but it doesn't? !

The reason is all electrical items are limited by they're resistance so a 100w lamp can only draw 2.3 amps from the supply A = V/W (230/100=2.3)

So even the breaker is rated at 40a the saw or whatever he was planning to connect to the circuit will be limited by it resistance and that will be worked out by the manufacturer and they will fit the correct size flex cable on the machine which if it's 16 a is 2.5mm. So the flex and socket won't burnt out because they can't draw more current than the equipment is rated at.

Then there is fault conditions which again I could write anther paragraph on but fault condition occurred in 0.4 of a second which isn't enough time to cause significant damage to the cable.

And there is the question of whether the circuit supply cables (inside the conduit) can take 40a but we don't know that.

I could go on...but I prefer getting away from this for the evening and talking about hand planes and playing in my shed!!

 

[n0legs]

Okay here we go again.
If you are going to post about electricity please make sure what you are posting is correct.

A 100 watt bulb does not pull 2.3 amps.
The formula is wrong . The formula is :-
Amps = Watts divided by the Voltage ( A=W/V ), which in the case of a 100 watt bulb equals .44 Amps.


P.S-
Welcome to the forum Jinxy

 

[n0legs]

 

[pcb1962]

Okay here we go again.
If you are going to post about electricity please make sure what you are posting is correct.

I'm glad someone said it

 

[Eric The Viking]

I was wondering about proposing a sticky thread on some electrical basics, given some of the thoughts higher up this thread (don't want to embarrass anyone). For example, explaining power, energy voltage and current, what fuses are for (how many people know they should blow at double the rated current, for example?), and three phase versus single.

The downside to this is those who will think a simple(-istic) explanation gives them enough information to wade into deep waters in their own workshops. Nobody wants to treat electrickery as a black art (wasn't that Worzel Gummidge?), but on the other hand...

I was under the impression, incidentally, that 16A circuits should have an individual breaker per socket. The good quality (i.e. really expensive) splitters and disty boards (64A and 32A inlets) I've seen for stage and broadcast use always have an associated MCB per outlet, and often an 100mA RCD too. Otherwise you could get pretty high fault currents (and switch contacts welding themselves 'on' at the load end).

E.

PS: Even the meticulous make howlers occasionally. In this case the devil's in the description. #-o

 

[graduate_owner]

OK Jinxy, I see your logic. So the fact that a fault condition would cause excess current for the cable capacity is not significant because the RCD would trip before any damage is done. Makes sense.
One more question - what happens if the fault is not an earth fault, say a burnt out winding causing excess current to flow without tripping the RCD? Again - just asking.

K

 

[n0legs]

An RCD does not protect against overload.
A fuse, MCB or RCBO protects in the event of overload.

If only an RCD was used in a circuit the amount of current passing through that circuit could be equal or more than the rated current of the RCD.
The amount of current would only be regulated at the origin of the supply, possibly 60/80 or 100 amps in a domestic supply even higher in industrial/commercial supplies.
IE, whatever size of fuse is installed by the DNO (electricity board) in the cutout/service fuse.
The cutout or service fuse is the fuse that is on the main cable to the dwelling, installed just before the meter.

Therefore taking into account what I have written above:-
If we use a radial circuit in the home workshop wired in a standard 2.5mm Twin & Earth cable as an example.
Then lets say to keep costs down there were two 16a sockets on this circuit. THIS IS NOT THE CORRECT WAY TO DO IT, THIS IS AN EXAMPLE. However I have seen this done.
2.5mm has a current carrying capacity of approx 24 amps.
If there is no fuse or MCB protecting the circuit the maximum current that could flow through our tiny, teeny weeny 2.5mm T&E could be 60, 80 or 100 amps depending on the load applied to the circuit via the appliances plugged into the sockets.

AN EXAMPLE, (I have seen this in practice, I think I've mentioned that)
We have two 16 amp (blue plugs) machines with 3.5kw motors both running together at full load.
Each machine is drawing 15.21 amps, using 230 volts as the input voltage (but beware the supplied voltage can be higher or lower depending upon certain factors and this in turn has an effect on current ).
So twice 15.21 amps equals a total load of 30.4 amps, on a cable rated at only 24 amps.
We have an overload situation for the cable in the circuit.
The RCD would not operate as there is no current leakage, well not until stuff starts to smoke.
The cable on the machine is okay because it's rated above 16 amps, it's what most manufacturers do.
The problem is our 2.5mm T&E. Possibly buried in the wall, under floors, in ceiling voids getting nice and hot, maybe smoldering a bit starting a small undetected fire. We lock up the workshop and go in for a cuppa' with the wife and kids.
999, direct line and staying at the in laws for a few weeks.
Electricity, you can keep it give me gas any day.

 

[PeteG]

Please follow Deejay's advice, from the photos it appears you may have some issues.

Hello gentlemen. I haven't replied sooner as I've been completely lost by all the information, but I'd like to thank you all.
n0leges I have to ask, what are the issues you see in the images? Would appreciate your comments!

 

[NickWelford]

Electricity, you can keep it give me gas any day.

Bugger trying to run the p/t on gas though........

 

[Myfordman]

AN EXAMPLE, (I have seen this in practice, I think I've mentioned that)
We have two 16 amp (blue plugs) machines with 3.5kw motors both running together at full load.
Each machine is drawing 15.21 amps, using 230 volts as the input voltage (but beware the supplied voltage can be higher or lower depending upon certain factors and this in turn has an effect on current ).

Your basic point about cable loading and lack of current protection by RCDs is well made but please don't perpetuate the myth that ohms law can be applied to the power, volts and current of reactive loads such as motors.

