Mhf

[theoldwizard1]

Under what conditions would 2-2-4-6 MHF be allowed/approved compared to 2-2-2-4 ?

 

[alfredeneuman]

You have to know the load on the feeder exactly beforehand and calculate how much current the neutral would carry in order to use the downsized neutral.

IMO, it's just not worth it just to save a little bit of $. Conditions change, and that's a lifetime decision.

 

[pattenp]

I believe in most cases if not all the #4 neutral is fine. The neutral can be sized at 100% of the max unbalanced load, so the #4 is good for 65A. Seeing how the feeder is rated at 90A you would be really pushing your 120V loads to reach the max neutral limit of 65A.

Here's some reading you can do. http://ecmweb.com/code-basics/characteristics-neutral-conductor

Edit: I've seen the 2-2-4-6 sell for more than the 2-2-2-4. So I agree with just using the 2-2-2-4.

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[theoldwizard1]

The neutral can [MUST] be sized at 100% of the max unbalanced load, so the #4 is good for 65A. Seeing how the feeder is rated at 90A you would be really pushing your 120V loads to reach the max neutral limit of 65A.

Thanks for the explanation. Seem perfectly logical. In a typical residential application, accurately calculating the unbalance load is nearly impossible ! However, unless you are silly enough to install your breakers in every other stab, it would be hard to exceed 65A.