For my garage, I have a 30A 240 breaker in my main panel. In the sub-panel in the garage there are 2, 20A 120 breakers.

Should I have 2, 15's then since I have 30A breaker inside?

 

The two pole 30A breaker is there to protect the wiring from the main panel to the sub panel. A 30A breaker tells us that the wiring between the panels should be #10 wiring.

 

Actually its #8.... electrician didn't have a 40. Told me 50 would be OK though. I told him I didn't think so and to put a 30 in instead.....

 

As PJ posted, the breaker protects the feeder or branch circuit wiring. You could really have 20 - 20 amp breakers in the panel and it would be fine. As long as the breakers are sized correctly to the wire size.

 

Is there any valid reason where 50A can be used with #8?

 

If it is a welder depending on duty cycle and in some cases some compressors with high starting current depending on run current.

 

If the circuit breaker and any other connections are listed for use at 75[SUP]0[/SUP] Celsius AND you are using type THHN or THWN wire (no type NM cable) then you can use the 50 ampere circuit breaker. Also, as Ray stated an arc welder (depending on the duty cycle), most motor loads that you would find applicable to a single phase circuit and some air conditioners.

 

Well thanks, then I look like a fool to the electrician when I insisted that #8 doesn't go with a 50A breaker... why is my Residential Wiring book telling me different... 10 with 30, 8 with 40, 6 with 50 etc??

 

For a couple of reasons. The primary one is that there are several different "Ampacities" for any given size wire depending on several factors. The most common factors are the insulation type and the temperature that will occur from either the ambient area or as a result of the current flow through the wire. The most common (at one time) of plastic wire insulation is type T and it is rated for a maximum operating temperature of 60[SUP]0[/SUP] Celsius. The T stands for thermoplastic insulation. The most common plastic insulation for individual conductors today is type THHN where the first T is as before, the first H stands for high temperature, the second H stands for extra high temperature and the N stands for a nylon covering. There are many different letter combinations to cover the different types of insulation.

Then there is the temperature of the connection itself. All (if I am not mistaken) terminals rated for 100 amperes or more are rated for use at 75[SUP]0[/SUP] Celsius and with the proper wire insulation allow for a greater Ampacity on conductors of size #8 and larger. In a very technical interpretation conductors smaller than #8 are also rated at a higher Ampacity but there is also an overriding footnote in the NEC that states something to the effect that ALL conductors of #10, #12 and #14 will be limited to Ampacities of 30, 20 and 15 respectively regardless of the insulation type or the terminal temperature ratings.

Then there are the special rules that apply to motor-driven equipment and to arc welders to further complicate the mess. Add in derating factors for more than three current-carrying conductors in a cable or raceway and also ambient temperature corrections and you see that the simple statements in publications geared to the layperson cannot always be trusted as the actual code.

 

So a fault between the main panel and the subpanel (this case #8 wire protected by 30A breaker) would trip the 30A breaker in the main.

Where a fault in the garage, at an outlet after the subpanel, would trip one of the breakers in the subpanel.

Is that correct?

Is there any circumstance where a fault in the garage would trip the breaker in the main panel?

 

Under normal circumstances..... no.
However, I have seen a larger breaker trip before a smaller one.