available fault current?

[zappy]

i do mostly residential wiring.i've never asked poco for this info before.i've done a few service changes and never been asked for this info.can some one explain why i would need this info and when would i need this info thank you for your help.

 

[don_resqcapt19]

You need that to select the correct breakers. The breakers must have an interrupting rating that equals or exceeds the available fault current.
Don

 

[coulter]

To insure the equipment you are installing has a high enough interrupt capacity to with stand and open a bolted fault (dead short).

Likely never be asked for a residential application. A 50kva xfm (3 - 4 houses), 1ph, 240V is about 200A FLC. At 5% Z (impedance), that gives 4000A fault current - well below most residential grade equipment.

carl

 

[dcspector]

I recommend contacting your local POCO and that information will and should be available.

 

[jim dungar]

To insure the equipment you are installing has a high enough interrupt capacity to with stand and open a bolted fault (dead short).

Likely never be asked for a residential application. A 50kva xfm (3 - 4 houses), 1ph, 240V is about 200A FLC. At 5% Z (impedance), that gives 4000A fault current - well below most residential grade equipment.

carl

A utility oil-filled transformer of only 50kVA would probably have an impedance drastically lower that 5%. At 1.8% the available fault current would be in excess of 11kSCA which is more than the typical 10k residential breaker.

The requirements, NEC2005 110.9 and 110.10, for equipment to be rated for the fault current available at its line side terminals has been in the NEC for more than 20 years.

 

[zappy]

but wouldn't the main brkr with a 22,000 interup rating

but wouldn't the main brkr with a 22,000 interup rating

protect the branch circuit brkr's?also how does a brkr trip if the fault happens on the line side of it?doesn't poco have fuses before it hits the service enterance conductors?i'm very ignorant about this stuff.i never seen this called by a inspector before.

 

[dcspector]

protect the branch circuit brkr's?also how does a brkr trip if the fault happens on the line side of it?doesn't poco have fuses before it hits the service enterance conductors?i'm very ignorant about this stuff.i never seen this called by a inspector before.

Here in DC POCO or commonly known as Pepco provides that information.

 

[Pierre C Belarge]

The inspector is thinking about the breakers in your service panel.
There will be X-amount of fault current available to circuit breakers from the source supplying those breakers. As one moves further downstream from the source, the available fault current will become less as there is more impedance added to the circuit.


Some of the newer (larger homes) by us have their own transformers, closer to the home and lower impedances available. For some of those homes, I am in question of the available fault current available.
Last week, looked at a 3-phase, 1200 amp service to a home...that is no small service. Transformer pad is about 15 feet from the corner of the house, and the service disco is directly on the inside of the wall.

 

[iwire]

but wouldn't the main brkr with a 22,000 interup rating protect the branch circuit brkr's?

No.

When a fault happens the current goes up in the circuit immediately throughout the entire circuit.

It does not get to one section first.

So a 22000 amp fault would hit the branch breaker at the same time as the main breaker.

also how does a brkr trip if the fault happens on the line side of it?

It does not unless it has undevoltage protection.

doesn't poco have fuses before it hits the service enterance conductors?

No, but even if it did that would not change the available fault current.

The availer fault current is going to be high until something opens and that will be fast but not immediate.

 

[zappy]

a 3ph 1200amp service to a home??

a 3ph 1200amp service to a home??

i thought 3ph was for comm.or indust.only.

 

[Pierre C Belarge]

there is no such restriction by code, there may be some restriction by the local utility company.