# Does this need a 16 amp supply?



## stuartpaul (13 Sep 2010)

Just purchased one of these:

http://www.startrite.co.uk/index.php?se ... f=Bandsaw#

Click on the specification tab on the left.

Came with a 16 amp plug on the end and this caused me minor palpitations as I suspect my supply probably isn't up to it. No previous mention in any of the blurb that it might need 16 amp.

Put a 13 amp plug on the end and it ran up OK but no blade fitted yet (that's another story :evil: ) so not the mass of both wheels to get moving.

Any views?


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## 9fingers (13 Sep 2010)

Probably borderline and could depend on the voltage of your supply and cable length from the consumer unit. (longer lead and or lower voltage will need more current hence risk of blowing a 13 amp fuse.
In colder weather it will draw more power too.

If you have a properly installed 13 amp ring in your workshop (if not why not ?:lol: ) This should be fused at 30 amps
There is no reason not to install a 16 amp socket protected by a 16amp MCB from your workshop CU.

If you are running on a glorified extension lead then now is the time to do a proper install. Nothing to stop you doing a diy install all but the final connection which should be done by a spark who can test/inspect the rest.

hth

Bob


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## dannykaye (13 Sep 2010)

its a 2 hp motor 1 hp = 750 w or about 3a if the load is balanced and it will be so a 13a plug should be fine. motor current goes up under load and if you jam the blade it will peak but a 2hp motor should never take near 13a


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## 9fingers (13 Sep 2010)

dannykaye":1fzckvyd said:


> its a 2 hp motor 1 hp = 750 w or about 3a if the load is balanced and it will be so a 13a plug should be fine. motor current goes up under load and if you jam the blade it will peak but a 2hp motor should never take near 13a



Sorry Danny, Ohms law (which you have used above) does not apply to reactive loads such as motors. The peak starting current could well exceed 13 amps and if the motor does not get up to speed quickly then the plug fuse could blow.

However, once running, it is exceptionally unlikely to blow a fuse. 

I have a Jet planer with a 1.5hp motor and that will blow fuses on startup on about 1 in 3 attempts.


Bob


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## stuartpaul (13 Sep 2010)

9fingers":8iaimupz said:


> Probably borderline and could depend on the voltage of your supply and cable length from the consumer unit. (longer lead and or lower voltage will need more current hence risk of blowing a 13 amp fuse.
> In colder weather it will draw more power too.
> 
> If you have a properly installed 13 amp ring in your workshop (if not why not ?:lol: ) This should be fused at 30 amps
> ...



Thanks Bob,

By 'borderline' do you mean that it's just a case of it might or it might not? And if it does it might one day just decide not too?

I'm close to the house and the CU isn't far from the saw so I might be lucky (yeah right!).

Guess I'll have to fit a blade and see what happens. If not then a 16 amp supply will have to be fitted. 

I'm left feeling a bit of a plonker with £800 quids worth of saw and not knowing if it will work or not. I still feel that they should make the point in their material (like Axminster do) but perhaps I should have asked?


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## 9fingers (13 Sep 2010)

stuartpaul":uba1clbt said:


> By 'borderline' do you mean that it's just a case of it might or it might not? And if it does it might one day just decide not too?
> 
> I'm close to the house and the CU isn't far from the saw so I might be lucky (yeah right!).
> 
> ...



Yes that is exactly what I mean.

The fact that they supply it with a 16amp plug should be enough of a clue. As I say your supply voltage might be high enough for it to be OK. On another day you might find the voltage lower, the machine colder and the fuse will pop before it gets going.
Try it and see - you may be lucky but make sure you have a few spare fuses handy. To be for certain sure, you know what to do.

Bob


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## PeterSk (13 Sep 2010)

As much as anything seemingly "random" fuse blowing by kit with big motors is directly related to whereabouts in the mains AC cycle you are when you throw the switch.

If you hit the switch when the voltage is swinging up, then all is good. The power available to the motor is increasing as it tries to spin up, so once the initial inertial load is dealt with everything goes swimmingly.

However, if you switch on the motor on the downswing, the power available is dropping. As the voltage drops, the current draw of the motor will increase, and this is compounded by the fact that if the load does not start spinning pretty quickly then the motor will find it very hard to overcome the inertia and start spinning. Motors at 0rpm are essentially short circuits, so the fuse blows.

I first came across this when working on high power amplifiers, the power draw of the capacitors in the power supply behaves very similarly to a motor.


