who derates conduit on a roof......?

who derates conduit on a roof......?


  • Total voters
    20

Fulthrotl

~~Please excuse the mess. Sig under construction~~
there is that spiffy derating that now takes place with
conduit on a roof, depending on how close to the roof
the conduit is....

THWN #6 copper stranded is rated for 65 amps,
according to my spiffy iphone app.....

in conduit 6" above the roof, that becomes 48.8 amps.
it's not till you get to 37" above the roof you get the
full rating back.

who's derating, and who's not? i'm just curious....
maybe i should make a poll... never done one of those before.... :dunce:
 
I can't respond to your poll given the choices you you gave, pity since it is your first.
I can give you an honest answer - I wouldn't if I didn't have to.
I haven't been on all of the rooftops in my fair city, but I have been
on a lot of them. No conduits more than four inches above the deck. No failures to report.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
This is an example where a "worst case" study funded by a party with an economic interest was used to write a code rule. There was never any real world examples of a problem with all of the past installations that were made without derating.
 

renosteinke

Senior Member
Location
NE Arkansas
I've applied the derating several times, and the end effect has been : no change in wire size.

How can that be? Well, the calculations start in the 90 degree column, while you size starting with the 75 degree column. That gives you plenty of 'excess amps' to calculate away. There's also the matter of breaker sizing for the smaller wires: the calculations for #12 start at 30 amps, but you can't use more than a 20 amp breaker in most cases.

I am a bit bothered by the application of the rule, though. Does it matter WHEN the load is used?

Let me describe one customer. This customer had combined several condo units into a single home. The end result was that the panel was as far as possible from the yard. When he added air conditioning and an outdoor hot tub, the easiest routing was up the wall, across the roof, and down the other side. Two pipes in the sun.

Now, pipe #1 was for the air conditioning- which was very likely to be fully loades when the sun was brightest. No question about ambient de-rating there.

Pipe #2 was for the jacuzzi panel - and the tub was certain to be used after sunset, and most likely in cooler weather. In short, the tub would be used when the rooftop ambient was a good 40 degrees cooler (or more) than when the air conditioners are used.

I do not see any code provision that would allow you to consider these different ambients.
 

Fulthrotl

~~Please excuse the mess. Sig under construction~~
This is an example where a "worst case" study funded by a party with an economic
interest was used to write a code rule. There was never any real world examples
of a problem with all of the past installations that were made without derating.
my thoughts exactly.
and of course we do what we have to do to comply with code....

my poll question probably should have been....
"do you think this is absurd and silly?"
 

iwire

Moderator
Staff member
Location
Massachusetts
I've applied the derating several times, and the end effect has been : no change in wire size.

How can that be? Well, the calculations start in the 90 degree column, while you size starting with the 75 degree column. That gives you plenty of 'excess amps' to calculate away. There's also the matter of breaker sizing for the smaller wires: the calculations for #12 start at 30 amps, but you can't use more than a 20 amp breaker in most cases.

I am a bit bothered by the application of the rule, though. Does it matter WHEN the load is used?

Let me describe one customer. This customer had combined several condo units into a single home. The end result was that the panel was as far as possible from the yard. When he added air conditioning and an outdoor hot tub, the easiest routing was up the wall, across the roof, and down the other side. Two pipes in the sun.

Now, pipe #1 was for the air conditioning- which was very likely to be fully loades when the sun was brightest. No question about ambient de-rating there.

Pipe #2 was for the jacuzzi panel - and the tub was certain to be used after sunset, and most likely in cooler weather. In short, the tub would be used when the rooftop ambient was a good 40 degrees cooler (or more) than when the air conditioners are used.

I do not see any code provision that would allow you to consider these different ambients.
You must have a different. NEC than i do as derating hits me as soon as i am beyond a 30 amp circuit
 

renosteinke

Senior Member
Location
NE Arkansas
Now, Bob, youknow better than that. You know that rooftop temps are only a tiny slice of the 'derating' pie, and that not every circuit is sized the same way.

FWIW, the circuits I used in my example were well over 30-amps. The example stands. The air conditioners are quite likely to draw their fullly-rated 42 amps through those rooftop wires on the hottest, sunniest days of the year- while that hot tub isn't likely to be drawing the full heater load (50 amps) for any period of time untill the New Years' 'after the party' party ... where slipping on the ice is a real peril!
 

Stevareno

Senior Member
Location
Dallas, TX
I voted no, because I usually don't have to worry about that scenario.
When I do, it's usually just a roof penetration for an A/C RTU. So far, never been red tagged on that.
 

iwire

Moderator
Staff member
Location
Massachusetts
Now, Bob, youknow better than that. You know that rooftop temps are only a tiny slice of the 'derating' pie, and that not every circuit is sized the same way.

FWIW, the circuits I used in my example were well over 30-amps. The example stands. The air conditioners are quite likely to draw their fullly-rated 42 amps through those rooftop wires on the hottest, sunniest days of the year- while that hot tub isn't likely to be drawing the full heater load (50 amps) for any period of time untill the New Years' 'after the party' party ... where slipping on the ice is a real peril!
I would love to see the calculations to support your post.
 

bullheimer

Senior Member
Location
WA
i would think that going one size up in wire for the distance from that panel, to avoid any voltage drop, (std practice, isn't it?) would eliminate the need. but still, you have to calc it out, as posted, there are inspections. how long does it take to figure it out? 430 gives you so much lee way.
 

Fulthrotl

~~Please excuse the mess. Sig under construction~~
i would think that going one size up in wire for the distance from that panel, to avoid any voltage drop, (std practice, isn't it?) would eliminate the need. but still, you have to calc it out, as posted, there are inspections. how long does it take to figure it out? 430 gives you so much lee way.
it takes about ten seconds to figure it out...
there's an app for that.

http://www.carry-onsoftware.com/page2/page2.html

the electrician's helper is about the best app for sparkies i've found.
put in the particulars, and the distance above the roof the horizontal
run is at, and it gives you the answer.
 
Top