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would being added to spouses credit card help my credit if her payments have always been on time and has a less than 30% balance?
That would depend on the age of the account and the %util. < 10% util would be better and if you are revolving balances 0% would be ideal.
All history wil be reflected in your CR and scoring, so other factors such as AAoA and credit mix, will also be impacted.
Is your current % util higher than hers, and expected to remain so?
Is it older than your current AAoA?
Do you currently have other revolving, or will this be your only revolving account?
If the account history results in a net positve, then it will help your FICO score.
However, apart from improvement in your three-digit score, how it will affect your overall credit is also dependent upon the extent to which any creditor may choose to discount your score if it includes credit history that is not yours. That is the potential downside of addition of an AU account.