ECN Forum Forums General Topics of the Trade General Discussion Area Sizing HVAC circuits.

Other than calling the HVAC contractor for a cut sheet, is there a way to calculate breaker sizes and wire sizes for HVAC units such as roof top units?

Example: 20 ton RTU 480V 3 Phase
(Gas heat)

What should the breaker size be and what should the wire size be? Also, what will the running amps be? (I do realize that it may vary slightly from brand to brand.)

M-66,
Don't do it. There are a dozen calcs you can use, none of which will match the nameplate. The only time I'd feel comfortable with this is if their package stores, or something similar, and they use the same thing over and over.

OR.......you could run parallel 500s over to it......that oughta do it.

Now ya gotta decide to use fuses, or take a chance it'll accept an hacr breaker, ah go ahead and use fuses, their always accepted.

I know that this is going to sound wierd but, I don't like to see fuses on 3-Phase units for fear of "single-phasing"

As for the 500's, aluminum or copper?