Temperature Correction: When 90 != 90

[viciousesque]

Referencing NEC 2020
Can someone explain why a conductor in the 90C column of Table 310.16 gets corrected in Table 310.15(B)(1) even when the ambient temperature is below 90?
For example, 8 AWG THHN copper is rated at an ampacity of 55 amps in the 90C column. Why then do I need to apply temperature correction for temperatures BELOW 90?
I would think an ambient temperature of 90C would have an adjustment of 1.0.
Why do we apply temperature correction factors for ambient temperatures above 30C even for conductors with temperature ratings (from Table 310.16) well above 30C?

Thanks, all!

 

[Dennis Alwon]

The 90C of the insulation is not ambient temperature. The table for temperature is of ambient air temperature where the other is the insulation temperature.

The table is based on ambient temperature of 86 F so that is why the correction factor is 1 at that temperature. If the room or area where the conductors are going thru is very hot then lower correction values are used. The hotter the ambient temperature the lower the ampacity of the wire. Good wiring is all about keeping the conductors at a reasonable operating temp.

 

[Carultch]

Referencing NEC 2020
Can someone explain why a conductor in the 90C column of Table 310.16 gets corrected in Table 310.15(B)(1) even when the ambient temperature is below 90?
For example, 8 AWG THHN copper is rated at an ampacity of 55 amps in the 90C column. Why then do I need to apply temperature correction for temperatures BELOW 90?
I would think an ambient temperature of 90C would have an adjustment of 1.0.
Why do we apply temperature correction factors for ambient temperatures above 30C even for conductors with temperature ratings (from Table 310.16) well above 30C?

Thanks, all!

In an ambient temperature of 90C, no person could survive. The standard temperature for the ambient temperature in the testing lab is 30C, which slightly less than 90F, and is much more realistic as an ambient temperature on this planet. It is just a coincidence that 86F is very close to the number 90, but it is completely different than 90C. The values in Table 310.16 are based on an ambient temperature of 30C or 86F, and the conductor is rated to safely carry up to its rated ampacity without exceeding its maximum rated temperature of 90C.

I would recommend familiarizing yourself with the square root formula, instead of the correction tables. It means a lot fewer numbers to look up, and can simplify a spreadsheet. The NEC permits you to use either the square root formula or the tables for this calculation, whichever works for you.

 

[winnie]

The conductor has an insulation temperature rating of 90C.

Current in the conductor causes it to heat up. The greater the current the greater the temperature rise above the surrounding temperature.

The ampacity is set by the available difference between ambient temperature and allowed conductor temperature.

If you have 90C conductors in a 90C environment there is no room for the conductors to heat up; their ampacity in those conditions is _0_.

Jon

 

[Fred B]

To add to Jon's statement the temperature correction is based on ambient temperature of surrounding and the ability of the surrounding to dissipate the heat induced within the conductors by the loads. Thus the higher the ambient temperature is outside of the conductors, the less the ability for cooling of the conductors, and you would reduce the current carrying ability of the conductors to prevent overheating of the conductors. Also why if ambient is less than the tested values it can increase the cooling of the conductors and you can thus allow for a higher current carrying capacity on the conductors. Same reason to derate for bundling of conductors, heat dissipation.
Think of it in relationship to your body, you dress for working outside at 50 deg (ambient tested rating). as the outside temperature rises to nearer 75 deg, you get more uncomfortable trying to work and begin to sweat, you can either work less (derating) or remove some clothing to keep doing the same amount of work without sweating. So in the electrical conductor situation normally there isn't the ability to reduce the ambient temperature (remove some clothing) so it has to be lower (derate) the load capacity (work less).

 

[viciousesque]

The 90C of the insulation is not ambient temperature. The table for temperature is of ambient air temperature where the other is the insulation temperature.

The table is based on ambient temperature of 86 F so that is why the correction factor is 1 at that temperature. If the room or area where the conductors are going thru is very hot then lower correction values are used. The hotter the ambient temperature the lower the ampacity of the wire. Good wiring is all about keeping the conductors at a reasonable operating temp.

Thank you, Mr. Alwon. Very nice explanation and now it is very clear in my mind.

 

[viciousesque]

The conductor has an insulation temperature rating of 90C.

Current in the conductor causes it to heat up. The greater the current the greater the temperature rise above the surrounding temperature.

The ampacity is set by the available difference between ambient temperature and allowed conductor temperature.

