Wiring vent/light/heater combo in bathroom

I am wanting to replace my light/vent combo in my bathroom to a light/vent/heater combo. My curent switch runs the light and vent at the same time.
How would I wire the new heater cobo? Will I have to run a new circuit or can I just use the existing wire?
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Don wrote:

You can use the existing wire *IF* you are willing to have the fan and light and heater all come on at the same time *every* time. But...I would look closely at the heater rating and see if the wire size is adequate. You'd have to calculate what other loads are already on that circuit too.
Doesn't sound practical to me... Jim
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Most Heat/Vent/Light units are 1200-1500 watt heaters and the manufacturers state "Use on dedicated circuit only."
If you're gonna go through with this, now would be a good time to bring in 2 -20a circuits, 1 for the heater and the other for a new 20a bathroom GFCI outlet.
H-V-L units (w/o the "night light") require a 2-wire cable and a 3-wire cable to have seperate switches for each function.
I used a 12/2 for the heater on a spring-wound "Broan" timer, on it's own circuit, and a 14/3 from a local lighting circuit for the light & fan. The fan is also on a timer, and the light comes on with the vanity lights.
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