rooftop unit

[kingelectric]

3-480v 30amp rooftop units with 3 # 10s and one common # 10 ground wire in a 1'' conduit ,the inspector said we could not do this. Any code violation here??

 

[augie47]

can you provide a bit more detail ?
RTU Min Clt amps ? Does the conduit run on the roof ?
On the surface there seems to be no problem but the devil may be in the details.
Did the inspector state why it was not compliant ?

 

[kingelectric]

min circuit amps are 19.5 ,conduit is ran inside and sealtite through the bottom of the unit to the disconnect.

 

[Flooglebinder]

310.15 (B)(2)(c) means your wire is probably undersized

310.15 (B)(2)(c) means your wire is probably undersized

Good article about rooftop conduit installations here: http://www.ecmag.com/?fa=article&articleID=9401

This is the important part from the link, especially the last sentence in the paragraph:

The new text in 310.15(B)(2)(c) will result in much larger conductors where conduit is installed in direct sunlight on a rooftop. For example, if a conduit containing conductors supplying a 65-ampere continuous load (already calculated at 125 percent) is installed on a rooftop where the ambient temperature is 104?F using 2-by-4-inch redwood support blocks to support the conduit, the temperature adder would be an additional 40?F. Where normally a 4 AWG THWN copper conductor would be permissible (85 amps in the 75?C column of Table 310.16 with a 0.88 ambient temperature correction factor), the new correction factor would be 0.33, requiring a 3/0 copper conductor (200 amperes ? 0.33 = 66 amperes). This conductor size difference would certainly cause problems with wire terminations and enclosure sizing for a 100-ampere disconnect. The change in the 2008 NEC makes installing conduits in direct sunlight on rooftops prohibitive.

 

[infinity]

You said 3 units. Are these all 3 phase units and all 9 phase conductors are in one 1" raceway with a single EGC? Trying to decipher exactly how you have this setup because so far I can't see a problem.

 

[augie47]

nor can I. Did the inspector state why it was in violation ?

 

[cowboyjwc]

My guess would be that the inspector believes that 3 EGC's are required.

 

[squaredan]

Whats type of wire is being used ,THHN/THWN? what type of pipe is being used? single phase or 3 Phase? I am guessing 3 phase And are you feeding all three units with 3 # 10s or is each unit is getting that set up?

 

[btharmy]

the inspector said we could not do this

What specifically is "this" that you speak of? He should be able to narrow down what exactly he has a problem with, and then cite a code violation so you can correct it. So, what is the specific violation?

 

[kingelectric]

correct.3-3phase circuits with 3 # 10s thhn to each and 1-# 10 egc in a 1'' emt conduit, inspector said to many current carrying conductor and need seperate ground wire to each.

 

[augie47]

sounds as if you have encounter an inspector who may not know the NEC.
If you are using THHN, the derated value for the 9 wires (assuming no ambient problem) would be 28 amps.
With a 19.5 MCA, you would actually be legal with #12s ( #12 @ 90?C =20 x .7 = 21) (based on '08 ratings)

 

[infinity]

correct.3-3phase circuits with 3 # 10s thhn to each and 1-# 10 egc in a 1'' emt conduit, inspector said to many current carrying conductor and need seperate ground wire to each.

Inspector needs to spend some time here. IMO these conditions (derating and EGC's) are inspection 101.

 

[kingelectric]

same thing i was thinking,he needs to go back to class

 

[jusme123]

sounds as if you have encounter an inspector who may not know the NEC.
If you are using THHN, the derated value for the 9 wires (assuming no ambient problem) would be 28 amps.
With a 19.5 MCA, you would actually be legal with #12s ( #12 @ 90?C =20 x .7 = 21) (based on '08 ratings)

30 x .7 = 21

 

[augie47]

typo in '08, #12 90? = 30

thanks

 

[Dennis Alwon]

Although the inspector is incorrect I wonder if he is not using 90C or possibly is using 90C and sees an issue with fill. 40 amps*.7= 28 amps. He may not realize this can still be on a 30 amp breaker.