How to wait for a frame to load before locating an element?

There are a couple of things you need to consider :

The line of code to switch to the frame looks perfect which doesn't throws any error :

var wait = new WebDriverWait(driver, 15);
wait.Until(ExpectedConditions.FrameToBeAvailableAndSwitchToIt(By.Id("frameA"));

In the next line you have tried the ExpectedConditions method ElementExists. As per the API Docs ElementExists Method is defined as :

An expectation for checking that an element is present on the DOM of a page. This does not necessarily mean that the element is visible.

Selenium can't interact with elements until the element is visible. Hence you need to use the method ElementIsVisible as follows :

var wait2 = new WebDriverWait(driver, 15);
wait2.Until(ExpectedConditions.ElementIsVisible(By.Id("elementA")));

Here you can find a detailed discussion on Ways to deal with #document under iframe

You can wait for the frame itself to be clickable:

wait2.Until(ExpectedConditions.ElementExists(By.Id("YOURFRAMEID")));