Write SQL query to solve given problem: Among the coaches whose team has over 30 wins in a year, how many of them are born in the USA?. Keep the solution inside sql tag ```sql [SQL-Query] ```
SELECT COUNT(T2.coachID) FROM Master AS T1 INNER JOIN Coaches AS T2 ON T1.coachID = T2.coachID WHERE T2.W > 30 AND T1.birthCountry = 'USA'