My garage has an existing electrical supply via an overhead-slung single cable (2.5mm twin-and-earth) from the house consumer unit: the relevant module is labeled "B32 - NSB32".
Inside the garage this cable feeds a small consumer unit which has a 30mA RCD and two modules: a B-6 and a B-16. The 6A circuit feeds two fluorescent lights, the 16A a single double-socket.
I'd like to add a few more sockets (or have them added by a professional, if necessary) but I wonder if it's also possible to increase the rating of that circuit: at present my chop saw draws too much current and trips it.
I haven't looked closely at the garage unit as yet, but I'm hoping that the modules are removable. Can the B-16 one simply be replaced by one of a higher value?
I would wonder why a chop saw trips a 16A breaker. It may just be too quick and trip with the starting current. I don't recall what type of 16A breaker I have on my outside circuit but my chop saw doesn't trip it even when the extractor vac is running.
Thanks for that: interesting. I can run the chop saw on an extension lead plugged into a domestic socket inside the house with no problems at all. Obviously I don't want to increase the rating of the circuit if it isn't necessary, but on the other hand I do want to be able to use my saw in the garage.
I'll also be using a medium-size metal-turning lathe (currently in storage, so I can't try it) on the same circuit though not, of course, at the same time as the chop saw or anything else. I don't want to run into the same problem with that.
A type B mcb isn't usually much good for motors. Change it for a C16 and you should be ok. The difference is in the tripping curves, the type B trips much faster for the same current. I'd be tempted to change the house end for a C20 too. A 32A mcb is too big to protect a 2.5mm t&e spur. The C16 at the garage end will still trip first and your cable will have better protection. Under ideal conditions a 2.5 cable will carry 27A.
That MCB is too big for that cable. You should have a 20A maximum.
That sounds alright, as diversity means you wont be drawing more than the 20A that the circuit should be protected by.
Trips what? The MCB or the RCD? Inside the house CU or garage CU? Have you got an RCD in the house CU?
Not really. 20A is possible with no other circuits, though I wouldnt advise it. If one item is causing a 16A MCB to trip, then there is either a fault with the MCB, or the item being used. What is the Wattage rating of the saw?
This is generally true - although normally applies more when using induction motors with high inrush currents.
Actually the "curves" for a B and C type breaker are identical. Its the non curved bit of the response (i.e. the magnetic action responsible for handing the much larger fault currents) that differs.
Compare:
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speed of response of both is the same for any most overload scenarios. However the rapid 0.1 to 5 sec response for the C type will tolerate 160A of inrush before tripping compared to the 80A for a B type.
Yes good point - I was not paying attention to the size of the feed to the garage in my previous response. 2.5mm^2 is a tad on the small side for a submain really (although more than adequate for the original intended use of providing for lights and one socket).
The 32A house MCB will provide adequate fault protection for the submain, but not overload protection for it. Hence it will fall to the garage CU to provide this for its supply cable. So the garage CU is going to be limited to a 6 and a 20A MCB maximum.
ok, we will ignore the unsuitability of T&E being slung from centenary wires for the moment...
Also are you sure its 2.5mm^2?
Check the dimensions here:
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single 2.5mm^2 cable would not be adequately overload protected by a
32A MCB (although it would offer adequate fault protection) - this will limit options in the garage at bit.
You can add the sockets yourself. Extending an existing circuit is not a controlled activity, and hence does not come under part P.
What sort of motor does the chop saw have? If its an induction motor (there are a small number of chop saws that do) then its not actually too surprising it trips a 16A breaker, since the inrush current on induction motors is quite high (five to nine times their nominal current draw is often quoted).
If its a "normal" universal motor (these are the normal electric drill style noisy motor - compared to the very quiet and smooth running induction motors common on workshop tools), then its possible that the circuit breaker in the CU is faulty and a tad too sensitive.
Almost certainly - so long as its a MCB that is still available or or least a design that will accept a competing brand if not.