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## stuartpaul (14 Sep 2010)

Startrite's response to my concern:

'The 16 amp should not be a problem on start it will blow a 13 amp fuse but will run below 10 amp, you should just be able to replace your 3 pin socket with a blue 16 amp'

Not sure it's a solution, - would just appear to bypass the fuse issue.

Any thoughts from those with electrical knowledge?


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## 9fingers (14 Sep 2010)

My view as a non qualified spark is as follows
1)Fuses/MCBs are there to protect the cabling not the appliance.
2)The existing ring cable will be unchanged by fitting a 16amp socket in place of (or in addition to ) an existing 13amp (or double 13amp) socket. Therefore the cable protection is unchanged by the change.

The bandsaw will have protection in the NVR switch to protect the motor and the cable supplied with the saw will have been selected for the purpose.

So all is hunky dory? BUT there is no control over what else might get plugged into the 16amp socket by you or someone else in the future and that load might not be protected fully. My suspicion is that the regulations will frown upon this and so prevent the fitting of a 16amp socket on a 30amp ring.

I would be very interested in the views of a professional spark.

Bob


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## Mike.C (14 Sep 2010)

Stuart I hope that you do not mind me hyjacking this thread for a moment. Bob why don't 16 amp plugs have fuses within the plug like a 13 amp do? In this case are my machines only protected by my mcb's?

Cheers

Mike


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## Stoday (14 Sep 2010)

stuartpaul":ifmts75w said:


> Startrite's response to my concern:
> Any thoughts from those with electrical knowledge?


Yeah, that's the question to ask. I can't stop laughing at the electrical "explanations" so far in this thread.

"Probably borderline and could depend on the voltage of your supply and cable length from the consumer unit. (longer lead and or lower voltage will need more current hence risk of blowing a 13 amp fuse)" :roll: CEGB must have got it wrong then, when, in the days of power shortages, it reduced the voltage to reduce consumption. :lol: 

"If you hit the switch when the voltage is swinging up, then all is good. The power available to the motor is increasing as it tries to spin up, so once the initial inertial load is dealt with everything goes swimmingly" :roll: :roll: That motor's gotta spin up to speed in less than 10 milliseconds because the voltage will then reverse and swing down. Whoops! :lol: 

Oh — the answer to stuartpaul's question: Just stick a 13A plug in place of the 16A and you'll be OK. 

The starting current transient of an induction motor started direct on line is around 6 times the full load current. So for a 2HP motor, FLC ~ 8.1A (note this takes account of a power factor below unity) and starting transient ~ 50A. This is OK for a 13A plug because the fuse in it is made to BS1362. The specification of the fusing characteristics is that it may not blow with a load of 30A in less than 1 second and that it must blow in less than 400 seconds. Correspondingly, for 50A, between 0.1 and 20 S; 100A between 0.01 and 0.2 S.

I'd expect the starting transient of your motor, starting off-load, to be below the fusing current characteristic of a 13A BS1362 fuse. My tablesaw's 2kW motor runs off a 13A plug without ever having blown the fuse.


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## 9fingers (14 Sep 2010)

Mike.C":25jql0wo said:


> Stuart I hope that you do not mind me hyjacking this thread for a moment. Bob why don't 16amp plugs have fuses within the plug like a 13 amp does?
> 
> Cheers
> 
> Mike



Our 13 amp UK plugs are about the only plug in use (Europe and USA at least) that does have a fuse and relates to our use ring wiring.
'Normal' plugs/sockets are wired on radials which is the correct way to fit commando sockets of whatever rating.

Not a very good answer but a bit better than "because they don't" :lol: 
Bob


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## Stoday (14 Sep 2010)

Mike.C":gpzciyuh said:


> why don't 16 amp plugs have fuses within the plug like a 13 amp do? In this case are my machines only protected by my mcb's?


Because 16A sockets are only fitted to a circuit that is protected by a 16A MCB. 13A sockets are fitted to a ring circuit protected by a 32A so must have their own 13A fuse.

It follows that it would be dangerous to connect a 16A socket directly to a ring circuit protected by a 32A fuse, quite apart from it not complying with BS7671 (the wiring regulations). You could connect a fused spur to the ring, and the 16A socket to the fused spur, but as the maximum fuse you can install in the fused spur is 13A, it rather defeats the object...


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## Mike.C (14 Sep 2010)

9fingers":1ayq3qwp said:


> Mike.C":1ayq3qwp said:
> 
> 
> > Stuart I hope that you do not mind me hyjacking this thread for a moment. Bob why don't 16amp plugs have fuses within the plug like a 13 amp does?
> ...



