Do I really need a 20amp supply?

[RogerM]

I'm thinking of buying a bandsaw that Axminster says requires a 16amp supply. This is the Axminster Plus AP4300T/1 which is rated at 1500W (6amps), or the Jet JWBS-16 MK2 rated at 1900W input (8 amps), 1100w output, for which Axminster recommends a 20 amp supply. The only other piece of kit that I will have running at the same time is the dust extractor at 750W (3 amps). I am familiar with the concept of a greater current being required on start up until the motor is turning to create a back emf. The bandsaw will be the only piece of kit requiring an uprated supply as I run my t/s and p/t successfully off the 13amp ring.

I am a simple person! The garage has it's own dedicated 13amp ring running from an old style (30 years old) consumer unit (also in the garage) with fuses rather than MCBs. As the ring cable is rated to take 13 amp, that must mean that as a ring it could handle twice that at the socket. So arithmetically at least, surely all I need do is add a piece of 5amp and 2amp fuse wire in the same holder as the existing 13amp, and hey presto - I have a system that would take a transitory load of 20 amp before settling down to around 9 amps continuous. Hey - I might even put my hand in my pocket and buy some 20amp fuse wire.

Or if that doesn't suit, how about wiring in to the immersion heater supply which we never use with a new suitably sized circuit?

Please note this idea comes from one who at school 40 years ago instigated an experiment that involved wrapping silver foil across the terminals of a 3 pin plug with the winner being the one who created the longest soot mark up the wall without putting all the lights out. I came in a close second behind a lad who went on to become an electrical engineer. And a very fine one!

 

[woodbloke]

Roger, yes I'm afeard so, for that Jet b/s (which is the one I'm interested in) you will need a 20A supply (confirmed by the technoweenies at Axminster) The reason is that when the cast iron wheels spin up to begin with, it generates a 'blip' in the mains, which then settles down once it gets going.

However, all is not not lost. On Bob9fingers advice I recently changed the old Volex CU in my 'shop (with 16A ring main and 6A lighting type B breakers) for a Wylex 8 way (from screwfix) CU complete with Wylex 16A and 6A breakers (as before) plus the addition of two 32A type 'C' breakers, one for the bandsaw and the other for the p/t. Cost was around £70 or so for the new CU and breakers and took around an hour to fit in the 'shop - Rob

 

[Mike.C]

Roger I have the Axminster Plus AP4300T/1 (brilliant piece of kit, and so quiet) and it certainly needs a 16amp circuit.

IMHO as Rob says you really do need a new CU, one that comes with a RCD and allows you to do a spilt load. This will let you to put everything except the lights on the RCD side, which means that if the system is tripped you are not left in the dark.

Cheers

Mike

 

[wobblycogs]

I've just been through exactly the same thought processes with my Jet P/T which states it needs a 16A connection. After blowing a number of 13A fuses trying to get it to run off a regular plug I wired it up properly to a spare 32A Type B breaker (I'm going to get a 20A Type C when I get a chance) and I've not had any problems since.

I'm curious about your ring main though, I'm guessing its pretty old from the figures you give. A modern ring would be on 2.5mm csa cable which will handle 20A giving a safe maximum ring load of 32A. What you could probably do is wire a 20A junction box into the ring and spur a 16A or 20A socket off that. If your junction box is a of the re-wireable variety though I'd recommend replacing it with a new 17Ed one which shouldn't cost much and will be much safer over all. Note, I'm not a sparky, wire at your own risk

Edited to correct typo of 1.5 mm to 2.5mm.

 

[jhwbigley]

These are the thickness wire are used in my instalations

32 amp Ring main 2.5 mm

16 amp on 4mm

32 amp on 6mm

 

[RogerM]

Interesting. I've just been ferreting about around the fuse box and 13amp sockets. I have a 2.5mm ring feeding two double 13amp sockets, one of which is in a position where it is never used. This is just for the garage. The fuse holder shows 240volt 30 amp. Not sure whether it is in fact 30 amp fuse wire in there as I've never blown it.

