I have been down this road...you have to pay tax in installments if the tax you will owe is $1,800 or more. The amount of CPP is included in the installment payment but is not part of the factoring into whether or not you have to pay instalments.

The thinking is that people will make a little bit more money each year. With inflation and raises. This is not always true, especially for our job, but that doesn't matter.

Here is how it works. The CRA will estimate what your tax bill will be for 2015 and send you a notice in the summer, probably July 2015. They will divide that estimate (say $5,000) by 2 payments. Sept 15th and Dec. 15th. So $2,500 each.

Then, they will estimate what your whole tax bill will be for 2016 (say $5,500) and send you a notice in Jan. 2016. They will take that estimate and divide it in four instalments. (Mar. 15, June 15, Sept. 15 and Dec. 15) each for $1,375.

Things to know, even if they don't send you a notice, you are responsible for paying instalments if the tax part or the tax bill is more than $1,800 for a year. It is your responsibility to know.

Also, if you KNOW your tax owing will be less than $1,800 for 2015, you don't have to pay instalments. If you end up owing only $1,799, then there will be no penalty come tax time. However, if it's going to be close, and you can't figure out in advance how much you will owe, you are taking a chance.

Penalties are charged on each instalment that is late or doesn't cover the amount owing.

Money you send to CRA in 2016 cannot be used as an instalment for 2015. (Found that out the hard way.)

You can pay more often than just 4x per year if you want. Online banking makes this easier.

Oh, and btw, I don't know your accountant, but s/he should know all of this info and should have explained it to you.

Good luck!