It is one of those simple, easy to understand wrong answers so often applied in discussion of workshop electrics.
In reality, the current is greater than stated even if the applied voltage is the same.
MM

 

[mjcann]

+ 1 for MM comments.

There is I think a wide spread misunderstanding about motor current, and protection. Even dare I say amongst sparks.

With regard to the pic of workshop electrics, I would recommend getting an electrician to look at it, as the 13a sockets appear to be protected by a 40a breaker, and they may or may not be on a ring, there is no way of telling from photo.

Maybe I am old and over cautious, but throughout my 50Yrs in elect trade, my advice has always been the same, if you are unsure get qualified guidance, from somebody that has looked at the job first hand.

Martin

 

[n0legs]

Your basic point about cable loading and lack of current protection by RCDs is well made but please don't perpetuate the myth that ohms law can be applied to the power, volts and current of reactive loads such as motors.

It is one of those simple, easy to understand wrong answers so often applied in discussion of workshop electrics.
In reality, the current is greater than stated even if the applied voltage is the same.
MM

Absolutely MM, thank you for the comments.
I indeed do not wish to create any more myths than there are all ready out there.
My example is simplified as an illustration. I wrote it so that anyone can check my figures and come to the figures shown just to show how easy it is to create overload.
I don't want to insult anyone or come over as the big I am also I hope if there are others who can offer some insight they can jump in.

 

[n0legs]

+ 1 for MM comments.

There is I think a wide spread misunderstanding about motor current, and protection. Even dare I say amongst sparks.

With regard to the pic of workshop electrics, I would recommend getting an electrician to look at it, as the 13a sockets appear to be protected by a 40a breaker, and they may or may not be on a ring, there is no way of telling from photo.

Maybe I am old and over cautious, but throughout my 50Yrs in elect trade, my advice has always been the same, if you are unsure get qualified guidance, from somebody that has looked at the job first hand.

Martin

Hi Martin,
I couldn't agree with you more.
There is no limit to being cautious and with 50 years in the trade you have obviously been doing something right.
Your last line is my whole point, these forums are very good but there is a huge possibility that the information supplied by someone can be misunderstood as the situation is being viewed from afar. Then we have the whole can of worms of how competent the person who is asking the questions is to carrying out the work/repairs/improvements.


PeteG

Martin has given you the answer to my concerns.
Course of action should be to ask the electrician who did the work for you to explain how the circuit is wired and what cable he has used.
If his answer seems unsatisfactory get back to us with your concerns, I'm sure between us we will manage to keep you safe.

 

[Myfordman]

Workshop electrics is one of those subjects where the otherwise arrogant sounding repost of "if you have to ask then you don't know what you are doing" really does apply.
Even a perfectly correct response to an electrics question can be read by someone else as being appropriate in their similar situation thus leading to over confidence and mistakes being made and risks taken.

Best thing is to make friends with a local spark who can look at the installation and is willing to advise in exchange for woodworking favours.
Bring back barter!

MM

 

[graduate_owner]

Well as I said previously I'm not an electrician so I can't give the definitive answer, but I still think a separate, suitably rated MCB (say 20Amp?) to protect the cable feeding the 16A socket and installed by a qualified person would be the safest way to proceed. I can't see any disadvantages with this, apart from the cost. Also, as I said before, bite the bullet and be safe.

K

 

[PeteG]

Again, thank you all for your help.

This was how the wiring looked before the refurb last year, the box had already been fitted by the previous owner and seemed quite new, and I'm sure it had a 16 amp fuse fitted.



After the refurb, my cousin and an electrician that he works with, who also happens to be his brother-in-law, put all the new wiring in for me. Everything seemed OK and I didn't have any problems, but there again, it was before I had started kitting the shed out. The first item I bought was a 50 litre 3HP compressor which I'd planned to use for spraying the walls, as it turned out I was pretty useless at spraying and the compressor was pushed to one side for a while. After I was ready to start building benches etc, and at this point the only things plugged in were the freezer, radio, phone and the Record AC400 air filter, I brought the compressor out, can't remember what for but every time I switched it on the fuse on the main board in the house would trip. If I turned the air filter off, the compressor was OK. I think this is when my cousin swopped the 16 amp for a 35 amp!
Once I had the work station built for the Bosch mitre saw, I bought a Nilfisk 30T 1800W vacuum, hoping the Bosch would work off the Nilfisk, but every time I switched the saw on, the fuse in the house would trip again. The Nilfisk went back to the shop, and I think that was then the electrician came back and replaced the RCD. During all this time I'd used the Axminster TS200 connected to the Lidl's vacuum cleaner without any problems.
Leading up to my original 16 amp question, I mentioned to my cousin a couple of weeks back that a few of the planner/thicknessers I'd looked at required a 16 amp upgrade, that's when he swopped the 35 amp fuse for the 40 amp. I switched pretty much everything on in the shed that wasn't already on, radio, phone, freezer, six strip lights, AC400 air filter, BS350 bandsaw, 1800W mitre saw, the BIG Triton router, and a 750 belt/disc sander, the only thing I didn't switch on was the table saw and vac. Just in case!

I'll try and find out tomorrow if the sockets/breaker are on a ring!
Pete.

 

[PeteG]

A quick up date, apparently everything is on a ring.

 

[markturner]

The ring should be protected by a 32 amp fuse really, 40 amps is pretty near the maximum loading ( each piece of 2.5mm cable can carry approx 22 amps )

if you look at any domestic fuse board, a ring circuit is 32 amp protected and a radial ( single 2.5mm wire) is usually 20 amp.

Also, check the maximum loading that the circuit may carry ( total Kilowatt rating of the appliances divided by 230) if its over 32 amps, then you should really look at splitting it into 2 smaller rings or rewiring with heftier cable like 6mm.