Conductor above rooftop ampacity

[Jpflex]

This NEC practice test question makes no sense as can be seen in the photo, the roof installed AC unit requires a 60 AMPERE 240 VOLT supply

But then says the unit has a nameplate rating of 45.5 amperes and will operate at a rooftop reaching 145 degrees free height but the conduit with THWN-2 wires will be installed 40” above the roof

(so no temperature adder for wires above 7/8” of the roof)

So are they wanting a 60 ampere feeder to a disconnect and then asking for the minimum branch circuit size?

If the unit required a 60 ampere circuit then we just stop here and find a THWN-2 wire capable of 60 amperes after temperature correction

Or as what I think what they are asking for is the minimum size branch circuit tapped or coming from the 60 ampere feeder.

This would be #6 THWN-2 wire rated for 75 amperes at 90 degree

Then ambient derating of 0.65% (with ambient temp 145 degree) is 0.65 x 75 amperes = 48.75 amperes so this is what the wire is good for?

But wire under 100 amperes should be rated for 60 degrees?

 

[Jpflex]

Picture did not upload

 

[don_resqcapt19]

This NEC practice test question makes no sense as can be seen in the photo, the roof installed AC unit requires a 60 AMPERE 240 VOLT supply

But then says the unit has a nameplate rating of 45.5 amperes and will operate at a rooftop reaching 145 degrees free height but the conduit with THWN-2 wires will be installed 40” above the roof

(so no temperature adder for wires above 7/8” of the roof)

So are they wanting a 60 ampere feeder to a disconnect and then asking for the minimum branch circuit size?

If the unit required a 60 ampere circuit then we just stop here and find a THWN-2 wire capable of 60 amperes after temperature correction

Or as what I think what they are asking for is the minimum size branch circuit tapped or coming from the 60 ampere feeder.

This would be #6 THWN-2 wire rated for 75 amperes at 90 degree

Then ambient derating of 0.65% (with ambient temp 145 degree) is 0.65 x 75 amperes = 48.75 amperes so this is what the wire is good for?

But wire under 100 amperes should be rated for 60 degrees?

Yes, unless the terminations are listed for 75°C you use the 60°C ampacity, however, all ampacity adjustments and/or ampacity corrections are applied to the 90°C ampacity for the conductor specified. Had the ampacity correction factor been .87, the resulting ampacity would have been 65.25. In that case you would be limited to the smaller of the adjusted and/or corrected ampacity and the ampacity shown in the 60°C column if the equipment was not marked for 75°C ampacities.

 

[augie47]

As shown in the explanation the #6 THWN-2 is a 90° conductor and the derated value is 48.75 so that is the allowed ampacity,
The 60° amapcity for the #6 is 55 amps so that's well above the calculated 48.75
If the terminations restrict you to the 60° rating then 55 amps would be the max load allowed, but you are only required 48.75
All is good,.

 

[Jpflex]

As shown in the explanation the #6 THWN-2 is a 90° conductor and the derated value is 48.75 so that is the allowed ampacity,
The 60° amapcity for the #6 is 55 amps so that's well above the calculated 48.75
If the terminations restrict you to the 60° rating then 55 amps would be the max load allowed, but you are only required 48.75
All is good,.

I guess more of my question was whether the question was asking for a wire for the 60 ampere supply or nameplate 45.5 ampere load. The structure of question was confusing

I beleieve they meant size the branch circuit for 45.5 amperes but not the 60 ampere feeder?

 

[wwhitney]

This NEC practice test question makes no sense as can be seen in the photo, the roof installed AC unit requires a 60 AMPERE 240 VOLT supply

But then says the unit has a nameplate rating of 45.5 amperes

I'm going to say that's some really bad wording for MOCP = 60A, MCA = 45.5A.

Cheers, Wayne

 

[Jpflex]

As shown in the explanation the #6 THWN-2 is a 90° conductor and the derated value is 48.75 so that is the allowed ampacity,
The 60° amapcity for the #6 is 55 amps so that's well above the calculated 48.75
If the terminations restrict you to the 60° rating then 55 amps would be the max load allowed, but you are only required 48.75
All is good,.

You are installing a conduit on a rooftop to supply power to an air conditioning unit. The unit requires a 60 ampere 240 volt supply, with a nameplate load amperage of 45.5A. The rooftop will regularly reach 145°F in the summer, as you are installing above dark rooftop material, though your conduit will be 40" above the rooftop. What is the minimum size THWN-2 copper conductor that you can use?
	​	a. #8
		b. #6
	​	c. #4
	​	d. #3

 

[Dennis Alwon]

Often times the max overcurrent protective device is larger than the circuit that feeds the unit. Imo all you need is a conductor that is good for 45.5 amps or better and you can still use a 60 amp ocpd

 

[augie47]

As wwhitney states it's a bad question.
"The unit requires a 60 ampere 240 volt supply" is, IMO, misleading or, at best, incomplete.
If the MCA is 45.5 then that's the requirement for the conductor.