Conduit Exposed to Sunlight table 310.15.(B)(3)(c)

[KarlS]

I am running RGS conduit up to the roof of a metal building and across to an HVAC system. I am running 3#4 with gnd (Thwn) from a 70A breaker. My load for the HVAC is (63.1A) with recommended MCB at 70A from the manufacture. It looks like I need to derate the cable based on table 310.15(B)(3)(c). I have a very short run across the roof to the HVAC unit. I see min distance requirements in the article. Am I missing something ? It looks like I will need to change to 3#2 base on detrating. The building will be located in Corpus Christi Texas. Design temp would be about 98 degrees.
Thanks in advance for the help!

 

[texie]

I am running RGS conduit up to the roof of a metal building and across to an HVAC system. I am running 3#4 with gnd (Thwn) from a 70A breaker. My load for the HVAC is (63.1A) with recommended MCB at 70A from the manufacture. It looks like I need to derate the cable based on table 310.15(B)(3)(c). I have a very short run across the roof to the HVAC unit. I see min distance requirements in the article. Am I missing something ? It looks like I will need to change to 3#2 base on detrating. The building will be located in Corpus Christi Texas. Design temp would be about 98 degrees.
Thanks in advance for the help!

Before getting into calculated ampacity due to factors such as ambient temps, I'm wondering if you are saying that your unit has an MCA of 63.1 and a MOCP of 70? This seems an unlikely combination. The nameplate or the spec sheet will have the exact MCA stated and this is what you base the ampacity of the conductors on-not the breaker size.

 

[kwired]

I am running RGS conduit up to the roof of a metal building and across to an HVAC system. I am running 3#4 with gnd (Thwn) from a 70A breaker. My load for the HVAC is (63.1A) with recommended MCB at 70A from the manufacture. It looks like I need to derate the cable based on table 310.15(B)(3)(c). I have a very short run across the roof to the HVAC unit. I see min distance requirements in the article. Am I missing something ? It looks like I will need to change to 3#2 base on detrating. The building will be located in Corpus Christi Texas. Design temp would be about 98 degrees.
Thanks in advance for the help!

I agree with texie, those figures seem a little abnormal.

Next question is how did you arrive at 98 deg design temp - especially in Texas?

I could see the design temp of other than what is on the roof being 98 degrees.

If you only have a short distance on the roof consider what is said in 310.15(A)(2) exception:

(2) Selection of Ampacity. Where more than one ampacity applies for a given circuit length, the lowest value shall be used.

Exception: Where two different ampacities apply to adjacent portions of a circuit, the higher ampacity shall be permitted to be used beyond the point of transition, a distance equal to 3.0 m (10 ft) or 10 percent of the circuit length figured at the higher ampacity, whichever is less

If the portion of the circuit on the rooftop is less than 10 percent of the circuit length you can use the minimum ampacity necessary for the remainder of the circuit. I think the idea here is the lower temp portion of the conductors can sink the heat from the higher temp portion of the circuit.

 

[KarlS]

Kwired that is exactly what I was looking for. I did not look far enough back to 310.15 (A)(2). I have the NEC workbook which makes for many pages of explainations.

To clarify the other information, I checked the 20ton York unit nameplate. The MCA is 63.10 amps and MOCP is 70 amps. I know it doesn't seem likely because 63A x 125% is about 78amps. I deal with HVAC units alot and this is typical. We usually wall mount and pad mount these units and cable size is not a problem. We have HVAC vendors doing the work but we need to supply the control and power conductors. Regarding temp, if you guys couldn't help me find a length min then I needed to look up the exact temp for the area. I was just using 98 as the ambient to get my point across. NEC actually points you to ASHRAE for the site temp and gives a great example on how to do the calc.

 

[david luchini]

I know it doesn't seem likely because 63A x 125% is about 78amps.

The MCA already includes 25% of the largest motor, so there is no need to multiply 63 by 125%.

 

[kwired]

Kwired that is exactly what I was looking for. I did not look far enough back to 310.15 (A)(2). I have the NEC workbook which makes for many pages of explainations.

To clarify the other information, I checked the 20ton York unit nameplate. The MCA is 63.10 amps and MOCP is 70 amps. I know it doesn't seem likely because 63A x 125% is about 78amps. I deal with HVAC units alot and this is typical. We usually wall mount and pad mount these units and cable size is not a problem. We have HVAC vendors doing the work but we need to supply the control and power conductors. Regarding temp, if you guys couldn't help me find a length min then I needed to look up the exact temp for the area. I was just using 98 as the ambient to get my point across. NEC actually points you to ASHRAE for the site temp and gives a great example on how to do the calc.

What is voltage and aux loads? I am working on installing line to 2- 20 ton units right now. They are rated MCA - 108 and MOCP - 125. They are on a 208 volt system, and have two compressors, indoor and outdoor blower motors all in one unit.

 

[KarlS]

The Voltage is 480V. THe units are York standard factory units. No aux loads etc. The descripton reads- two stage cooling, 36KW heat, man fresh air damper, 5hp static belt drive blower. Nothing fancy. I actually have three HVAC units on the roof of a metal interlocking (PDC) industrial building. I have three 70A breakers feeding them. We also use alot of Specific systems custom units and the MCA goes up.

Great help! Since my total RGS conduit is 14ft and 4ft is on the roof it looks like I will need to do the calcs and derate. A little bigger cable will be fine.