'Best' MCB rating for boiler + washing machine?
 

For a 2.5mm2 radial circuit supplying a washing machine (via a double 13A socket) and a boiler/pump/valves (via 3A FCU) would a 16A or 20A MCB be 'best', and why?

If the spare 13A socket might end up being used for something, e.g. an occasional iron, does that sway the decision towards the 20A MCB?

'Best' MCB rating for boiler + washing machine?
 

A double socket is usually rated to 20 amps.

Do you know if it is a ring main with 2
lots of 2.5mm2 or a single 2.5mm2 or a single 4.0mm2 cable?

If it's a ring main, you can use up to a 32A MCB but clearly no point if it's one double and a spur for boiler. So a 20A MCB would be sufficient.

If it's 4.0mm2 then a 20A MCB is also ok.

If it's a single run of 2.5mm2 then you will need to check how it's been clipped.between the consumer unit and the double socket.

Depending on what it's been mounted onto and what surrounds it to determine if it's a 16A or 20A MCB.

'Best' MCB rating for boiler + washing machine?
 

Mathew Newton wrote :
For a 2.5mm2 radial circuit supplying a washing machine (via a double 13A
socket) and a boiler/pump/valves (via 3A FCU) would a 16A or 20A MCB be
'best', and why?

Radial circuit in 2.5mm you can use a 20amp MCB.

'Best' MCB rating for boiler + washing machine?
 

On 12/07/2019 20:53, Mathew Newton wrote:
For a 2.5mm2 radial circuit supplying a washing machine (via a double
13A socket) and a boiler/pump/valves (via 3A FCU) would a 16A or 20A
MCB be 'best', and why?

If the spare 13A socket might end up being used for something, e.g.
an occasional iron, does that sway the decision towards the 20A MCB?

Either would adequately protect the cable, but the 20A will give more
flexibility, so that would be my choice.


--
Cheers,

John.

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'Best' MCB rating for boiler + washing machine?
 

On 12/07/2019 23:04, wrote:

If it's a single run of 2.5mm2 then you will need to check how it's been clipped.between the consumer unit and the double socket.

Depending on what it's been mounted onto and what surrounds it to determine if it's a 16A or 20A MCB.

You should not have to do that. It should be installed so that it's
current carrying capacity is greater than 20A.

If not then it's a deviation from the regs.


--
Adam

'Best' MCB rating for boiler + washing machine?
 

But would you agree with me that in certain cable run situations, the cable cross sectional area is increased if it's going through thermally insulated routes instead of surface clipped to the wall and the MCB is not uprated as a result....

Ditto for extra longer cable routes to keep the voltage drop to less than 3%?

It's not clear from the OP if this circuit was already present or whether he created this new circuit?

'Best' MCB rating for boiler + washing machine?
 

On 13/07/2019 10:54, wrote:
But would you agree with me that in certain cable run situations, the cable cross sectional area is increased if it's going through thermally insulated routes instead of surface clipped to the wall and the MCB is not uprated as a result....

Ditto for extra longer cable routes to keep the voltage drop to less than 3%?

It's not clear from the OP if this circuit was already present or whether he created this new circuit?


I totally agree that if the CCC of the cable is exceeded then the MCB
rating must be lowered (subject to known loads) or the CSA of the cable
must be increased.

Your example is a good one. A 4mm radial is good for 32A but if the
cables passes through insulation then it might have to become a 20A radial.




--
Adam

'Best' MCB rating for boiler + washing machine?
 

On Friday, 12 July 2019 23:04:22 UTC+1, wrote:
A double socket is usually rated to 20 amps.

Do you know if it is a ring main with 2
lots of 2.5mm2 or a single 2.5mm2 or a single 4.0mm2 cable?

As mentioned it's a 2.5mm2 radial.

If it's a single run of 2.5mm2 then you will need to check how it's been clipped.between the consumer unit and the double socket.

Depending on what it's been mounted onto and what surrounds it to determine if it's a 16A or 20A MCB.

It runs loose up inside an partition wall for ~1.5m, through joist centres for ~8m and then down behind dot-and-dab skimmed plasterboard on a block wall for ~1.5m. No insulation anywhere along the run.

'Best' MCB rating for boiler + washing machine?
 

On Saturday, 13 July 2019 10:54:48 UTC+1, wrote:

It's not clear from the OP if this circuit was already present or whether he created this new circuit?

Apologies; it was already present - spurred off the origin of a 2.5mm2 ring circuit inside the consumer unit and I was contemplating putting it on its own MCB.

'Best' MCB rating for boiler + washing machine?
 

On Saturday, 13 July 2019 22:37:59 UTC+1, Mathew Newton wrote:
On Saturday, 13 July 2019 10:54:48 UTC+1, wrote:

It's not clear from the OP if this circuit was already present or whether he created this new circuit?

Apologies; it was already present - spurred off the origin of a 2.5mm2 ring circuit inside the consumer unit and I was contemplating putting it on its own MCB.

