Derate cables - solar pickup 4

[JGoldberg]

When running power cables outdoors exposed to the sun, is there an additional ampacity derating factor due to solar energy pickup? The NEC only addresses ambient temperature considerations for ampacity derating, not the extra energy absorbed by the cable due to solar energy radiation.

 

[peebee]

There's nothing in the NEC that I'm aware of that specifically addresses solar heat loading beyond the considerations you mentioned for ambient temperature. Some cities have specific derating requirements, though -- Phoenix, for example, requires derating to 82% of the 75 degree ampacity.

Assuming you're using 90 degree wiring, the limiting factor will most likely be the temperature rating of the terminations, not the wire.

 

[ElectroKing]

Go with the insulation rating and insulation type. Nromally 75 deg C works well for outdoors. But you must use raceway or channels for routing.

 

[peebee]

Regarding: "But you must use raceway or channels for routing" -- why? Why couldn't other cables rated for outdoor use, such as type SE, sunlight-resistant-listed TC, or any other conductor meeting 310.8(D) and other relevant sections of the NEC be used in this application?

 

[cuky2000]

Dr Goldberg:

The NEC does not address directly the effect of solar radiation for the ampacity of power cable. However, permits the use the Neher-McGrath (NM) formula as state on article 310-15.

Please notice that this formula is a simplify version of the NM formula 9A. For outdoors cables application this formula contain an additional term ([sub]DELTA[/sub]T[sub]ini[/sub]) that account for the temperature increase do to the sunlight radiation as follow:

I = [(T[sub]c[/sub]-(T[sub]a[/sub]+[sub]DELTA[/sub]T[sub]d[/sub]+[sub]DELTA[/sub]T[sub]ini[/sub]))/R[sub]dc[/sub](1-Y[sub]c[/sub]Yc)R[sub]ca[/sub]][sup]1/2[/sup] kA
[sub](Ref.: NM formula 9A)[/sub]

[sub]DELTA[/sub]T[sub]ini[/sub] = 4.3 D[sub]s’[/sub].R[sub]c[/sub]/n’ [sup]o[/sup]C
[sub](Ref.: NM formula 47A).[/sub]

R[sub]c[/sub] = 3.5n’/D[sub]s[/sub]’[(V[sub]w[/sub]/D[sub]s[/sub]’)[sup]1/2[/sup]+0.62e] thermal Ohm-ft
[sub](Ref.: NM formula 42B)[/sub]

[sub]NOTES: For black surface should be increase in 75%.

D[sub]s[/sub]’ = effective (circumscribed circle) of several cable. (in)
n’ = number of conductors within stated diameter.
V[sub]w[/sub] = velocity of the wind (MPH). …Typ. value = 2 ft/s.
e = coefficient of surface emissivity….Typ value = 0.4[/sub]

A second approach used by some utilities in the South and large industrial facilities for estimate ampacity on outdoor cable application is to increase the ambient temperature (typically 10 to 20 oC above 40[sup]o[/sup]C) and derate the cable ampacity in accordance with the NEC ambient temperature table.

Other source to account for solar head gain on overhead bare conductors is the IEEE std 738. [sub] Check with Sarah in Nuclear for the work I submitted last year for the LANL project.[/sub]

I understand that the IEEE working group 14-7 has work in progress to publish derating factor such as hot pipe proximity and solar radiation effects not covered in the IEEE Std. 666. Similarly there are other parallel effort on ICEA and IEC.

I hope this could help.

 

[beghtesady]

What you need to know is that the ambient temperature is the guideline for the derating. With that said, if a cable is installed within a raceway in outdoor location vs. installed without a raceway would have different ambient temperature, hence a different derating factor. Also what you need to be aware of that the ambient temperature varies for each local. So, before you design your installation, check with the local authority to see what is (if any) their applied derating factors.