# Interest group forums > Electrical Contracting Industry Forum >  Sub DB Rating

## clintdk

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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## hartdev@hotmail.com

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  :Smile: 
...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

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## clintdk

> 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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## bergie

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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## hartdev@hotmail.com

> 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.
> 
> Sent from my BV6000 using Tapatalk


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  :Smile: 
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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## AndyD

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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## Gtfast

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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## adrianh

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?

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## Gtfast

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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## hartdev@hotmail.com

> 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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## adrianh

> 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)



Thanks. I am a Diamond member because I have been a part of this group for many years. I am not an electrical contractor and I really appreciate your insight. And yes, I fully agree that everything should be done properly.

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## AndyD

> A couple of stupid questions: 
> 
> 1. Should a sub DB have an earth leakage breaker?


 Depends on the circuits in the SubDB. If there's socket circuits or any other types of circuit that require earth leakage protection then yes, there should be an earth leakage breaker installed (or RCBO's used instead of MCB's).



> 2. Will that breaker be rated at the sum of the other breakers (provided of course that all wiring is appropriately rated)


 No, highly unlikely. We work on a system of 'diversity' where it's assumed (gambled) that every circuit would never simultaneously be under full load conditions. There are rules for working out diversity in the regs.



> 3. Will the tripping of the sub DB EL breaker save the main DB EL breaker from tripping?


 It's poor electrical design to have an earth leakage breaker that's fed from another earth leakage breaker unless one is differently rated to the other ie one is a faster type or they both have different mA ratings otherwise it's always a lottery which one trips first and often both will trip during a fault.  It's also bad design to have the supply for a SubDB run from an earth leakage breaker in the main DB.

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Dave A (20-Sep-17)

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## adrianh

It's poor electrical design to have an earth leakage breaker that's fed from another earth leakage breaker unless one is differently rated to the other ie one is a faster type or they both have different mA ratings otherwise it's always a lottery which one trips first and often both will trip during a fault. It's also bad design to have the supply for a SubDB run from an earth leakage breaker in the main DB. 

- Does this mean that A sub DB board is wired in parallel to the main board rather than being fed off the main db board breaker - so that each board has its own separate supply, main breaker and earth leakage?

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## AndyD

The supply for the Sub DB would be via the main breaker in the main DB and a second breaker in the main DB for the subDB cable.

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## adrianh

Why are the two breakers in series with the same break current?

Which one will break if a fault occurs in the garage at 60A?

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## AndyD

This was just a drawing I had kicking around so I edited it for illustration.

The garage circuit breaker would be sized according to the size of cable feeding the garage and several other factors including the installation method used for the cable and the length of the cable etc.

It could be okay to use the same size breaker for the garage as the main breaker if the cable size was sufficient but it would be good practice to introduce 'discrimination' by making the garage breaker a faster curve than the main breaker ie the garage breaker might be a 'B curve' and the main breaker could be a 'C curve'.
0600DB0105.pdf (first page for brief explanation)

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## Sparks

What non-electrical people need to understand is that a circuit breaker is selected to match the thickness of the wire used along with other factors. It's purpose is to protect that cable. Usually in the main DB the breaker is stronger for the supply to a sub-DB because the cable is thicker. The sum of the breakers is never a consideration. Many sub-DBs' have a supply rated at below 60A in the main board, but the main switch in the sub-DB is rated at 60A. This is because a 60A Double pole isolator is cheaper. The pupose of the main switch is to isolate the sub-installation. Each circuit fed from the sub-DB should be protected by a lower rated CB than the Sub-DB supply CB in the main board. This will localise tripping to the sub-DB. 

The only device in the DB that protects the user is the ELCB. It is good practice to once a month switch off all electronic devices and press the test button on the ELCB. This will be an indication that it it still functioning. It is also wise to have it tested every two years to determine whether it still functions within the rating.

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