Need help with electrical Question??

My friend is currently upgrading his 100 Amp fuse box in our small house to a breaker box. There is, I believe, #4 copper or #2 Aluminum service from a 100 amp meter setting with a max 240 V meter. What is the highest amp breaker box he can go with? He needs 20 Spaces. Is an undersized service the hazard or oversized, or both? Any help on these questions would be wonderful.

With the info from your post it is an undersized service that poses a hazard. The #4 copper would be too small for a 100 amp service. It should be at least a #3 copper. If it was a #2 aluminum, that would be ok for a 100 amp service. The panel itself is usually rated for 125 amps, but comes in many different sizes. They can hold 12, 16, 24, 32, 48, all the way up to 64 breakers depending on the brand you purchase. If the meter is only rated for 100 amps, that appears to be the smallest link in the chain, which would determine the maximum size of disconnect breaker it would allow. BUT…make sure its not a #4 copper supply as that would only be enough for a 70 amp system. It’s always determined by the smallest rated link in the system, and does’nt matter if other components are rated higher than that.

I’m not the electrical wizard by any means, but this is how I understand it. I’m sure if I’m incorrect one of the specialized fellows will correct us.

Darrell, the info you provided may be correct for Canada, but the OP is in the US so not all of it is accurate.

#4cu (or #2al) is standard issue for a 100 amp service in the US. ALso, our panels are limimited to 42 circuits/spaces.

Ryna, the largest panel you could install would be a 32 space Square D QO series. I don’t know of anyone else who makes a 32 space or larger.
Typical would be a 20 space, or 20/24, or 24 space. Depending on what is available in your area.


Thanks for reminding us that CAN and US codes are similar but sometimes different in important areas for US inspectors.

A Green box for you my friend.

Thank you for the information!! I will send this link to him!
So, Can he put a 125 amp breaker on the #2 al wire coming?? Thanks everyone. I love this NACHI family!

No, the # 2 AL is only rated for 100A ( 310.15.B.6 )…in order to place a 125A breaker on it ( if we are talking about the main breaker ) then it would need to be a 1/0 AL… again if we are talking aluminum conductors.

What Paul said.
The panel may very well be rated at 125 amps, but the main breaker must be 100 (or less).