Could this be as simple as taking a spur off one of the existing sockets (or a junction box) using 4mm cable to a new 16amp socket with an RCD? From a pragmatic standpoint I'm struggling to see how this would do any harm.

 

[wobblycogs]

I'm pretty sure you would struggle to spur 4mm cable from one of the existing sockets as it would be too tight. It's hard enough getting an additional bit of 2.5mm in there for a regular spur. A 30A junction box from screwfix is 71p (a 20A is 101p - more really is less) so it's hardly going to break the bank and is a neater solution.

An RCD socket would be a good investment but you really should consider replacing the main fuse box so the whole place is protected. It sounds like your power requirements are pretty modest so something like the Volex 2 Way RCD CU @ £32 would probably cover your needs (although it doesn't provide the benefit of split load obviously)

Ideally you want a friendly sparky to bring his testing kit round to check the RCD. Occasionally the earth connection on old installs isn't terribly good and occasionally it can result in an RCD that doesn't trip in the required time. If you were closer I'd check it for you.

Edit, just noticed MK do a 2 way garage unit for just £31. The MK stuff is generally nice to work with too.

 

[woodbloke]

the Volex 2 Way RCD CU @ £32 would probably cover your needs (although it doesn't provide the benefit of split load obviously)

Should you wish to Roger, my Volex CU plus all the breakers is available for the exchange of a moderate amount of vinovouchers. It's the one shown here. Either post or collect from Salisbury if you fancy a run out one day (could give you a demo on veneering if you fancy?) PM if interested - Rob

 

[RogerS]

No-one else has mentioned it yet but Part P?

 

[wobblycogs]

If you wire it as a new 20A circuit you would need to get it checked and signed off. A spur from an existing ring is a (oddly enough) a permitted modification but I'd have to check the regs to be sure about this case as I have a niggling feeling that the spur has to be to a single, fused, 13A socket.

Obviously replacing the CU would very much come under part p.

Having said that, it's not rocket science...

 

[RogerM]

Should you wish to Roger, my Volex CU plus all the breakers is available for the exchange of a moderate amount of vinovouchers. It's the one shown here. Either post or collect from Salisbury if you fancy a run out one day (could give you a demo on veneering if you fancy?) PM if interested - Rob

Aaaggghhh! No! Travelling from Plymouth, Axminster would be on the way! Get thee behind me temptation! PM to follow.

 

[tjwoodms]

all motors will draw approx 3 times the current stated on the plate on start up and the settle down once up to speed ususaly about 2/3 of that stated on the plate when running with no load.

 

[OLD]

I have a ring cct. in the garage that has a 20 amp fuse and feeds the table saw (blue plug and socket unfused) and very little else, works fine for me. The protection is there for a 16amp m/c and anything plugged in to the ring is protected at 13amp or less to a max combined load of 20amps so the cct is running at a lower load than originally designed for.

 

[andycktm]

I'm not an electrician,but i think people are getting carried away with the 32amp breakers.
You've got a motor that requires say 8 amps running(forget startup for a minute)
When its running where's the protection if your allowing it 32amps anytime.
If it can only ever handle 18 amps say for startup and it decided to take 20amps due to say a fault and you've a 32 amp trip on.......

 

[wobblycogs]

I suspect you are reffering to the fact I have my P/T on a 32A breaker at present. I fully agree it should be on a 20A Type C as 20A is what the cable is rated at but I didn't have such a beast to hand so I stuck it on a 32A Type B temporarily.

Anyway, if your motor draws 8A while running the initial draw for a few fractions of a second will be much greater than this. As I understand it, in theory at least motors draw infinite current at zero RPM and the current draw drops as the motor spins up to it's working speed at which point you have it's base load. Since their will be a heavy draw for a short while at spin up you put motors on either a Type C or D breaker, both of which will handle a higher instantaneous tripping current (e.g for type c it's 5 to 10 times it's rated current) and generally will have a longer trip time than Type B.

