Incoming and outgoing breakers

[Rio]

Dear All,

I am attaching a single line diagram. I have the following reservation but I could be wrong. Just want some expert advise on this.

In my opinion the incoming breaker is very small as compare to the outgoing breakers.

Please comment on this. If there are any other points please do share.

Note: I am doing an internship at the moment and this is kind of an assignment. My colleague (Internee) as well think that it is right. He says that 125A TP means 125A on each leg and the total current capacity is 375A and the total outgoing current is 270A. is he right??

Kind regards,

 

[glene77is]

Rio,
Can't see the details in your jpg.
However, Would the "diversity" of the load allow this situation.
"Diversity" is a common factor in calculating loads in residential planning.

 

[jim dungar]

Note: I am doing an internship at the moment and this is kind of an assignment. My colleague (Internee) as well think that it is right. He says that 125A TP means 125A on each leg and the total current capacity is 375A and the total outgoing current is 270A. is he right??

No.
He is wrong.

A protective devices rating is done 'pole by pole'. There is no addition involved.

 

[ggunn]

Note: I am doing an internship at the moment and this is kind of an assignment. My colleague (Internee) as well think that it is right. He says that 125A TP means 125A on each leg and the total current capacity is 375A and the total outgoing current is 270A. is he right??

He is wrong. The current in each conductor in a balanced multiwire circuit and the total current are one and the same.

 

[Rio]

I have re-attached the file. Hope it is more convenient to read now.

Kind regards

 

[petersonra]

I don't see anything obvious that would bother me any.

Having said that, I know nothing about what codes, specs, and/or standards would apply in India on this particular project either.

 

[Rio]

Hi Bob,

I was concerned with the incoming breaker size. Its 125A. The outgoing breakers amount to be around 270A. Is that right? I mean, I am still in my first year, I could have been missing something but it doesn't seem right to me..

As per my supervisors instructions he said to use British standards when analyzing this circuit.

 

[jim dungar]

Hi Bob,

I was concerned with the incoming breaker size. Its 125A. The outgoing breakers amount to be around 270A. Is that right? I mean, I am still in my first year, I could have been missing something but it doesn't seem right to me..

As per my supervisors instructions he said to use British standards when analyzing this circuit.

The 'sum of the handle ratings' of the branch breakers has absolutely nothing to do with the sizing of the main device.

 

[topgone]

As posted by others above, you size your mains by calculating the load it handles. Please post your schedule of loads instead so others can get a clearer picture. As mentioned, you need to know if the loads run at the same time (there are loads that when active requires another related equipment to be put off).

 

[broadgage]

Although codes and regulations vary significantly between the USA and the UK, the basic principles remain the same.
There is nothing wrong with the total capacity of branch circuit/subcircuit MCBs considerably exceeding the size of the service or feeder.
The total load should not exceed the available capacity.

As an example, in both the UK and the USA it is common practice to place fire alarms on a dedicated circuit, with another dedicated circuit for intruder alarms. Neither system is likely to use more than one amp. The provision of these two dedicated circuits which might be 15/16/20 amps each certainly does not mean that the main or feeder needs to be 30/32/40 amps larger than without the dedicated circuits.

As an example, here is the circuit list for an area where I work, all single phase 240 volts.
The feeder is 100 amps

1) 32 amp, outlets in boiler room
2) 32 amp, outlets in corridoor
3) 32 amp outlets in my office and workshop
4) 32 amp, outlets in store rooms
5) 16 amp, gas leak detector
6) 16 amp, outdoor outlet

7 through 18 are all 10 amp lighting circuits, 12 circuits in total.

Now consider the actual likely load
The outlets in my office and workshop are typicly loaded to about 3 amps in total, maybe 20 amps if I use a power tool and a space heater at the same time.
The outlet circuits for the boiler room, store rooms, and the corridoor are not normally used, but are provided to allow for use of power tools or cleaning equipment, normall load load is zero, maximum likely perhaps 20 amps, at very long intervals.

The gas leak detector uses almost nothing.
The outdoor outlet is seldom used, normal load is zero, might exceptionly be fully loaded to 16 amps.
The 12 lighting circuits have an average connected load of about 2 amps each, but are mainly on presence detectors and therefore little used. Normal load for all 12 circuits might be 2 amps, with a maximum under rare conditions of about 25 amps.

The maximum demand on the 100 amp feeder is therefore about 80 amps, and that is under most improbable conditions. Is it really likely that I and at most 2 other employees would use a space heater, two heavy duty power tools, and a vacuum cleaner at the same time, AND whilst someone runs in and out of each store room in order to turn on all the lights.
The load seldom exceeds 20 amps in practice.

 

[Rio]

Thank you very much broadgage. That was really helpful. It all makes more sense now.

Regards