# Home AC confusion I must be missing something ?

#### TowerBlockTechnician

Joined Aug 27, 2018
8
Hi

I suppose this post is referring to big AC circuits rather than small battery DC circuits but I am hoping some of you clued up folks can enlighten me.

I was recently considering taking a spur of my socket ring circuit for a new plug outlet in a large space at the end of our hallway. While learning how to do this correctly it has given me a fair interest about the layout of the electronics in my home and how it can be altered.

I'm pretty new to understanding AC but have quite a good idea of the distribution board and Breakers etc and how it all works.

A few things still confuse me. When I look at all the RCBO switches on the bus bar.. I notice some are 16amp for the lone circuits then like 32 or 40 amp for bigger circuits.. my confusion is this

if the main service cable entering the property carries all the current into the meter then comes out the meter powering the busbar which carries the full load supplying to the RCBOs.

Well If the entire load being pulled by every breaker together adds up to more than say 16amp, does this then mean that the current in the busbar and mains cable entering the property is also above 16 amp... and if so HOW does this not effect or trip the 16amp RCBOs on the same busbar ??

I assume I am missing something about the workings of AC... if somebody could help me with this confusion Id very much appreciate it

cheers

#### wayneh

Joined Sep 9, 2010
17,201
I’m having trouble understanding your question. Your service may be rated to, say, 200A. You would not approach that unless each individual circuit was approaching its rated max. They are all in parallel, branched off the main, and contribute to the main service total amperage.

An expert can chime in but I believe the sum of all the branch capacities can exceed the main capacity, because it’s highly unlikely you’d ever be maxing out multiple circuits at the same time.

Joined Feb 20, 2016
4,268
The total max current the house connection can handle is not usually the total of all the circuits combined.
It is quite lower. One would never run all the circuits at max power. Here at my place, the total circuit current, if you go by the breaker rating, is a bit over 200Amps. But my service current breaker is only 50 Amps.
(I'm in Oz, so it is 240VAC)

#### AlbertHall

Joined Jun 4, 2014
12,199
Well If the entire load being pulled by every breaker together adds up to more than say 16amp, does this then mean that the current in the busbar and mains cable entering the property is also above 16 amp
Yes, indeed it does.

and if so HOW does this not effect or trip the 16amp RCBOs on the same busbar ??
Because that total current flows in the bus bars to/from the meter. It does not flow through the individual circuit breakers.

#### TowerBlockTechnician

Joined Aug 27, 2018
8
thanks for the replies guys.

Because that total current flows in the bus bars to/from the meter. It does not flow through the individual circuit breakers.
ALberthall I sort of understand, but If say the 32amp and 40 amp breakers were drawing a heavy load through the bus bar that slightly exceeded say 16amp or even 10amp.. my light ciruit is on a 6amp breaker on the same bus bar.. does this not mean a higher current in the bus bar should trip the 6amp breaker before it can even power the light circuit... this is the part I fail to understand

do the ac circuits after breakers only draw the current they use or are the circuits automatically carrying whatever current is in the bus bar, ?

i probably missing a crucial factor and sound like an idiot, i just can self figure this part out, much more a dc guy

#### Travm

Joined Aug 16, 2016
363
The bus bar is carrying the current, not the breakers.
Each circuit only carries whatever current is being required by the load on that specific circuit, puts it onto the busbar, and out the main breaker to your service meter. That current isn't travelling through every breaker.

#### wayneh

Joined Sep 9, 2010
17,201
As above. If I turn on my stove and it draws 30A, and nothing else in the house is on, 30A are passing thru the stove circuit's breaker, the main bus and it's breaker, and the service line entering my home. No current whatsoever is passing through all the other breakers. They could be gone and it would make no difference.

#### BR-549

Joined Sep 22, 2013
4,928
TowerBlockTechnician......Electricity does not work that way. I suggest that you stay away from line voltage. Study a book and find a friend to teach you. Line voltage needs to be taught in person for safety reasons. It's dangerous to you and others.

How would the main bk, bus bars, and circuit bks work....... if it were DC?

Same difference.

#### Stuntman

Joined Mar 28, 2011
221
You seem to be caught up on some kind unfounded behavior of AC circuits. They operate with many of the same principles of DC. Consider the attached DC schematic. Consider that the fuses are just DC circuit breakers. (This is not an obsure comparison, homes were wired with fuses instead of breakers for decades).

If I closed SW2 and SW3, would not all of the fuses (including F0) operate properly? Yes

If I closed only SW2, would F1, F3, or F4 blow just because F2 is sourcing nearly 50A? No, the fuses will be all be fine.

If one closed SW2, SW3, SW4 and SW5, would the current exceed F0 and blow the fuse? Yes.

And yes, it is quite common (more common than not in my experience) for the sum of all the individual breaker capacities to exceed that of the main feed breaker. IE, a 100A panel may have 150A worth of combined breakers. The premise is that all breakers will not be loaded to their rated capacity at all times.

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#### MrSoftware

Joined Oct 29, 2013
2,069
Before you go changing anything, check if your house is wired using multiwire branch circuits, in which a single neutral is shared between two out of phase hots. This is common in some areas of the USA (not sure where you are). If your house is wired this way and you make changes without understanding what you're doing then there is risk of overloading the neutral and creating a fire hazard. Similarly there is risk of getting 220v instead of the expected 110v if you work with the wrong pair of wires.