To clarify this further, the current spur feeds only the boiler but if I were to move it to its own MCB (turning it into a radial circuit) it was then that I'd move the adjacent washing machine on to it also. My thinking was that'd it'd help spread the current between circuits, and also help keep potentially leaky devices on their own RCD protection if I opted for an RCBO instead of the MCB.

'Best' MCB rating for boiler + washing machine?
 

It is recommended not to have 3 or more wires coming out of a MCB or RCBO.

The simple reason is because one of the wires can work loose as it's not to get the screw terminal to grip all three wires with equal force.

I would put that single radial run on its own MCB.

In fact, I would use a RCBO for 3 very good reasons but you would have to wire the RCBO upstream of the RCD. (The main one being the boiler still continues to work stopping the house from freezing if the main RCD trips so you don't come back to a flooded house. The second reason is if the washer develops a water leak, it does not take out the RCD and affect half the house.

'Best' MCB rating for boiler + washing machine?
 

Mathew Newton wrote:

To clarify this further, the current spur feeds only the boiler but
if I were to move it to its own MCB (turning it into a radial
circuit) it was then that I'd move the adjacent washing machine on to
it also. My thinking was that'd it'd help spread the current between
circuits, and also help keep potentially leaky devices on their own
RCD protection if I opted for an RCBO instead of the MCB.
If I was considering making a circuit into a dedicated RCBO protected
circuit for a boiler, the last thing I'd share the circuit with would be
a device with heating element and water ... well an oven would probably
be the *real* last thing, but you probably wouldn't do that for other
reasons.

'Best' MCB rating for boiler + washing machine?
 

Agree totally.

:-)

S

'Best' MCB rating for boiler + washing machine?
 

On Sunday, 14 July 2019 19:00:45 UTC+1, wrote:
It is recommended not to have 3 or more wires coming out of a MCB or RCBO..

I didn't even know it was permissable to spur of the circuit origing, until I looked it up in the regs. Good point about the 3-wire aspect.

In fact, I would use a RCBO for 3 very good reasons but you would have to wire the RCBO upstream of the RCD. (The main one being the boiler still continues to work stopping the house from freezing if the main RCD trips so you don't come back to a flooded house. The second reason is if the washer develops a water leak, it does not take out the RCD and affect half the house.

This is a high availability CU with four spare non-RCD ways so I might well move it onto its own RCBO. I note Andy's point about a boiler and washing machine sharing the same RCD/RCBO protection however I wouldn't be running the washer whilst out/away and, if it did start causing trips I just wouldn't use it until fixed (I am assuming the most common failure mode would be when it is running).

'Best' MCB rating for boiler + washing machine?
 

My own boiler is on a RCBO so provided there is still power, even if all the other circuits trip, my boiler still works during a cold snap unless of course it itself breaks down or causes the RCBO to trip.

My alarm, smoke dets, CO dets and CCTV system is also on its own RCBO. And ditto all outside sockets and outside lights so the internal sockets and internal lights are not affected by outside events.

'Best' MCB rating for boiler + washing machine?
 

On Monday, July 15, 2019 at 7:42:00 AM UTC+1, wrote:

My alarm, smoke dets, CO dets and CCTV system is also on its own RCBO. And ditto all outside sockets and outside lights so the internal sockets and internal lights are not affected by outside events.

That's the direction I am heading in as whilst I've never had any issues, nuisance trips etc, I may as well design in robustness whilst I am undertaking other changes to the system.

'Best' MCB rating for boiler + washing machine?
 

I have a curve CU with 16 RCBOs in it and have room for two more RCBOs.

Ever since it was installed 6 years ago, we have never had a nuisance trip.

With RCDs often protecting several circuits, it does not take many leaky devices to add up and the total leakage current go over 30mA which is the trip threshold for RCDs.

With RCBOs you have a trip threshold of 30mA per circuit rather than of half the house circuits in the case of a split load CU.

We did have a real trip due to a faulty iron but it only took out the lounge and the master bedroom. (Wifey was ironing in front of the lounge wood burner).

Worth every penny even though RCBOs do not cost much more than MCBs and the saving in not buying the RCD helps too. :-).

It will not surprise me if the next wiring regs mandate an all RCBO CU instead of current split load CUs

'Best' MCB rating for boiler + washing machine?
 

'Best' MCB rating for boiler + washing machine?
 

On 14/07/2019 19:01, Andy Burns wrote:
Mathew Newton wrote:

To clarify this further, the current spur feeds only the boiler but
if I were to move it to its own MCB (turning it into a radial
circuit) it was then that I'd move the adjacent washing machine on to
it also. My thinking was that'd it'd help spread the current between
circuits, and also help keep potentially leaky devices on their own
RCD protection if I opted for an RCBO instead of the MCB.
If I was considering making a circuit into a dedicated RCBO protected
circuit for a boiler, the last thing I'd share the circuit with would be
a device with heating element and water ... well an oven would probably
be the *real* last thing, but you probably wouldn't do that for other
reasons.

I would say an oven it not going to cause any real problem.If the oven
fails and you switch it off at the DP switch.

--
Adam