Heater calculation

[Greg121305]

I have a 3 phase, 208, 15kw, 41amp reheat in an attic that can reach temperatures up to 133 degrees worse case. It's about a 60-70 foot run. It is considered to be continuous duty . So having that said how would I go about calculating for my wire size?

 

[Dennis Alwon]

Take 41 amps and multiply by 1.25 for continuous duty. Then use Table 310.15(B)(2)(a) and divide by the correction factor. Then look up the wire that matches that ampacity or higher

 

[drcampbell]

If it's 100-plus degrees in the attic, electric heaters shouldn't be running. Talk to the HVAC people and see if a more-sensible approach than reheat is possible.

 

[JFletcher]

erm, what's an attic reheat, or reheat in an attic?:?

and welcome to the forum

 

[drcampbell]

In days of old, when knights were bold and electricity was too cheap to meter, simultaneous humidity & temperature control was sometimes achieved by chilling air to ~40-45 degrees F to remove moisture, then reheating it to ~65-70 degrees before returning it to the room. The reheat element is inside the air handler; it's not intended to heat -- or reheat -- the attic itself.

It's every bit as inefficient as it sounds.

When you're actually paying for electricity, more-economical methods, such as blending chilled & warm air or matching the fan speed to the evaporator capacity, are usually preferred.

Reducing the summertime attic temperature with roof ventilation and/or radiant barriers is probably also a good investment.