Main breaker has two different sized cables going into it

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#272795 16-Jul-2020 13:10

Our switchboard has a two pole main circuit breaker with one larger and one smaller cable coming into it. I'm wondering what this smaller cable is for. Both cables are 235V as I measured. Is this smaller cable like a 'supplementary' cable as the larger one is not enough?

I have attached a picture below:

Any ideas on this smaller cable's purpose? Thanks

mdooher

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#2524137 16-Jul-2020 13:15

probably for the ripple (controlled hot water)

Matthew

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#2524973 17-Jul-2020 20:27

That's your hot water supply. It's controlled by a ripple relay so the network can switch it off at peak times and goes through a separate meter (usually combined into one unit on modern meters) so it can be billed at a different rate. As the ripple relay and meter are in the meter board you need a separate wire coming into the switchboard, and since you still want the main switch to turn everything off, you have the two separate poles on the main switch. The cable is smaller because it's only 16 or 20 A for the hot water only, whereas the main is likely sized for 63 A to feed everything else in your house.

UsernameNotAllowed

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#2525132 18-Jul-2020 11:16

Thanks for the information. I found the ripple control receiver in the power meter box.

petermcg

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#3352847 11-Mar-2025 21:14

I have one of this type of 2 pole main switch. My mains cable from the street enters on the right side top terminal 3, my house load exits from terminal 4 bottom right, I have my ripple wire connected to terminal 1 top left,, now there is a spare terminal, not used bottom left labelled terminal 2,

Its the same as this one pictured, but just using top instead of bottom terminals, I understand that this is not the style of 2 pole switch that will isolate Live and neutral.

My question is this, Can I use spare terminal 2 to feed a 10mm mains out to my garage?

tweake

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#3352861 11-Mar-2025 21:52

no.

the hot water feed is 16 or 20 amp while garage submain is 30 amp so the wiring is undersized to start with. edit: also there is the ripple control to deal with.

also that main breaker is 63 amp and you need a 30 amp. so if the garage overloads it will cook the garage feed before tripping that breaker.

you need to put in a separate breaker wired to the bus bar etc.

also none of the above your allowed to do. its sparky only job.

Handle9

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#3352919 11-Mar-2025 22:29

To reinforce above: A homeowner may not work in a switchboard. Only appropriately registered electrical workers may do this kind of work.

Don't do it. If you have to ask on here you don't know what you are doing.

If you want to save some money find a sparky who will let you do the donkey work and then come and charge you for testing and terminating, don't take the risk of winging it.

petermcg

77 posts

Master Geek

#3352926 11-Mar-2025 22:56

Thanks guys, an electrition is going to do the work, I was just trying to satisfy in my own mind how this switch works. Still a bit unsure its quite technical, It appears that the 2 poles are connected together within the switch itself.

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tweake

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#3353255 12-Mar-2025 17:07

petermcg:

Thanks guys, an electrition is going to do the work, I was just trying to satisfy in my own mind how this switch works. Still a bit unsure its quite technical, It appears that the 2 poles are connected together within the switch itself.

its actually 2 separate breakers joined together. in the photo its actually the gray bit linking them. i'm not a huge fan of this because i have had it where one will trip but it hasn't got the strength to fully trip the other. however there is purpose made duals (afaik) so it depends on what you have.

an option is to delink the two breaker and replace one with a 30 amp for the garage feed (assuming board is full).

larknz

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#3353261 12-Mar-2025 17:30

You will find that it is 2 switches joined together with the link in the handle, they aren't circuit breakers. There will be separate circuit breakers to provide the protection.

tweake

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#3353263 12-Mar-2025 17:46

larknz:

You will find that it is 2 switches joined together with the link in the handle, they aren't circuit breakers. There will be separate circuit breakers to provide the protection.

doubt it.

pretty common to use the circuit breakers as a switch.

larknz

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#3353264 12-Mar-2025 17:49

The symbol shows a switch, not a circuit breaker and there is no Type number.

Daynger

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#3353752 13-Mar-2025 20:30

tweake:

doubt it.

pretty common to use the circuit breakers as a switch.

You can doubt all you like but that is a main switch, not an MCB, as larknz said.

petermcg

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Master Geek

#3353755 13-Mar-2025 20:52

My one is a dual pole like this one, one side has the main in cable and feeds the rcd and circuit breakers but there is nothing coming from the other end of the switch that has the ripple wire, in fact I dont have an electric hot water heater I have gas. I was thinking that these 2 switches were linked inside the unit, but maybe not.

larknz

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#3353757 13-Mar-2025 20:55

Definitely not linked internally. Maybe your house had an electric water heater in the past.

John19612

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#3353872 14-Mar-2025 07:04

larknz:

The symbol shows a switch, not a circuit breaker and there is no Type number.

To be specific, it is the symbol for a double pole on-load isolator.

Some (most) of the unqualified advice in threads like this is more than enough reason for Section 79 of the Electricity Act to be repealed ASAP.