Using a "standard" breaker as a "main" breaker

[Michael Thomas]

Ran across this on a DIY board ... one more thing to think about:

Using a standard breaker as a main breaker may not provide the interrupting capacity you'll need to open should a fault occur.

Most main breakers, if not all, have a 22 kaic rating and branch breakers usually have a 10 kaic rating.

These are allowed to be used per the "2 tier system" because the 10 is being protected upstream with the 22.

I'm referring to 110.9, 110.10 and 240.89. Also, this provides some information as well http://ecatalog.squared.com/pubs/Ele...4100DB0301.pdf (http://ecatalog.squared.com/pubs/Electrical%20Distribution/Load%20Centers/4100DB0301.pdf)

I would check with the POCO to see what the available fault current is so, in the event of a fault, the breaker is capable of interrupting it.

[John Steinke]

It's something of a 'distinction without a difference.'

In the 60's, Ford had an advertising coup when they showed the advanced process they used to paint cars; what the ads failed to mention was that the same high-tech process was used by every car maker. Such is the case here.

Apart from some brands no longer made, every breaker maker grossly exceeds the AIC minimum requirements. In a similar manner, so does every panel maker exceed the bussbar mounting requirements. I gather that it's not so much from deliberate desire, as the difficulties in making a breaker that only 'just' meets the requirements. It's just much easier to over-do it a bit.

Naturally, this has not prevented one maker (Square D) from touting how they exceed specs. This claim, while true, omits mention that their competition also exceeds specs - just like in the Ford example I gave earlier.


As for available fault current .... lots of luck getting that from the power company! It's not that easy; a proper site-specific engineering study is required for that. In practical terms, unless the PoCo transformer is within 10 ft (wire length) of the breaker, then you're not likely to have a problem.

[Donald Farrell]

The reason you do not use a "standard breaker" as a main breaker is that it will trip simultaneous with the branch breaker, killing the entire panel instead of just the failing branch circuit.

[Robert Rolleston]

As long as the breaker has some form of hold down it can be used as a main breaker to feed the panel. In situations where the main breaker goes bad and can't be replaced due to availability sometimes an affordable fix is replace the 100 amp main and feed the panel from a 100 amp breaker with a hold down.