Multiple conductors and derating

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kbsparky

kbsparky

Senior Member
Location
Delmarva, USA
I am designing the installation of several circuits for multiple HVAC rooftop units.

It would be easier to run a single conduit, and install (8) #8 THHN conductors. This would supply power to (4) separate units.

The units have a min. circuit ampacity requirement of 36 Amps, Max breaker of 50.

According to the NEC, #8 THHN is rated for 55 Amps, before derating factors. I figure applying 70% to that amount equals 38.5 Amps. Which should be sufficient for the connected loads.

Section 440.22 allows me to use a 50 Amp breaker on these circuits.

Does anyone concur with my assessment?
 
Remember the a/c units have overload protection so the circuit breaker is only protecting from ground fault not overload.
 
Lcdrwalker said:
Remember about derating for rooftop temp. also...
Click to expand...

Of course. But that does not mean further derating is required in this case.

The conductors will all be in the same conduit from the load center to a junction box above the ceiling (below the roof) adjacent to the planned pitch pocket.

From that box, 3 sets of seal tight will pass thru the pitch pocket to the roof, and terminate in their respective rooftop mounted disconnects. The freon line sets will all be installed in the same pitch pocket as well.

You see, there won't be more than 3 conductors present in each line after the junction box, so cumulative derating won't apply ... :happyyes:
 
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