16amp circuit

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--Tom--

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I use H07RNF for loose flex, is much nicer than arctic particularly in a workshop. Worth shopping around for as price per metre varies wildly and often cheaper to buy slightly more than you need to get the reduced unit price. All my sockets are wired with individual cores in trunking specced by spark. In retrospect I should have taken the hit and got better sockets. I went with MK and they aren’t the nicest, but budget was stretched already at the time.
 

Jacob

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Thanks for answers and so much information.
It's now on my things to do list, item 376b!
Get there sooner or later.
 
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It'll probably be fine - it depends on the specifics of the installation. Reading between the lines it appears your electrician is installing or replacing a second consumer unit in your garage, makes a lot of sense especially in a detached garage.

It may seem an odd question but how is your garage decorated? If it's bare brick and the cable is simply being clipped to the wall it's fine. The cables need to be heavier gauge in e.g. studded walls or especially if amongst insulation since they can't cool as easily.
Yup - he is replacing a consumer unit in the garage. The cable to the garage is rated at 100A. The box in there at the moment is quite small with only two 'circuits' (thats probably the wrong word) one for a series of 13A sockets and one for the LED lights.

The new box has a couple of extra spaces to run two separate single 16A sockets - one to each side of the garage.

The garage is framed out and plasterboarded. The 16A cable will run in plastic conduit on the surface.
 

Sideways

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I use H07RNF for loose flex, is much nicer than arctic particularly in a workshop. Worth shopping around for as price per metre varies wildly and often cheaper to buy slightly more than you need to get the reduced unit price. All my sockets are wired with individual cores in trunking specced by spark. In retrospect I should have taken the hit and got better sockets. I went with MK and they aren’t the nicest, but budget was stretched already at the time.
With you there Tom.
I moved away from the MK / ABB industrial connectors from screwfix as I used more of them. They don't hold together very well, backs and fronts can come unclipped if you use them with thick and stiff cables.
Mennekes are much the best of the ones I've used but you pay for quality.
Lewden / Palazzoli aren't bad and can be picked up on ebay when someone's selling off odd ones and two's. You see those with black bodies sometimes as they are used for wiring on stages and at events where you don't want people to pay attention to all the temporary cabling.
 

Sideways

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Yup - he is replacing a consumer unit in the garage. The cable to the garage is rated at 100A. The box in there at the moment is quite small with only two 'circuits' (thats probably the wrong word) one for a series of 13A sockets and one for the LED lights.

The new box has a couple of extra spaces to run two separate single 16A sockets - one to each side of the garage.

The garage is framed out and plasterboarded. The 16A cable will run in plastic conduit on the surface.
What Spectric said. In a small CU like this, better to have "RCBO's" than waste slots on two RCD's and then use MCB's for each circuit.
6 circuits are pretty easily used up in a small workshop.

Lights
13A ring
2 odff 16A sockets - you might want a dust extractor and a planer or a saw running together - or just a machine and a big fan heater.
One 32A socket - if you ever get into welding or buy something with a 3kW motor
One for spare - or maybe to a bunch of 13A sockets to power the battery chargers and stuff that you want to leave on while you isolate all the other sockets as you leave.

16A cable in plastic conduit - I can just imagine arctic flex inside sticky backed mini trunking that he's bought from B&Q . Amateur hour !
 

Spectric

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No one has asked where this new distribution board will be supplied from or talked about a supply cable, maybe this guy is going to use that artic flex as an extension and just plug it in somewhere or maybe take it off as a spur.

Has anyone noticed badly formed screws in electrical connectors lately, I have had two different items and both had wire retention screws that could not be tightened because the head would not accept any form of screwdriver.
 

thikone

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I don`t have wall switches on my 16 amp blue sockets but they are on individual type C mcb`s in the main consumer unit.

Ollie
I'll second that! Don't forget to use type C for inductive load, like a motor. It should be written "C 16A" on the miniature circuit breaker, not "B 16A".
 

