Aluminum wire - rephrased

[Al D. Bolt]

According to NACHI Online Ed., when using aluminum wire a 30A breaker requires wire size #8, a 20A #10.... Am I correct in believing a 25A would also require #8?

Anybody have any documentation regarding what a 25A requires?

An electrician quotes the NEC (200 that #10 aluminum wire is good for 25A (60 degree C) & 30A (75 degree C) breakers. This does not correspond with any of the other aluminum wire/breaker charts I have. Any help in this?

Note: In the photo you will see breakers manufactured by Bryant. On my report I recommended an electrician evaluate further the Bryant breakers and he said they are OK.

[Michael Larson]

Circuit breakers do not require a particular wire size.

A particular wire size and material does require protection not to exceed the specified value for the wire size and material used

[Paul W. Abernathy]

It is important to understand what you are asking. If you want to know the "ampacity" of a given conductor you would look at the 90 degree column. However, since we are limited to the ratings of the terminals if known or the allowances of 110.14[C][1]. So this can be a loaded question depending on the answer being seeked.

based on the 2008 NEC the Ampacity of a 8 AWG AL is 45 Amps. But of course terminals are usually limited to 60 or 75 degree's so you have adjust it based on that information. So in reality lets say your terminals are 75 degree and the terminals of the piece of equipment is 75 degrees....the adjusted ampacity is then 40A based on 75 degrees.

In your question you pose a 25A ampacity need...based on lets say a piece of equipment. With that said we have to assume 75 degree terminals so the size conductor being needed would be 10 AWG AL....since under the 75 degree column it is good for 30 Amps....Right........Right......

Then of course you need to check 240.4[D] which in the 2008 NEC reminds us that the overcurrent protection for the 10 AWG AL can't exceed 25A OCPD unless of course you bring in the allowances of 240.4[E] or [G] like for a motor in which you could use the ampacity of 310.16.

Hope this was a little helpful....Gotta Run !

[Paul W. Abernathy]

Al,

When you see 8 AL listed as 30A you have to understand that when no information if given on terminals for example you have to use the 60 degree column and indeed under the 60 degree column an 8 AWG would only be good for 30A.

This is why it is so important to understand how the columns apply and knowing the terminal ratings and what to do if you dont know...if you dont know then you have to use the tables and degree columns as referenced in 110.14[C][1] unless otherwise specified.

Hope that clears it up for you.