No Bob that's a good enough answer. In other words if 90% of everyone else do not use them then I an happy enough to trust their judgment. What protects it then, just the MCB?

While on the subject. The sparky who installed the CU in my workshop put the 16 amp circuit (along with the lights) on the unprotected side. In other words it is not protected by the large main circuit breaker. I understand why the lights are there but I cannot understand why he did not protect the 16 amp saw?

EDIT: Crossed posts. Has Stoday already answered my question above?

Cheers

Mike


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## 9fingers (14 Sep 2010)

Stoday, 

Please can you tell me what would the wiring regs view of fitting a 32amp socket on a ring main?

Cheers

Bob


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## PeterSk (14 Sep 2010)

Stoday":1j9dr07o said:


> "If you hit the switch when the voltage is swinging up, then all is good. The power available to the motor is increasing as it tries to spin up, so once the initial inertial load is dealt with everything goes swimmingly" :roll: :roll: That motor's gotta spin up to speed in less than 10 milliseconds because the voltage will then reverse and swing down.



You mock but clearly miss the point. It's about the initial starting condition, once the load is moving even at a fraction of full speed the required power to spin it up to full speed drops exponentially.

Plus I (and quite a lot of other people, EEngs among them) have proven this experimentally with power consumption monitors and oscilloscopes. It's even to the point whereby using a generated AC sine wave you can blow 100 fuses in a row by controlling the phase angle on switch on.


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## Stoday (14 Sep 2010)

9fingers":y6oyhjat said:


> Stoday,
> 
> Please can you tell me what would the wiring regs view of fitting a 32amp socket on a ring main?
> 
> ...



It's not permitted. Regulation 433.1.5 says you can only connect accessories that comply with BS1363 to ring circuits. That means only 13A sockets and fuse connector units.

You should connect a 32A socket to a 32A MCB in a consumer unit using 6mm² cable.


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## 9fingers (14 Sep 2010)

OK thanks.

Can I assume that you are a qualified spark? If so it will be useful here because as I often state, I am not qualified or up to speed with regs.

Cheers

Bob


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## Stoday (14 Sep 2010)

PeterSk":1hiw3yjx said:


> You mock but clearly miss the point. It's about the initial starting condition, once the load is moving even at a fraction of full speed the required power to spin it up to full speed drops exponentially.



Yes, I mock, but only because I thought a full explanation a bit esoteric for a workshop forum. Nevertheless, here goes:

The starting transient is made up of two separate components. One is the magnetising current (which is what PeterSk is getting at) and the other is the locked rotor current (which PeterSk has ignored).

*Magnetising current:*
This applies to all sorts of electric machine (particularly motors and transformers) with an iron core that is magnetised by the electricity supply to the machine. Assuming the core to have no residual magnetism, an increasing voltage on the windings will create a magnetic field in the core of increasing flux. As the voltage is alternating, after 5 mS it will reverse and the flux reduced. After another 10 mS the voltage will be at a negative maximum and the magnetic flux will have reversed.

If the device (and particularly a transformer) is switched off when the magnetic flux is not passing through zero, the core will retain some magnetism when the circuit's switched off. Switching on again may occur when it reinforces the residual magnetism. Then the induced magnetic flux may exceed the capability of the core. The core becomes saturated, the magnetism no longer increases, so the constraint on the voltage no longer occurs (i.e. there's no "back e.m.f."). You then get a very short-lived current transient.

*Locked rotor current:*
A squirrel cage induction motor (You'd only expect to find this type of induction motor in a home workshop) will run at nearly its synchronous speed when not loaded. That's normally 3000 or 1500 rpm, (from the mains frequency in cycles per minute — 50 cycles * 60 seconds/min = 3000). As the mechanical load is increased, the speed reduces and a current is induced in the rotor of the motor, which creates torque. This torque would bring the motor to a standstill if it did not get electric power to maintain rotation.

If the motor is overloaded mechanically, the speed will fall (typically below 85% of synchronous speed), the power it consumes will rise and eventually trip the overcurrent protection. 

When the motor starts, the motor's rotor is stationary and so the motor takes a current that's higher than its full load current. This current does not decrease exponentially. It falls to the FLC when the motor has speeded up to its working speed, around 90% of synchronous speed. The inertia of the motor's rotor plus the intertia of the machenery it's powering will slow the speed-up. If it powers up on load, full load speed could take a few seconds during which a starting current, higher than the full load current, must be supplied.

