It depends on the ambient temperature, how many conductors in a cable or conduit, local code requirements, etc. About 6 AWG, with possible correction for these various conditions. Please consult your local regulations.
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Without knowing the original poster's location, it is not possible to advise further. Were the poster in the UK, then BS7671 covers this issue and many more, and the poster needs to be aware of it.
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"Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856