Heat Lamps

I have 7 piglets that were born approx all middle of October. I live in Northern WI, and, right now, the average temperature is in the 30s (F). They have a heated waterer, and I make them heated vegetable mashes everyday for food. My question is, when should I start using a heat lamp to keep them warm? None have shown any signs of sickness, but I don’t want them to freeze to death. Also, they get a bundle of fresh hay every 2-3 days to burrow in. Thank you!

Hi Deanna;  Welcome to Permies!
So you have seven piglets. Is there a mom or are they alone?
What kind of building are they in?   Dry , out of the wind?
Hay is OK as bedding but straw is far superior. Straw stalks are hollow and they hold heat much better than a solid stalk of hay.
As far as heat lamps.  In norther Wisconsin I suspect you have some weather coming in the next  3 months where it gets much colder.
I suggest setting up your lamps and then day by day decide if you want to turn them on.
They have plenty of water, you are feeding them warm food.   Do they have dry grain to snack on between feedings?
For winter raising I suggest both your warm chow and some dry pig feed as well. Piggy's and humans need to eat if its cold and snowy outdoors!
Keep us posted.
We like photos.  

Heating pads are much more efficient and safer, much less likely to start a fire.  Fleet and farm used to sell them.

Thank you! I must say first, that I really enjoy Permies, and use some of the advise on my farm currently and hope to use it in future endeavors.
The piglets are alone. They are in a wooden structure that is dry, and closed off from the wind. I will look for straw and add that to their bedding for sure.
I do not feed dry grain snacks, but am now planning to as that makes sense that they'll need a higher caloric intake to sustain in the winter.
I will post pictures soon!

Beware of the metal fixture  for heat lamps.  I go with fire resistant ones.   There are a good many variables in the answer, but I go with 32 degrees. Depending upon the size and if mom is with them, you can drop down below that. Of course, if the piglets are small, you might run with warmer temp. Then, the breed of pig can also come into play. I have Kunekunes, and they tend to tolerate colder weather.