If the circuit supplying the motor isn't adequate to provide the starting transient, the motor may take too long to speed up; the motor takes a high current for longer and causes the circuit protection to operate (I think that's what is happening to one respondant's 1½ HP motor that blows 13A fuses)

I'm sorry if you think this a bit heavy for the forum; if so, just laugh at the "rotor of the motor" quote from "Hello hello". :lol:


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## Stoday (14 Sep 2010)

9fingers":1rk5qokt said:


> Can I assume that you are a qualified spark? If so it will be useful here because as I often state, I am not qualified or up to speed with regs.



Well, er yes and no. I used to design and supervise electrical installations in factories in the 1960's but then moved on, but always in electrical engineering. Finished up as an international consultant until I retired 5 years ago. Used to work in crappy countries – three started wars after I left (Côte D'Ivoire, Lebanon and Pakistan) :shock: Built myself a nice double garage sized workshop for retirement, but have only now got to use it because I had a heart attack and my kidneys failed. For the rest of my life I have to spend 4½ hours in hospital every other day on dialysis.


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## stuartpaul (15 Sep 2010)

So, - let me try and get this right.

Stoady thinks I'll be all right on a 13 amp plug.
9Fingers reckons a 16 amp socket on a 16 amp circuit
Startrite say stick a 16 amp socket on a 13 amp circuit.

Stoady says that what startrite have said is 'dangerous' (and illegal?). I also don't like the idea of doing that so that's out.

Clearly the 13 amp plug appeals and I'll be having a go at that as soon as I get my mits on a blade and can spin the whole lot up.

If that fails then a 16 amp socket/circuit or send the bloody saw back!

I'm still miffed that their advertising material didn't allude to the fact that 16 amp required otherwise I wouldn't have bought the saw!! A fact that so far Startrite have refused to acknowledge.

I'm going to bob around the mediterranean for a couple of weeks so will come back to this on my return.


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## 9fingers (15 Sep 2010)

Stoday":3ufeeqyc said:


> 9fingers":3ufeeqyc said:
> 
> 
> > Can I assume that you are a qualified spark? If so it will be useful here because as I often state, I am not qualified or up to speed with regs.
> ...



That explains your technical contributions and I'm very sorry to hear of your kidney problem. That must a real PITA.

Bob


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## Mike.C (15 Sep 2010)

Mike.C":23lliq7y said:


> 9fingers":23lliq7y said:
> 
> 
> > Mike.C":23lliq7y said:
> ...



Has the sparky done this right?

Cheers

Mike


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## 9fingers (15 Sep 2010)

Depends what you mean by unprotected.
If the 16a socket is truly unprotected then I think it is wrong but you are possibly confusing RCD protection with MCB protection??

Bob


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## Mike.C (15 Sep 2010)

9fingers":3gu5lio3 said:


> Depends what you mean by unprotected.
> If the 16a socket is truly unprotected then I think it is wrong but you are possibly confusing RCD protection with MCB protection??
> 
> Bob



Thanks for the fast reply Bob. Here is a photo of the CU







The 16 amp circuit is where it says table saw (right next to the lights) and as you can see it is not protected by the RCD. It has it's own MCB but not knowing anything about electrics, I have always thought that the greater protection comes from a RCD.
As I have said I understand why the lights are not on the RCD side, because if something happens and the RCD is tripped you have still got lights, but I cannot see why he has put the 16 amp over on that side too.

Of course it could be that the RCD is not as superior as I think it is and the MCB is just as good, but if that if that is the case why would you need RCD's?

Cheers

Mike


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## 9fingers (15 Sep 2010)

MCB gives overcurrent protection and is essential in any circuit to protect downstream wiring not the appliance.

RCD is an 'optional' extra and gives protection against leakages to earth and will indicate a 'leaky circuit or appliance as well as offer protection against shock to users. 

An RCD does not give any overload protection and must be followed by an MCB 

On the downside, RCDs can sometimes suffer from nuisance tripping and maybe your spark opted to wire your saw circuit this way. alternatively, he may just have run out of spare ways on the protected busbar inside the CU?

Just to help maintain confusion, manufacturers are coming out with combined RCD/MCBs for each circuit at high prices to help protect their margins. These get used where the earth loop impedance is not considered low enough to trigger MCBs in the required time.

hth

Bob


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## Mike.C (15 Sep 2010)

Once again Bob thanks for putting my mind at ease.

Cheers

Mike


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## Stoday (19 Sep 2010)

The wiring regulations were updated in 2008 and now require a RCD if the circuit wiring is hidden underneath plaster. 

The regulations are not retrospective, so no need to change existing wiring. However, if you are installing a new circuit and want to comply with the current regulations, you should either run new circuits on the surface, in conduit or use a RCD.


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