Commercial dishwasher on a 32amp ring main

bwallum · 18 Aug 2014

I have been looking at a commercial dishwasher attached by a 13amp fused spur to a 32amp rcbo protected ring circuit. The site is a village hall kitchen with a fridge, water heater, 6 double sockets and the commercial dishwasher on the ring.

The dishwasher had not worked for 4 years. I found the 13amp fuse in the spur dead, replaced it, tried the dishwasher and the fuse blew again. I contacted the manufacturer and discovered that it has a 2.8kw water heater and a 735 watt water pump. Both can operate together giving a 3.535 kw load for short periods (20 second bursts, about half a dozen before the washer gets up to temperature in about 40 minutes). My sums say this will generate 15 amps or so hence blown fuses.

I fitted a 20amp dp switch in place of the fused spur. Dishwasher works fine and the rcbo stands up with fridge, water heater, 3kw kettle on as well.

I'm uncomfortable with the loading however because the dishwasher is at one end of the ring. The ring is cabled in 2.5mm T&E.

I'm at a loss as to how best to protect the ring from overload with just the dp switch in place. I'm thinking that I may need to run a radial cable to serve the dishwasher. I would prefer to keep the job simple however.

Am I being over concerned? How can I simply introduce over current protection at say 16amps at the dishwasher?

OwainDIYer · 18 Aug 2014

I believe (although coulnd't cite the regulation to prove it) that a 16A MCB (not fuse) can be connected to a ring final circuit as a point load.

However it should really be on its own circuit. As a village hall then public liability and possibly Electricity At Work Regulations apply; the usage pattern may be higher than it would be for domestic; there may be several kettles or urns on simultaneousnly; and it might easily be replaced in future with an even higher power rated appliance.

Installing a new circuit would be advisable and I'd do it with cable rated 32A to allow for future use. The extra cable cost would be minimal. Check with the mfr if the appliance would need 16A protection on its flex.

JohnW2 · 18 Aug 2014

bwallum said:
The dishwasher had not worked for 4 years. I found the 13amp fuse in the spur dead, replaced it, tried the dishwasher and the fuse blew again. I contacted the manufacturer and discovered that it has a 2.8kw water heater and a 735 watt water pump. Both can operate together giving a 3.535 kw load for short periods (20 second bursts, about half a dozen before the washer gets up to temperature in about 40 minutes). My sums say this will generate 15 amps or so hence blown fuses. ... I fitted a 20amp dp switch in place of the fused spur. Dishwasher works fine ....

I wouldn't expect short bursts (or even long ones, come to that) of 15A to blow a 13A fuse - but the start-up current of the pump might be a lot more than the "735W" rating suggests, so maybe that's what's doing it.

bwallum said:
I'm at a loss as to how best to protect the ring from overload with just the dp switch in place. I'm thinking that I may need to run a radial cable to serve the dishwasher. I would prefer to keep the job simple however. Am I being over concerned? How can I simply introduce over current protection at say 16amps at the dishwasher?

The existing 32A RCBO should give adequate protection to the circuit as a whole, so you don't really need to worry about that. However, you clearly have a load which can be appreciably in excess of 13A (witness the blown fuses), and that shouldn't really be run off a ring. As you say, the 'proper' solution is to run a dedicated radial for the DW, protected by a 16A MCB or RCBO. If, despite that, you did decide to run it off the ring, your only option would be a 16A MCB (in an appropriate enclosure) in the spur feeding to the DW - but, despite what Owain has suggested, I don't think that is really compliant, and certainly not advisable/good practice.

Kind Regards, John

JohnW2 · 18 Aug 2014

OwainDIYer said:
I believe (although coulnd't cite the regulation to prove it) that a 16A MCB (not fuse) can be connected to a ring final circuit as a point load.

I don't think that you'll find anything in BS7671 which says that. Indeed, Appendix 15 generally recommends that fixed loads >2kW (let alone 3.6 kW) should not be run from a ring final. Also, for the pedantic, the regs state only that 'accessories to BS1663' (which obviously doesn't include MCBs!) to be supplied by a ring final, and some argue that means that nothing else (like switches) may be connected to such a circuit!

