Immersion heater switch before or after FCU?

[bonebill]

I'm putting in a hot water tank with an immersion heater.

The hot water tank will be located in a loft.

I want to put an on/off switch on the wall in the utility room, so I have an MK 20A 'water heater' switch.

But I figure the immersion heater should have a local isolator right next to it, and have a 13A fuse between it and the final circuit. Also I need somewhere to terminate the (presumably heat resistant) one metre immersion heater cable. So I have a switched 13A FCU to go next to the immersion heater.

The power for this whole deal is supplied by a 16A final circuit assigned for heating (pumps & controllers etc) and immersion heater.

As it is only a 16A circuit, the 20A remote switch could be put in circuit either before or after the FCU.

If the 20A remote switch is before the FCU, then it makes the wiring simple. We just link as: CU - 20A remote switch - FCU - immersion heater, using the existing terminals on each accessory.

If the 20A remote switch is after the FCU, then we need to go: CU - FCU - 20A remote switch - junction - immersion heater.

The junction is needed because the cable coming up from the 20A remote switch will be 2.5mm² T&E, but the immersion heater needs to be fed with immersion heater cable. So this would need either a bit of 15A terminal blocking inside the FCU enclosure , or a separate junction box.

So I was wondering if there was a standard recognised approach for doing this.

 

[holmslaw]

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[ColJack]

you havent said what rating the immersion heater is.. they are generally 3KW..

it's not very common to have a FCU on an immersion heater circuit as it's protected by the 16A breaker..

I would also consider not putting the FCU in the loft, merely for the convenience of changing the fuse should it go..

immersion flex is usually 2.5mm also..

if you're on a TN-S or TN-C-S supply, then a FCU can be used as a means of isolation. If you are not however, you need to use a double pole means of isolation to disconnect the neutral also..

 

[RF Lighting]

Is an FCU not a DP isolator then?

 

[GaryMo]

Is an FCU not a DP isolator then?

Not an unswitched one

 

[holmslaw]

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[holmslaw]

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[GaryMo]

So I have a switched 13A FCU to go next to the immersion heater.

Whooosh!

 

[holmslaw]

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[holmslaw]

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[ColJack]

fuses on heavy current using equipment can sometimes blow due to fatigue.. prolonged use at near to maximum load weakens the wire eventually..

see it a lot on factory machines..

and DP isolators are required by the regs NOW unless you have a TN-S or TN-C-S supply..

 

[Taylortwocities]

Per the above. If the circuit is protected by a 16A CB in the consumer unit and all wiring is 2.5mm² then all you need is the 20A DP switch. No need for a 13A fuse, at all.

 

[holmslaw]

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[ColJack]

he'd be spending less money since he's not paying for the 13A SFCU.. the breaker is already 16A..

it does pain me to see the words "16A final circuit assigned for heating (pumps & controllers etc) and immersion heater" though as it suggests that there is other loads on the circuit...

a 3KW immersion will draw 13A ( 13.04A, but close enough.. )

 

[JohnD]

Per the above. If the circuit is protected by a 16A CB in the consumer unit and all wiring is 2.5mm² then all you need is the 20A DP switch. No need for a 13A fuse, at all.

completely agree

furthermore, adding an FCU gives you an additional potential point of failure. the 15A mcb is enough and is correct

quite likely one, or both will go if the element fails, then you have twice as much work tracing and resetting it

barmy.

A 20A DP switch with neon is ideal