A little will depend on the wiring in the garage. If the circuit is a radial, wired in 2.5mm T&E, then you can replace the breaker with a B20. Alternatively you could add a wire to the end of the radial, and return that to the same breaker in the garage CU so that it forms a ring, and change the breaker for a B32 - however the difficulty there is it sounds like the supply cable to the garage is not really large enough to support this configuration.
As it sounds like you are planning on using the garage as a workshop, it might be worth revisiting the RCD arrangements. The way you currently have it described, the RCD in the garage would if tripped cut both lights and power. This may not be desirable if you are holding a spinning power tool and suddenly have no lights. Assuming the main house CU feed of the garage submain is not RCD protected, then you could change both MCBs in the garage with RCBOs (i.e. MCBs that include their own RCD), and replace the RCD with a normal switch. That way a trip of the power will not kill the lights. A simpler less sophisticated solution might be to add a non maintained emergency light (
Thanks to everyone for the responses. I'll try to collect my answers into one reply.
Alan asked,
It's 1800W.
and,
It's the garage CU which trips, but I don't recall if it's the MCB or the RCD.
Yes, there's an RCD > ...we will ignore the unsuitability of T&E being slung from centenary
Yes, I'm well aware that this is far from ideal, and I plan to change it as soon as possible. I don't think it's going to be practical to bury the cable, but I can at least raise the wire a fair bit (it's not much above head height at present) and run the cable through plastic trunking.
I think I was mistaken. A closer look today reveals this on the cable: BASEC 300/500V 6242Y 4.0. So: 4mm^2, yes?
Ah, excellent.
I don't know, and there's nothing on the spec plate or in the manual to enlighten me. It's a Nutool MSR 250, bought about ten years ago.
It does kick on with an almighty surge, or seems to. And the tripping was instantaneous. But on the other hand...
It's a very noisy motor.
So... given that the feed cable is 4.00mm^2, what's my best course? Replace the B16 with a C16? Or something higher? The garage circuit is a radial in 2.5mm T&E. It would be possible to make it a ring, if that would be better.
Thanks for the thought about lighting. An emergency light will be the easiest arrangement, I think.
ok, that alters things a bit. You need to establish which trips before we can give any useful advice.
If the house RCD protects the garage submain, then no amount of fiddling on the garage will fix that. That really leaves two options; either moving the garage feed to the non RCD side of the CU in the house, or going the emergency lighting route in the garage.
If you use a flexible cable that is designed for the application, then you can do away with trunking. TLC's Hi-Tuf would be an ok choice for this.
Yup sounds like it. Should measure about 6.1 x 11.4 mm overall.
Almost certainly a universal motor then. Less of a problem regarding inrush current, although the nature of these saws can still mean a moderate starting current with any type of motor.
If it is, then you have more flexibility since the MCB on its origin will offer the submain both fault and overload protection. Hence no need to mitigate for that when making choices in the garage.
If you are using this as a workshop, then one 16A circuit may be a bit on the skimpy side - especially if you plan on needing some heating in the winter. I would be tempted to extend the socket wiring and form a ring, and protect that with a B32. That way you can stick in plenty of sockets, and have enough capacity available for heating, lighting, power tool, and dust extraction all concurrently. It may leave you with a discrimination problem - in that a fault or overload in the garage might result in a trip of the house MCB rather than the garage one. Having said this, if its not a long walk to reset that, and you have emergency lighting you may decide this is acceptable. (replacing the MCB in the house with a 32A fuse holder may resolve that)
The other thing to have a bit of think about is the distances involved and hence any voltage drop problems. Again unlikely to be an issue if the garage is not far away. If you are replacing the submain at some point anyway, then you could always go up a size then.
Right, I'll give it a try tomorrow and report back.
Assuming that I can get a compatible B32, that sounds like a good approach. The garage CU seems to be branded SAIP, not a manufacturer I know, or have found so far with Google.
That's the case. There are sockets in the house further away from the main CU than the farthest point of the garage.
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