As for using a 32A breaker in general it's probably not as bad as it appears as long as you are sensible about not overloading the cable (assuming it's 4mm). The fuse / breaker is there to protect the cable from over heating and burning the house down due to excessive current draw not to protect the machine from a fault.

If there is a short curcuit in the machine, for example, the PFC can easily be in the kA range for a few milliseconds till the breaker trips or a bit longer before the fuse blows (fuses are generally very slow). This sort of short term heavy draw wouldn't be enough to damage the cable. You could probably contrive a situation where there was a fault in the machine that caused a long term draw between 20 and 32A but I think we are talking highly unlikley.

 

[woodbloke]

to take 20amps due to say a fault and you've a 32 amp trip on.......

Axminster told me that the 32A type 'C' breaker would be fine with the Jet. The other thing is that there's no 20A type 'C' breaker sold by Screwfix...32A is the closest - Rob

 

[wobblycogs]

You might have to shop around a bit for a Type C breaker that fits your CU as they aren't as common as Type B. I'd be really surprised if you couldn't get one for a modern CU though. A 32A Type C will work but really you shouldn't have a breaker reated higher than your cables maximum safe limit if you can avoid it.

Having said that... when I moved into our current house I found a piece of 1mm lighting cable running a 3kW immersion heater. The cable reached about 50 to 60 deg C after an hour or so of being on - just touchable. Although this sounds hot it's actually still under the specified cable limit of 70 deg C for regular cable. Yes, I would be terrified if any of cable reached anything like 70degC and I changed that circuit straight away. For reference it was suface mounted so had passivle air cooling which is probably why it hadn't melted.

 

[wobblycogs]

I suspect this is what you are looking for Rob:

Wylex 20A Type C £5.80

http://www.neweysonline.co.uk/wylex...400v-ac/1050154415/ProductInformation.raction

 

[ondablade]

I've had some experience with stock 3pin 13A plugs which suggests that maybe it's not just about the cable and breakers - that plugs too don't seem to have a lot of spare capacity over their nominal rating.

In my innocence i ran my 2.2kW Robland combo saw off a 13A socket. It should draw around 10A when running. i.e. well below the rating of the socket.

Despite this it was the plugs/sockets (it happened in more than one plug socket combination) that caused me some bother.

At one level all was OK - it blew the stock 13A fuse once or twice under sustained load, no doubt because i forced it too heavily. (typically in rip cuts i think it was) On the other hand it didn't pop the 16A fuse fitted in the control panel. Perfectly as it should be you might say.

More ominously though it occasionally stopped during start up - it'd pop the reset on the contactor (?) in the saw's own panel. During the initial phase of start up before releasing the button in run mode i think it was.

Eventually i figured out that this seemed to be the result of some sort of minor arcing inside the socket - there were burn marks on one of the plug pins. A new socket and plug solved the problem for short while i ran the machine before selling it after this.

It may to some degree have a been a vagary of the specific sockets i had, but it brings two points to mind:

1. Despite 10A being the nominal full load current, the short duration start up surge seemed well able to cause problems on a 13A socket - without blowing fuses.

2. It suggests that 2.2kW on a saw motor is getting a bit above the upper limit for a 13A socket - regardless of whatever wiring is used.

I'm wiring the shop now for my Hammer machines. The conclusion i've drawn from the experience is that it seems to be as important to build some slack into the sizing of the plus and sockets as it is in the rest of the circuitry - that electrical componentry maybe does not have a great deal of spare capacity...

 

[wobblycogs]

Very interesting Ian. Personally, I would wire in a 16A or higher circuit for anything that drew 10+ amps a long period. A 13A socket should be able to cope but like you I've found that you can occasionally get some arcing although I've only found that with cheap plugs and sockets. We didn't have central heating for a while so were using 2kW fan heaters - the plugs on some got quite warm due to the contact resistance.