Derating Short Exposure Feeders

[EE_Intern_2]

Hi,

I have a design question.

I have 225A 120/208V 4 Wire feeders going to a panel via a 50 foot (one way) undeground run.

after the 50 foot run the cables come up to a disconnect switch and are exposed to high ambient temperatures before they go into a building and feed the panel.

If only a short section of cable, say less than 5 feet, are exposed to the high ambient temperatures, do I have to derate the cables for the ambient temperatures for this small section, or can I ignore derating for the ambient temperatures?

I understand since this is a 120/208 4 wire system, that I must also derate the cables for 4 current carrying conductors.

Thank You

 

[iwire]

You should check out the exception after 310.15(A)(2)

 

[LarryFine]

And, the neutral need not be counted as a CCC.

 

[EE_Intern_2]

@ Iwire

I thought that Exception No. 4, the one that I believe that you are pointing to after 310.15 (A) (2) applies only to derating of more than three current carrying conductors Table 310.15(B)(2)(a) and had nothing to do with derating for ambient temperatures or adjustments for conduits exposed to sunlight on or above rooftops.

The exception mentions conductors entering or leaving a trench and being in conduit less than 10 feet in length.

My run is going to be underground and in conduit for the entire length of the run.

Do you think this exception still applies to ambient temperature derating?


@LarryFine

What are your reasons for not counting the Neutral as a CCC?

BTW most of the load is for an HVAC unit and CFL lighting. Is the HVAC load considered harmonic or non linear?

Thank you for your help gentlemen.

 

[dbuckley]

Irrespective of what code says, the science says that if the ambient is high enough and the cable temperature rise significant enough, then you'll have cables operating outside of their design parameters. So I see it as entirely possible that as a minimum you may need to use larger conductors or a conductors of different construction for the 5 feet of cable exposed to high ambient temperature.

 

[LarryFine]

@LarryFine

What are your reasons for not counting the Neutral as a CCC?

Because it only carries the imbalance among the phases, presuming proper installation. Any current the neutral carries must be current not flowing in a line conductor(s), so no more than three conductors' worth of current will exist to generate heat.

It still must be counted for conductor fill, of course.

BTW most of the load is for an HVAC unit and CFL lighting. Is the HVAC load considered harmonic or non linear?

The HVAC will not impact the neutral, and the lighting somewhat, but probably not enough to upsize it.

The HVAC, as mainly motor loads, will be non-linear, but that's taken into consideration when the MCA and MOC are specified. For load calcs, use the RLA.



Oh, by the way, welcome to the forum!

 

[iwire]

@ Iwire

I thought that Exception No. 4, the one that I believe that you are pointing to after 310.15 (A) (2) applies only to derating of more than three current carrying conductors

This exception directly addresses your question.

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.

Irrespective of what code says, the science says that if the ambient is high enough and the cable temperature rise significant enough, then you'll have cables operating outside of their design parameters.

The code is using the conductors on each side of the hot area as a heat sink.

 

[EE_Intern_2]

@ Iwire

"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. "


Does this mean that I am supposed to splice two different size conductors for the feeder circuit? and have the larger size ampacity conductors go for an additional 10 feet past the high ambient section to act as a heat sink?

wouldn't it be a better design just to use the higher ampacity cables for the entire run instead of splicing things together? the run is like 50-60 feet one way.



@Larryfine

Thank you for welcoming me. It's great that forums like these exist for us new engineers.

I saw in the NEC that it says to count the neurtal as a CCC for harmonic and non-linear loads.

I did a google search and it said that both HVAC and the CFL lights are harmonic, non-linear loads? The HVAC and the CFL loads make up the majority of the loads in my situation.

I am confused, in which scenario would I count the neutral as a CCC?

Thank you gentlemen.

 

[kingpb]

Don't forget you can use 90 deg C cable, and the 90 deg C ampacity column and then derate from there, as long as the final ampacity is not more than that stated in the 75 deg C column. That may get you over the hump so to speak.

 

[dbuckley]

The code is using the conductors on each side of the hot area as a heat sink.

That's pretty funky.

I'm not sure (based on no science whatsoever but just a gut feeling) I agree the distance, but its an interesting idea, and I've gotta assume someone with a tall forehead has done the math.