If you have 90C conductors in a 90C environment there is no room for the conductors to heat up; their ampacity in those conditions is _0_.

Jon

Thank you for the additional detail. My understanding deepens! Thank you, Winnie.

 

[viciousesque]

To add to Jon's statement the temperature correction is based on ambient temperature of surrounding and the ability of the surrounding to dissipate the heat induced within the conductors by the loads. Thus the higher the ambient temperature is outside of the conductors, the less the ability for cooling of the conductors, and you would reduce the current carrying ability of the conductors to prevent overheating of the conductors. Also why if ambient is less than the tested values it can increase the cooling of the conductors and you can thus allow for a higher current carrying capacity on the conductors. Same reason to derate for bundling of conductors, heat dissipation.
Think of it in relationship to your body, you dress for working outside at 50 deg (ambient tested rating). as the outside temperature rises to nearer 75 deg, you get more uncomfortable trying to work and begin to sweat, you can either work less (derating) or remove some clothing to keep doing the same amount of work without sweating. So in the electrical conductor situation normally there isn't the ability to reduce the ambient temperature (remove some clothing) so it has to be lower (derate) the load capacity (work less).

Very good analogy. Thank you for adding onto the great responses already provided. Nice forum. Thank you, Fred.

 

[ramsy]

I would recommend familiarizing yourself with the square root formula, instead of the correction tables. It means a lot fewer numbers to look up, and can simplify a spreadsheet. The NEC permits you to use either the square root formula or the tables for this calculation, whichever works for you.

Unless you care to provide supervision, 310.15(C) Square-root formula would require a Licensed Power Engineer, for projects with building permits.

I am all ears, if you can idiot proof the variables, (Yc) skin affect, proximity effect, and (Rca) thermal resistance between conductor and ambient.

I assume knucklehead contractors are allowed to use formulas in Table 8 & 9 Notes.

Table 8, Note 2 formula is useful for temperature rise, and perhaps to modify the AC impedance formula, by mathematical interpolation with table-data points, although perhaps not evaluated for unqualified persons.

Table 9, Note 2 impedance formula seems useful for voltage drop, and ampacity at 75°C, if 0.85pf & other Table assumptions apply, but perhaps not evaluated for use by the unqualified or unsupervised.

 

[ramsy]

The NEC permits you to use either the square root formula or the tables for this calculation, whichever works for you.

310.15(B)(2) is another Square-root formula for ambient temperature correction factors, not requiring engineering supervision.

Although the code does not explore this formula, or evaluate its use with examples in Appendix-D, its critical for modifying resistance (R) when using Table 9, AC impedance formula.

 

[Carultch]

310.15(B)(2) is another Square-root formula for ambient temperature correction factors, not requiring engineering supervision.

Although the code does not explore this formula, or evaluate its use with examples in appendix D, I agree its very useful when combined with Table 8 & 9 formulas.

That's the one I had in mind, as opposed to the more detailed Nehr-McGrath formula. The 310.15(B) tables are based on this formula, and group its conclusions in 5C increments of air temperature (either ambient temperature, or conduit air temperature with the rooftop adder). It's a toss-up, as to which one is more conservative. From what I've typically seen, about 80% of the time, the tables are more conservative.

Temp correction factor = sqrt((Tc - Ta)/(Tc - T0)):
Tc = conductor rated temperature, typically 90C
Ta = actual ambient temperature in Celsius (or air temperature with rooftop adder where applicable)
T0 = test condition ambient temperature, typically 30C

 

[jimport]

Also, the terminals are not rated for use at the 90 degree, so the ampacity needs to be based on the lower temperatures of the lugs.

 

[ramsy]

Rather than solve ampacity, my spreadsheet solves 310.15B2 Square-Root formula for Temperature Rise, given load, environment, wire variables & Tbl.8, Note-2. As variables change so does Resistance (R2) in AC impedance formula from Tbl.9,Note-2.

This spreadsheet design monitors how loads affect conductor temperature, per 110.14(C), along with voltage drop, and impedances for SSC.

Since lookup tables are used for wire properties, the % conduit fill, Minimum EGC, GEC size, along with adjustments for CCC in conduit, % Motors for inductive load current rise, and % continuous loads. No one remembers all these requirements without forgetting a few.

If the critical parts are correct, my best possible result follows up to 1/0 where skin effect occurs. I will leave that correction, along with 3-phase adjustments to someone else.

 

[ramsy]

Please delete duplicate