Derating for pipe fill and ambient

[monkey]

I have a conduit run where I must use a 105-113 degree correction plus an 80% deration for pipe fill. Do I use both factors, which will result in over 40%, or can I use just the largest of the two?

 

[winnie]

You must use both, however they multiply rather than add.

So for example, if you have 75C rated wire in the 105-113 ambient, the derating factor is 0.82
If you have 4-6 current carrying conductors in the raceway, then the adjustment factor is 0.8

The net ampacity adjustment is 0.82 * 0.8 or 0.66

-Jon

 

[monkey]

Thanks Jon, one more question, if you were sizing residential feeders using table 310.15(B)(6) , how would you go about adjusting for ambient?

 

[iwire]

Good question and I do not see it addressed.

It does strike me that as this table is limited to dwelling units that the cable would not be exposed to high ambient temps.

 

[monkey]

Yes except we have many houses out here with flat roofs and no crawl space. So when a service gets re located sometimes the only choice is over the roof (from the meter to the panel).

 

[iwire]

So when a service gets re located sometimes the only choice is over the roof (from the meter to the panel).

I see what your saying.

However I don't see any NEC requirement forcing us to make ambient adjustments on that table.

If I felt like I should do that I would simply jump over to 310.16 and pick my conductor from there.

 

[tallgirl]

Good question and I do not see it addressed.

It does strike me that as this table is limited to dwelling units that the cable would not be exposed to high ambient temps.

Come to Texas. Ambient attic temperatures come summer can easily exceed 55C / 135F and it can be that hot all the way down to the top of the joists, much less up in the ridges where people here run powered attic vents.

 

[acrwc10]

Come to Texas. Ambient attic temperatures come summer can easily exceed 55C / 135F and it can be that hot all the way down to the top of the joists, much less up in the ridges where people here run powered attic vents.

That would keep me out of the attic.

 

[iwire]

Come to Texas. Ambient attic temperatures come summer can easily exceed 55C / 135F

I have no doubt of that whatsoever.

However the NEC does not address this situation when using Table 310.15(B)(6).

It becomes strictly a design issue.

I am not a fan of Table 310.15(B)(6) anyway, if the NEC knows these size cables are fine because the service calcs are to conservative than the service calcs should be changed to become more accurate and not so inflated.

 

[winnie]

As iwire says, there is nothing about adjustment or derating in 310.15(B)(6).

As the code is written, is 310.15(B)(6) permits the use of the specific tabulated conductors for the specific feeders ratings, without regard for any of the normal derating or adjustment factors.

I would argue that 310.15(B)(6) supplants any of the other requirements of 310.15 with regard to conductor ampacity, but does not supplant other parts of Article 310, eg. 310.10 "Temperature Limitations of Conductors". According to 310.10, you cannot use a conductor in such a manner as to exceed the temperature rating of the insulation. Normally, if you follow the rules of 310.15 you will meet the requirement of 310.10.

But 310.15(B)(6) permits the use of conductors without accounting for ambient temperature, which could mean that the conductors might exceed their temperature rating, if for example fully loaded during a hot day.

IMHO you are still required to prevent the conductors from exceeding their maximum temperature. The problem is that you are now left without any tools for calculating a conductor ampacity that is also in accord with the requirements of 310.10, except, of course, "engineering supervision"

-Jon

 

[winnie]

I am not a fan of Table 310.15(B)(6) anyway, if the NEC knows these size cables are fine because the service calcs are to conservative than the service calcs should be changed to become more accurate and not so inflated.

I agree with you big time.

Personally I believe that 310.15(B)(6) should at a minimum be replaced with something that uses conductor ampacity rather than specific conductor sizes. This way adjustments and derating factors would need to be included.

Going further, I believe that service calculations should give two separate numbers: OCPD trip rating and minimum circuit ampacity. There are many situations where the OCPD trip rating is permitted to exceed the ampacity of the conductors protected; 310.15(B)(6) implies that this is the case for residential services, why not make this explicit.

-Jon

 

[don_resqcapt19]

Jon,

I believe that service calculations should give two separate numbers: OCPD trip rating and minimum circuit ampacity. There are many situations where the OCPD trip rating is permitted to exceed the ampacity of the conductors protected; 310.15(B)(6) implies that this is the case for residential services, why not make this explicit.

I don't that is the case here. In the other cases where the OCPD is sized larger than the conductor ampacity, the OCPD is only providing short circuit and ground fault protection for the conductors, the overload protection is provided at the load end of the conductors by another device. The numbers in 310.15(B)(6) are based on actual residential loads as seen by the utilities and is based on utility numbers from many years ago. The reason it works without problems is the fact that Bob stated....the calcuations in Article 220 are very conservative and resulted in an inflated load calculation.
Don

 

[winnie]

Jon,

I don't that is the case here. In the other cases where the OCPD is sized larger than the conductor ampacity, the OCPD is only providing short circuit and ground fault protection for the conductors, the overload protection is provided at the load end of the conductors by another device.
Don

Is this always the case?

I know that this is true for motors, but I am not sure that it is true for welders.

-Jon

 

[Pierre C Belarge]

If you have more than 3 current carrying conductors (based on your OP), than Table 310.15(B)(6) is not to be used.
If you are paralleling conductors and have 2 sets of 3-wire, than I believe that you will still be required to meet 310.15(B)(2)(a).

The 80% is for the "more than 3 CCC" and if you are using a 90C rated conductor, than your temp correction factor would be an additional 87% based on your OP.

You can perform the 2 ampacity corrections either one first, but you will need to perform both... and yes it does equate to quite the adjustment.

 

[iwire]

If you have more than 3 current carrying conductors (based on your OP), than Table 310.15(B)(6) is not to be used.

I disagree. The limitation is for a 120/240 volt 3 wire service, not a 3 conductors in the raceway or cable. If it did limit us to 3 conductors than that table could never be used for a feeder which it clearly states it can used for a feeder.

The 80% is for the "more than 3 CCC" and if you are using a 90C rated conductor, than your temp correction factor would be an additional 87% based on your OP.

The NEC does not require that we make temperature corrections when using Table 310.15(B)(6).

If you feel that it is required can you point out the section requiring it?

 

[Pierre C Belarge]

The first sentence in my last post is absolutely correct. The first sentence is not related to the second sentence I posted, other than just being in front of it.

The 3 wire service in regards to 310.15(B)(6) is not including the EGC, so why could you not use it for the feeder? The fourth conductor I think you are alluding to is the EGC.

Where in the code do we see the wording that gives us relief from using the temperature correction table under 310.16?



As far as paralleling this type of service, I was throwing that statement out to see what responses I would get back.

 

[iwire]

Pierre I stand by comments.

The first sentence in my last post is absolutely correct.

No disrespect intended but IMO it is absolutely incorrect.

'3 wire' is describing the type of system supplying the dwelling unit. It is not a limitation as to the number of conductors in any raceway or cable in any installation.

It is just telling us the voltage and system configuration that the table applies to.

Where in the code do we see the wording that gives us relief from using the temperature correction table under 310.16?

I think a better question is where in the code do you see anything telling us to apply notes and corrections from one specific table to any other table in the code?

Those correction factors apply strictly to the table they are located in.