13A supply to 16A (3.2kW) machine. It works fine, but is it illegal?

Not "illegal", but potentially unwise.

What is the machine, and for how long is it typically used?

Dena Thompson said:

but it has been suggested this is illegal

Forgetting BS 7671 for a moment, if any safety incident occurs, which could range from burns to damage to electrical equipment causing a fire or shock, if this is in a workplace, it could be considered as a potential breach of legislation ... i.e. 'illegal'. Worth having a look at, in particular the Electricity at Work Regulations:

Regulation 5: Strength and capability of electrical equipment

Regulation 11: Means of protecting from excess current

In terms of BS 7671, in my opinion, it's not conformant if you are using 13 A accessories, or a 13 A overcurrent protective device, to supply the equipment. Regulation 132.2.2 in particular:

133.2.2 Current
Electrical equipment shall be selected with respect to the maximum steady current (rms value for AC) which it has to carry in normal service and with respect to the current likely to be carried in abnormal conditions and the period (e.g. operating time of protective devices, if any) during which it may be expected to flow.

If you are supplying through a BS 1363 series plug and socket-outlet combination, or any type of connection unit to BS 1363-4, it's worth remembering that the likelihood of damage from overheating, particularly around the fuse, increases as you go above 13 A load current. BS 1362 fuses are required to have a maximum power dissipation of 1 W at their rated current. A 'calibrated test link', that is designed to dissipate 1 W, is used in temperature rise tests, during type testing in the relevant parts of the BS 1363 series of standards.In addition, the  conditions of use in BS 1363-series should be considered.

For example, it is common to see BS 1363 plugs overheating around the fuse, and externally the line pin  of the plug (which is close to the fuse) if used with welding equipment with peak input current of 14-15 A, even if this is short-term.

So, what to do?

If there is no evidence of overheating at the moment, recommend any 13 A equipment is checked regularly (if possible, by users) for signs of overheating.

Recommend, if possible, connection by an appropriate-rated isolator, connection unit, or socket-outlet ... if a workplace, for example, BS EN 60309-2 or BS EN 60309-4 socket-outlet ('commando socket') could be used. Even if this requires a new circuit, it's probably still going to be in the ballpark of 'reasonably practicable' ?

Chris Pearson said:

Not "illegal",

Are we sure about that? It does depend on whether the machinery actually takes that much power, but if it does and something happens ... see above, Electricity at Work Regulations.

Agree - its not illegal in the sense the police will break down your door.

Nor is it against the wiring regs. 

But it may be the sort of thing that insurance companies might decide to point to if there is a fire, to justify not paying in full. In a place of work it may be seen as endangering the workforce.

Things like ovens and hot plates are a classic for this  - where the sum of all the heaters greatly exceeds the supply rating, but so long the situation can be avoided where it is all switched on at once for a cold start, in practice it runs without issues.

note that 13A *230V is 2.99kW and 13A * 240V is 3.12kW.  Look hard at that plate - what is the voltage is has been tested at ?

3.2kW is not really a full fat 16A - that is much closer to 3.7 (230V) or 3.8kW (240V).

You may be overloading things, but not by an amount likely to be any more dangerous than full load. And  I say it like that because there are plenty of designs of 13A plugs, and even more 13A extension leads, that actually run very warm at 13A and don't take much in the way of restricted ventilation or a warmer than normal  environment, to tip them over the edge at currents below the nominal  13A anyway.

This example of a 13A plug and socket were on a ~ 3kW heated catering service counter - the fuse did not blow, but as you can see, it ran 'really quite warm'. My warning is that there really is not a lot of slack in some examples of nominally '13A' rated hardware.

Mike.

Thankyou so much for your prompt answer!

BS 7671 does have a requirement to ensure that "small overloads of long duration" are unlikely to occur (433.1). It's not clear whether you actually have a (small) overload in this case - 3.2kW might be just a rounded up version of 13A * 240V, or the way the machine is used may mean it never draws the full load for any significant time (but that could change of course).

  - Andy.

13A supply to a 16A machine (kitchen cooker could be an example)

Points to consider

A dedicated circuit rated appropriately with a 4mm2 or 6mm2 CSA dependent on VD (Volt Drop) and clipping reference

A dedicated 16a or 20amp RCBO type A or higher certainly not type AC

Use a 16amp commando rather than a 13amp BS 1363 plug

Case in point for when a 16A unit is served by a 13amp would an immersion heater in boiler cylinder or electric shower the FCU generally tend to show thermal damage after prolonged use.  

Now the question is do you need a regulation or the use of engineering judgement?  Other points to consider would be the energy efficiency of an install if it is operating in the contingency zone. As we know a 13amp plugtop fuse will run at 16amp comfortably for hours.   The heat of the cable will make it less energy efficient.  The risk potential for an incident or fire are greatly increased.

Sergio Fernandez said:

As we know a 13amp plugtop fuse will run at 16amp comfortably for hours.   

The fuse might not operate ... but that doesn't mean the temperature in the fuse holder is classed as 'comfortable'. In addition, surface temperatures of equipment housing the fuse may well exceed the limits of Table 42.1 of BS 7671 and/or the product standard (see Regulation 423.1).

gkenyon said:

Are we sure about that?

I do not think that there is anything intrinsically unlawful about drawing 3.2 kW peak through a 13 A plug (and socket). After all, BS1363 (Parts 1 and 2) specifies a test current of 14 A for temperature rise tests.

That said, there is insufficient information to decide how safe the arrangement is.

Chris Pearson said:

After all, BS1363 (Parts 1 and 2) specifies a test current of 14 A for temperature rise tests.

Sadly, the 14 A is a type test, and is for specified conditions for the temperature rise test only.

The actual rating of the plug, fuse and socket-outlet is 13 A, NOT 14 A.

What's worse, is, yes there's is actually some tolerance built into the assumptions about the 13 A rating ... BUT product standards (e.g. BS EN 60335 series) permit the actual rating of a product at nominal voltage to have some tolerance ... in the case of BS EN 60335 series, this can be up to 10 % at nominal voltage (and in fact for immersion heaters it actually is 10 %).

Chris Pearson said:

3.2 kW peak

I think we mean '3.2 kW maximum' rather than 'peak' ... power, currents etc. are typically quoted rms according to the product and installation safety standards.

When you consider that the supply has a 10 % tolerance too, with linear loads you can have 10 % on top of 10 % ... which is actually 1.21 % more current/power.

So:

1. Perhaps in reality the 13 A socket-outlet might be OK with more than 13 A, perhaps creeping higher by over an Ampere for very short periods ... but I wouldn't calculate that current at 230 V, it's natural tolerance built into the standards; and
   
   
2. with a plate rating of 3.2 kW at 230 V, expect up to 16.8 A in use at 253 V supply, or a plate rating of 3.2 kW at 240 V, expect up to 15.5 A (linear load) at 253 V supply. This is a lot higher !

In my opinion, to try and use these tolerances to justify 'overloading' is not responsible, and quite possibly leads to the sorts of overload and equipment damage we see in the real world, as Mike alluded to earlier with some great illustrative pics.

Remember ... you might just be overloading, on a high supply voltage, with a positive tolerance on the power/load current also!