Eddy Current
Senior Member
How come in a normal panel the main breaker is less than the sum of all the branch circuit breakers in the panel?
Incoming amperage more than breaker amperage?
- Thread starter Eddy Current
- Start date
- Status
- Location
- Massachusetts
Because the sum of all the branch circuit breakers has nothing to do with the total load the main has to carry.
Because it is designed for a particular application, say, for a residence which may not be suitable for another residence.
Eddy Current
Senior Member
- Location
- Massachusetts
Yes.
But not every branch circuit breaker is going to be supplying it's marked value, likely none of them will be.
eHunter
Senior Member
- Location
- North Texas & Georgia
Or all concurrently.
- Location
- Massachusetts
Eddy,
The rating of the main is determined by the load calculations found in article 220.
Consider this.
You have a dishwasher that uses 5 amps, but we run a 15 or 20 amp circuit to it. How much current will that circuit add to the load on the main?
You have a electric dryer, it uses 21 amps, we run a 30 amp circuit to it. How much current will that circuit add to the load on the main?
So you have a lot of the situation above where the circuits run are actually larger than the load served.
Now when you have a lot of circuits in a panel there is very little chance all will be used at the same time, or all concurrently as eHunter put it.
In many cases the calculations in Article 220 address this with demand tables which basically allow us to count things less than they are for the purposes of sizing mains.
Now for another way of looking at it, consider the home you are in now. Would it need a larger main if for some reason you made every receptacle and lighting outlet a dedicated circuit?
The rating of the main is determined by the load calculations found in article 220.
Consider this.
You have a dishwasher that uses 5 amps, but we run a 15 or 20 amp circuit to it. How much current will that circuit add to the load on the main?
You have a electric dryer, it uses 21 amps, we run a 30 amp circuit to it. How much current will that circuit add to the load on the main?
So you have a lot of the situation above where the circuits run are actually larger than the load served.
Now when you have a lot of circuits in a panel there is very little chance all will be used at the same time, or all concurrently as eHunter put it.
In many cases the calculations in Article 220 address this with demand tables which basically allow us to count things less than they are for the purposes of sizing mains.
Now for another way of looking at it, consider the home you are in now. Would it need a larger main if for some reason you made every receptacle and lighting outlet a dedicated circuit?
Eddy Current
Senior Member
I understand all the circuits won't be used, and even if its at 80% or whatever the code says, that the sum is still larger, but i still don't see how its scientifically possible.
- Location
- Massachusetts
Well now you lost me. :?
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
Bob had a good example for you. But there are two things about your own comment that I believe are confusing you.
That was Bob's point: It's not 80%. What I mean is that the actual current flowing through a 20 amp breaker is very seldom as high as 16 amps (80% of 20 amps). You might have a 20 amp circuit supplying a bedroom, and all that is on is one lamp and the tv, for a total current of perhaps 3 amps (not 16).
The sum of the ratings of the breakers is larger. But the sum of the currents flowing at any moment through the entire set of breakers will still be far lower than the rating of the main breaker.
Strathead
Senior Member
- Location
- Ocala, Florida, USA
- Occupation
- Electrician/Estimator/Project Manager/Superintendent
Eddie, did you write this and post it before or after you read iwire's post? If it was before I would hope that his explanation now clears it up for you.
Another factor is heating and air conditioning in most applications you have separate air conditioners and heating elements (for houses with electric heat) that share the same fan. Never will they both be on at the same time, so the size of the service only needs to take in to account the heat or the air conditioning plus the fan, whichever is larger.
Eddy Current
Senior Member
OK i think i understand now, the breaker ratings are larger but, in actuality what is being used will not add up to being more than the main breaker. I guess that was kind of dumb question, if it was more the main breaker would trip.
Over here (Australia) the Main circuit breaker is there to protect the consumer mains only. the sub-circiut CB's don't have anything to do with it.
eg. if you have done a maximum demand for the installation which yields 80A with percentages on each appliance/final sub-circuit derived from the diversity assumed that everything will not be operating at the same time.
You use a cable size capable of carrying 100A and the sum of all your final sub-circuit protection equates to eg. 210A. Theoritically, there's nothing stopping that installation from pulling 200A under fault conditions causing the mains to burn out and short circuit. Also unprotected, the fault current may not be large enough to blow the LV sub fuse causing the mains to arc away and possibly catch the house on fire in a ceiling space or Main Switch Board. Not likely to happen... but more likely in older, high density, urban/industrial/commercial areas.
I have seen weird things eg, a fridge, without earth leakage protection, leaking 19A to earth without tripping tha 20A CB going back through the the main earth of a house nearly 100 yards down the street back into the network, this causing thier entire earthing system/appliances to become alive causing high shock voltages. I have seen sub-mains dead short to earth not blow re-wirable fuses, I would hate to tong the fault current on that!
IT CAN HAPPEN!
eg. if you have done a maximum demand for the installation which yields 80A with percentages on each appliance/final sub-circuit derived from the diversity assumed that everything will not be operating at the same time.
You use a cable size capable of carrying 100A and the sum of all your final sub-circuit protection equates to eg. 210A. Theoritically, there's nothing stopping that installation from pulling 200A under fault conditions causing the mains to burn out and short circuit. Also unprotected, the fault current may not be large enough to blow the LV sub fuse causing the mains to arc away and possibly catch the house on fire in a ceiling space or Main Switch Board. Not likely to happen... but more likely in older, high density, urban/industrial/commercial areas.
I have seen weird things eg, a fridge, without earth leakage protection, leaking 19A to earth without tripping tha 20A CB going back through the the main earth of a house nearly 100 yards down the street back into the network, this causing thier entire earthing system/appliances to become alive causing high shock voltages. I have seen sub-mains dead short to earth not blow re-wirable fuses, I would hate to tong the fault current on that!
IT CAN HAPPEN!
cuba_pete
Senior Member
- Location
- Washington State
Same Difference
Same Difference
Bing! That's the case in the States, too.
I think that is key to understanding the branch and service differences and similarities. A branch could have 6 receps at 15A apiece and still have a 15A breaker...a 150A main could have over 400A of breakers installed...same difference.
The breaker is there to protect the wires and devices.
Same Difference
Bing! That's the case in the States, too.
I think that is key to understanding the branch and service differences and similarities. A branch could have 6 receps at 15A apiece and still have a 15A breaker...a 150A main could have over 400A of breakers installed...same difference.
The breaker is there to protect the wires and devices.
- Status