tdsoldgit

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Hi, I don't often contribute to discussions, but I wonder if my experience might be useful.
If I understand correctly, the fundamental problem seems to be the high 'in-rush' current is tripping the breaker.
Mini lecture: When a motor is stationary it produces zero 'back emf'. It is just a low reactance load. As it starts to spin, the back emf (which increases with rotational speed) opposes the applied voltage so the supply needs to supply much less current as the speed increases.
I had this problem when I hooked a vacuum cleaner based extraction system to my band-saw. Adding the vacuum cleaner caused my breaker to trip on start up. I needed to reduce the start-up current surge. The solution I came up with was to use a Positive Temperature Coefficient (PTC) thermistor in series with the vacuum cleaner motor. PTC thermistors have a relatively high resistance when cold, but warm themselves up and when warm have little (= negligible) resistance. Its a bit like turning the voltage to the vacuum cleaner up (quickly, but not too quickly, to give the bandsaw time to get started - a matter of hundreds of milliseconds).
Ametherm make them, and have an excellent website with formulae etc, but I just sent them the motor data and they told me which one to use. (I tried to scrounge free samples, but they wanted something vast like $100 shipping). I got mine from RS.
Apologies if I'm telling you stuff you all know. I tend to feel a bit awestruck but the knowledge of everyone on this site, which I find absolutely brilliant.
 

gcusick

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Hi, I don't often contribute to discussions, but I wonder if my experience might be useful.
If I understand correctly, the fundamental problem seems to be the high 'in-rush' current is tripping the breaker.
Mini lecture: When a motor is stationary it produces zero 'back emf'. It is just a low reactance load. As it starts to spin, the back emf (which increases with rotational speed) opposes the applied voltage so the supply needs to supply much less current as the speed increases.
I had this problem when I hooked a vacuum cleaner based extraction system to my band-saw. Adding the vacuum cleaner caused my breaker to trip on start up. I needed to reduce the start-up current surge. The solution I came up with was to use a Positive Temperature Coefficient (PTC) thermistor in series with the vacuum cleaner motor. PTC thermistors have a relatively high resistance when cold, but warm themselves up and when warm have little (= negligible) resistance. Its a bit like turning the voltage to the vacuum cleaner up (quickly, but not too quickly, to give the bandsaw time to get started - a matter of hundreds of milliseconds).
Ametherm make them, and have an excellent website with formulae etc, but I just sent them the motor data and they told me which one to use. (I tried to scrounge free samples, but they wanted something vast like $100 shipping). I got mine from RS.
Apologies if I'm telling you stuff you all know. I tend to feel a bit awestruck but the knowledge of everyone on this site, which I find absolutely brilliant.
Spot on about inrush current. In the case you mention, I suspect that the problem that the combined inrush current of the bandsaw and vacuum cleaner were sufficient to trip the circuit breaker. An alternative approach in this setting is simply to delay the switch-on of one of the two devices.

There are other means of achieving soft-start. For brush-type motors (such as vacuum cleaners), a triac soft-start module can be used. There was a length thread on the forum about using these in mitre and table saws. For induction motors, there are soft-start controllers (our heat-pump uses one), but they’re pretty expensive.

One final point: the thermistor you are using must have a Negative temperature coefficient. The resistance of a PTC device would increase as it heats up, which is not what you want at all!
 

Spectric

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Don't forget to use type C for i
and don't forget you cannot just swap out a B for a C, you need to ensure that the loop impedance is low enough to handle the fault current of the type C . For a 16A type B is 48 to 80 amps whereas a type C which trips at 5 to 10 times the current will be 80 to 160 amps. This is where 2.5 mm twin and earth can cause problems because it only has a 1.5mm cpc compared to running singles where it can be 2.5mm.
 

thikone

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and don't forget you cannot just swap out a B for a C, you need to ensure that the loop impedance is low enough to handle the fault current of the type C . For a 16A type B is 48 to 80 amps whereas a type C which trips at 5 to 10 times the current will be 80 to 160 amps. This is where 2.5 mm twin and earth can cause problems because it only has a 1.5mm cpc compared to running singles where it can be 2.5mm.
In my case, I have 20 m of 4 mm² 5 cores wire on C 16A breaker. That is below 200 mOhm. Also have some WAGO connectors along the way, each probably adding below 5 mOhm. CE 400 V socket and plug might add some too. However, maximum is 1370 mOhm in some yellow book, so should be fine.

In another place, I have like 10 m of 1.5 mm² 3 cores wire, which amounts to about the same resistance, below 200 mOhm.

Do you actually measure it somehow? I can disconnect it but still scary to stick Ohmmeter there. Or maybe test it with short circute? :p
 

pe2dave

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and don't forget you cannot just swap out a B for a C, you need to ensure that the loop impedance is low enough to handle the fault current of the type C . For a 16A type B is 48 to 80 amps whereas a type C which trips at 5 to 10 times the current will be 80 to 160 amps. This is where 2.5 mm twin and earth can cause problems because it only has a 1.5mm cpc compared to running singles where it can be 2.5mm.
Thanks @Spectric - complete the picture please, what's a type A trip value? just the 'fuse' value?
 

