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Thread: Sub DB Rating

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    Sub DB Rating

    I have just bought a house where I received the issued COC from the seller. There is a Sub DB in the garage that has three 20A plug breakers and one 15A light breaker. These feed a new extension to the house. However when checking the main board in the house i see that this Sub DB is only receiving its supply feed through a 20A breaker. My question then is how compliant is this with regulations, as in theory all looks good when looking at the sub boards handling capacity for all the new extra circuits, but in practice it is severely limited to only 20A supply from the main board.



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    It truly is a small supply for the combined ratings of those circuit breakers but.... except for the workshop, all the other designated areas are not likely to draw much current.
    TV, Study and Lights don't use much at all although nothing is really stopping you from plugging in a heater in the study, while using a grinder and boiling a kettle at the same time etc
    ...but its very unlikely, have you actually had any tripping?
    As far as I know as long as the circuit breaker wire size ratio is correct it is compliant, but I'm continually learning new things in the sans book it seems!
    What may have happened is that there initially was a larger circuit breaker on the supply side, the wire may have been too thin and the electrician who signed it off decided to put a 20amp circuit breaker in to make it compliant.
    Looking at the picture though...it seems they may have used 4mm wire which means you could increase the circuit breaker size to 25/30amps (depending on length etc) but you would have to thoroughly check to make sure as there may be a joint somewhere where the cable size is reduced or some other issue.
    By the way, funny looking main DB!?
    Also quite surprised to see a bare neutral wire from the earth leakage to the ELP neutral bar
    Last edited by hartdev@hotmail.com; 06-Sep-17 at 06:44 PM. Reason: would like to add more

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    Quote Originally Posted by hartdev@hotmail.com View Post
    It truly is a small supply for the combined ratings of those circuit breakers but.... except for the workshop, all the other designated areas are not likely to draw much current.
    TV, Study and Lights don't use much at all although nothing is really stopping you from plugging in a heater in the study, while using a grinder and boiling a kettle at the same time etc
    ...but its very unlikely, have you actually had any tripping?
    As far as I know as long as the circuit breaker wire size ratio is correct it is compliant, but I'm continually learning new things in the sans book it seems!
    What may have happened is that there initially was a larger circuit breaker on the supply side, the wire may have been too thin and the electrician who signed it off decided to put a 20amp circuit breaker in to make it compliant.
    Looking at the picture though...it seems they may have used 4mm wire which means you could increase the circuit breaker size to 25/30amps (depending on length etc) but you would have to thoroughly check to make sure as there may be a joint somewhere where the cable size is reduced or some other issue.
    By the way, funny looking main DB!?
    Also quite surprised to see a bare neutral wire from the earth leakage to the ELP neutral bar
    Well that's my problem, I will be running more than just a grinder. Will be my 1.5kW CNC machine, pcb reflow oven that puts out about the same as a kettle, and a few other things so will be close to the 20A limit. So looks like I will be asking the family to switch off the lights and TV while I work (not going to be popular). I was also eyeing that suspect neutral connection out, and found it odd to find a 15 A breaker for the lights, normally its supposed to be a 10A breaker, but then again there are 14 down lights, and a whole bunch of other lights running off the single breaker. Unfortunately its a low angle flat roof, so I can't even try run a new cable to the sub. The main db is a old big one, has three rows for the breakers, the pic only shows the top row, that's why it looks strange.

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    it is a small supply but it does comply as long as the cable size and breaker match. thats a (messy)readyboard, a lot of them come prewired with that silly bare neutral. more than 1 neutral wire per terminal doesnt comply.

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    Quote Originally Posted by clintdk View Post
    Well that's my problem, I will be running more than just a grinder. Will be my 1.5kW CNC machine, pcb reflow oven that puts out about the same as a kettle, and a few other things so will be close to the 20A limit. So looks like I will be asking the family to switch off the lights and TV while I work (not going to be popular). I was also eyeing that suspect neutral connection out, and found it odd to find a 15 A breaker for the lights, normally its supposed to be a 10A breaker, but then again there are 14 down lights, and a whole bunch of other lights running off the single breaker. Unfortunately its a low angle flat roof, so I can't even try run a new cable to the sub. The main db is a old big one, has three rows for the breakers, the pic only shows the top row, that's why it looks strange.

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    Hey Clint
    As earlier mentioned, the supply to your DB looks like 4mm2 so if you are not qualified I suggest you get an electrician to check if you can up the breaker, I think you can from what i can see in the pictures.
    1.5mm wire can be on a 15amp breaker under certain conditions although I never do it myself, im the overkill guy
    Turning off lights and tv arent going to make much difference, and if you have the 50watt downlights or worse, 12v downlights I suggest youchange them to 230vac LED, the number of transformers and cables I see burnt around downlights in roofs is scary. the leds dont get too hot and the 230vac type obviously dont require transformers

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    Diamond Member AndyD's Avatar
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    As already covered above the circuit breaker is sized according to the wiring size and installation method, you can't just replace the breaker with a bigger one because you want to run more equipment.

    I'd suggest you get an electrician in to do a survey and advise you on the best way forward.
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    Hi all
    Should the main switch in this case in the main DB not be a double pole isolator / circuit breaker combo .Which should be on the left of the DB or Have a Double pole isolator main switch on the left of the Db with overload protection on its right.

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    Diamond Member adrianh's Avatar
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    A couple of stupid questions:

    1. Should a sub DB have an earth leakage breaker?
    2. Will that breaker be rated at the sum of the other breakers (provided of course that all wiring is appropriately rated)
    3. Will the tripping of the sub DB EL breaker save the main DB EL breaker from tripping?
    How easily someone is offended is directly proportional to how stupid they are.
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    Hi
    You are covering quite a portion of the regs with these questions .
    Sounds like you need an electrician.
    Recommend you get a competent electrician.


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    Quote Originally Posted by adrianh View Post
    A couple of stupid questions:

    1. Should a sub DB have an earth leakage breaker?
    2. Will that breaker be rated at the sum of the other breakers (provided of course that all wiring is appropriately rated)
    3. Will the tripping of the sub DB EL breaker save the main DB EL breaker from tripping?
    1. I would say it is always best that the sub Db have its own earth leakage breaker for discrimination , to fault find etc but don't believe it is a law, just good practice, especially since some places have their entire installations on earth leakage protection.
    2. The Main CB is very seldom rated at the sum of all other circuit breakers as we take the estimated load factors into account , ie: Your geyser won't always be on when your stove is on etc you would struggle to find a residential 60amp installation with only a sum of 60amps circuit breakers installed.
    3. It should if done properly.....
    but I just took a look at your logo (diamond member) and I'm wasting my time telling you things you already know x)

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