Are there any tax implications for a Canadian employee, working for a Canadian Corporation, paid in Canadian dollars? Total time in the US is about 20% of the annual hours.
You should file a US tax return (1040-NR) with the IRS to report the portion of your total wages that relate to the work that you physically performed in the US (i.e. 20% of total wages in your case).
On your Canadian return, you will be taxable on your worldwide income. In order to avoid double taxation, claim a foreign tax credit on your Canadian return for the American taxes paid.