Setting up rel="alternate" hreflang links: x-default vs EN?

[onlinesource]

If somebody has a better subject to describe this, feel free to change it.

So I have a .ca store and a .com store. Both are in English. Currently I have.

<link rel="alternate" hreflang="x-default" href="https://www.example.com" />
<link rel="alternate" hreflang="en-CA" href="https://www.example.ca" />

I've been told by people that you can't have en-ANYTHING without EN?

So should I be using:

<link rel="alternate" hreflang="EN" href="https://www.example.com" />
<link rel="alternate" hreflang="en-CA" href="https://www.example.ca" />

instead?

Others say, you need both, because .com is your x-default and EN so:

<link rel="alternate" hreflang="x-default" href="https://www.example.com" />
<link rel="alternate" hreflang="EN" href="https://www.example.com" />
<link rel="alternate" hreflang="en-CA" href="https://www.example.ca" />

Both sites are in English, so do I even need x-default? I mean if both sites are in English, the .ca site covers English-Canada and the .com site covers all remaining English, what is the purpose of the x-default? What is going to default to it, all languages remaining?

[edited by: Robert_Charlton at 7:13 am (utc) on Feb 16, 2017]
[edit reason] changed mysite.tld to example.tld [/edit]

[rainborick]

Plain "en" is acceptable. See Google's advice on hreflang [support.google.com].

Geographic Targeting is going to largely resolve this for you anyway. Your .ca site is going to be geo-targeted to Canada, which pretty much resolves the "en-ca" issue. If your sites are both hosted in Canada, you should make sure to use the Geographic Target tool in the Google Search Console to target another country - whatever your biggest target market might be outside of Canada.