Kind Regards, John

Spark123 · 20 Aug 2014

Please get an electrician to fit a new circuit for this. Currently you have connected an appliance directly to a 32A circuit with a 20A switch.
The circuit nor switch are suited for this and could cause danger or injury, I certainly would not say it is satisfactory on an EICR.

bwallum · 20 Aug 2014

Two 3kW kettles in a double socket would give a greater load. The sockets are tested (over hours) to give 20amp safe load (14 + 6 from memory) and the 20amp DP switch is similarly rated. Unconventional you might say, have I checked the load down the short leg and the cable fixing to check for over current in the cable, you might also say, but the 3.535kW of the dishwasher would be safer than two 3kW kettles in a double socket. That happens in village halls all too often.

Whilst I can't find anything specific to say that I can't connect the dishwasher to the ring I am mindful of others following on in the future not being aware and assuming the DP switch is on a radial and possibly putting a more powerful machine on it. So...I'll run a 4mm flat T&E from consumer unit to an above counter DP switch on the dishwasher with a 16amp rcbo for protection.

This has been a very informative chat for me, thank you all very much for the assistance.

RF Lighting · 20 Aug 2014

Don't know why we bother

bwallum · 20 Aug 2014

ar....so how do I 'thank you' so that it gives you all 'thanked x times' boost?

bwallum · 20 Aug 2014

found the button....

JohnD · 20 Aug 2014

bwallum said:
the dishwasher is at one end of the ring.

1) rings don't have ends

2) the maximum load of any appliance you are permitted to connect to a RFC is 13A

3) That large fixed appliance should be on a dedicated radial circuit.

JohnW2 · 20 Aug 2014

bwallum said:
Whilst I can't find anything specific to say that I can't connect the dishwasher to the ring ...

You may not find 'anything specific' which says that, but you'll find plenty which tells you that you cannot run an unfused spur in 2.5mm² cable (assuming that's what it probably is), protected only by a 32A RCBO, to an appliance (like a DW) which, under certain fault conditions', could result in an 'overload'.

bwallum said:
... I am mindful of others following on in the future not being aware and assuming the DP switch is on a radial and possibly putting a more powerful machine on it. So...I'll run a 4mm flat T&E from consumer unit to an above counter DP switch on the dishwasher with a 16amp rcbo for protection.

As has been said, a dedicated 16A circuit was always going to be the proper answer. You might want to use 4mm² cable in case you (or anyone) ever wants to upgrade the circuit (and RCBO) in the future, but for your present purpose, and so long as you have a 16A RCBO (or even a 20A one), you could use 2.5mm² cable.

Kind Regards, John

JohnW2 · 20 Aug 2014

JohnD said:
2) the maximum load of any appliance you are permitted to connect to a RFC is 13A

That's an interesting one. I don't think you'll find anything in the regs which says that explicitly (even though it obviously makes good electrical sense), although you might think it is implicit, if you are one of those who interprets the regs as saying that only 'accessories to BS1363 can be connected to a ring. However, as has been pointed out, electrically speaking a double socket can, quite legitimately, present a load of at least 20A to a single point on a ring.

JohnD said:
3) That large fixed appliance should be on a dedicated radial circuit.

bwallum said:
So...I'll run a 4mm flat T&E from consumer unit to an above counter DP switch on the dishwasher with a 16amp rcbo for protection.

Kind Regards, John

OwainDIYer · 20 Aug 2014

bwallum said:
the dishwasher would be safer than two 3kW kettles in a double socket. That happens in village halls all too often.

And suggests that double sockets (and possibly a ring) would be inappropriate for that location. Rings are for general purpose use; they are not always suitable, such as commercial catering locations.

Also having all the sockets on one ring gives no resiliency when it does trip. splitting the sockets over multiple circuits at least allows the bingo tea or whatever to continue albeit a little more slowly.

Spark123 · 20 Aug 2014

A normal kettle (or two plugged into a double socket) don't take 40 mins to get up to temperature though.
If it is blowing 13a fuses then a 16a RCBO probably won't be mush use

bwallum · 20 Aug 2014

JohnD said:
1) rings don't have ends

What I was clumsily referring to was the split of current down each cable of the ring, proportional to the inverse of the resistance from point of load. I suspect the dishwasher is not at the midpoint of the ring from the equal resistance down each leg perspective.

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Commercial dishwasher on a 32amp ring main

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