Sideways

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Do you actually measure it somehow? I can disconnect it but still scary to stick Ohmmeter there. Or maybe test it with short circute? :p
A proper multifunction installation tester can apply a controlled load directly between live and neutral and live and earth at the main isolator switch. It measures the loop impedances of the supply directly as well as calculating Potential Fault and Potential Earth Fault Currents which will be hundreds of Amps.
The local portion of the loop impedance is measured with the supply isolated using a low impedance ohmmeter that reads accurately down to 0.05 Ohms or better.
Circuit by circuit you add them up and do the maths for both L to N and L to E.
If there's a garage distribution board, you have the additional step of checking loop impedances at the DB and then measuring circuits from there.

These aren't typical DIY measurements that can be done with a normal multimeter. Those measure at most 10A of current and don't have the resolution and accuracy to measure very small resistances. A multifunction tester or set of separate installation meters cost some hundreds £ and plenty are over £1000.

Do you test it ? Yes. Every circuit must be measured like this and written up when it's installed, and sample tested again on least a good proportion of the circuits during periodic inspections. It's part of what you pay an electrician for.
The measurements are taken to ensure that enough current will flow in case of a live-neutral short or a live-earth short, so that the breaker protecting the circuit will trip inside the required time.

Also, the impedance of the local wiring used to work out how many volts will lost across the wiring at full load. This is to ensure that the wiring meets the minimum voltage requirement.

Changing the MCB type as Spectric says changes the current needed to trip it in time, so might require thicker wiring.

T&E has a thinner Earth conductor than it's live and neutral, so it might have a low enough impedance to be safe for a LN short but it could be unsafe in case of a LE short.


Changing the breaker type invalidates all the measurements and calcs done when the circuit was installed. It's notifiable work which few people apart from those qualified to do it will know how to do properly and have the test gear to prove it.
 
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Spectric

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A proper multifunction installation tester can apply a controlled load directly between live and neutral and live and earth at the main isolator switch. It measures the loop impedances of the supply directly as well as calculating Potential Fault and Potential Earth Fault Currents which will be hundreds of Amps.
This is why many people can do the physical wiring and installation but without the test equipment they cannot state the important information that qualifies that installation as being safe and as sideways has said this equipment is not cheap, it can get very expensive once you start talking three phase systems where you will easily spend upwards of three grand on decent kit from the likes of metrel, it is not just the tester but also all the adaptors and accessories where some are two grand on there own. With single phase the price is more reasonable, you can get one that will do all the test required for around a grand and this one from Fluke is good https://instrotech.com/fluke-1664fc...MIhZrA0bGv-QIVB7TtCh1j-wiLEAYYBCABEgLwGPD_BwE

Once upon a time we used low ohm meters to measure the R1 & R2 values on a dead circuit and then do some calculations to determine the potential fault currents along with wind up mega testers for insulation testing, now it is just connect to a live circuit and push a button, the tester runs the test and can print the results so you don't even have to fill in the certs, they have it easy these days if you also add in cordless drills, conduit benders and threading machines.
 

thikone

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By the way, does it mean that B 32A is basically the same as C 16A?

I would hope that even C 16A would eventually trip if there is current exceeds 16A for some time...
 

Sideways

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To understand it, you need to read about "time-current" or "trip" curves of MCBs.
This is the first site that would load for me tonight and it explains it well enough.

Over a period of say 10 minutes plus, a 16A breaker is a 16A breaker.
Over a period of 10 sec or less, a B type breaker will pass 3 to 5x rated current before it trips. A 'C' type breaker will pass 5 to 10x rated current before it lets go.
An AC induction motor has a big startup surge. 5x, 7x full load current, sometimes more but for only a few seconds. Depending how closely the full load current of your load is to the 16A or 32A rating of a blue socket, you may get away with a type B breaker or may need a type C to let the startup surge pass without tripping.

Note that MCB's are actually more sensitive than fuses. A UK plug top fuse meeting the British Standard will carry at least 25A and as much as 55A for 10 seconds before it melts.
In my experience you can run a 1.5 to 1.6kW motor off a 13A plug without blowing fuses.
Bigger motors will blow the fuse immediately or perhaps after a few start stop cycles, but because of that wide tolerance sometimes you'll chance on a tough fuse that will hang in there for a long time.

In designing electrical circuits and sizing the conductors, if you decide to protect a circuit with a fuse rather than an MCB you include a factor for the additional time and current needed to blow a fuse. This may force you to select thicker wires.
 
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