Current rating based on conditions of use

[Carultch]

So voltage drop and the inefficiencies it brings are of no concern?

Is the goal saving money or copper?

We're not talking about voltage drop here. We're talking about calculating what I like to call, the minimum local size.

I see voltage drop as a secondary issue, to calculate after the minimum local size is known.

 

[Smart $]

Not a home, commercial property. But the concept would still be the same imo.


So say I had a 3 phase 208 volt circuit for a bank of heat strips totaling 25kw. 70amps per phase, 75*C terminals. 70x125%= 87.5 or 90amps, giving me #3 cu. If I apply a correction factor of 50*F #4 cu I get 102amps. The wire is good, but technically I am on the 90*C column for the terminals so its no to code? :?

Don't forget you may get a break on ampacity up the wall, but on the roof you still have the "Temperature Adder" of 310.15(B)(3)(c). If conduit is on the roof or up to 0.5" spacing above, you get to add 60°F... followed by 40° for 0.5" to 3", 30° for 3.5" to 12", then 25° for 12" to 36". So your 50°F outside ambient amounts to correction factors of, respectively:
110°F@0.87
90°F@0.96
80°F@1.00
75°F@1.04

 

[mbrooke]

So voltage drop and the inefficiencies it brings are of no concern?

Is the goal saving money or copper?

I can go 350 feet before voltage drop goes over 5%. Check this out:

http://www.electrician2.com/calculators/vd_calculatoradv.htm

Also, what inefficiencies do you have in mind?

 

[mbrooke]

We're not talking about voltage drop here. We're talking about calculating what I like to call, the minimum local size.

I see voltage drop as a secondary issue, to calculate after the minimum local size is known.

How I normally do it. I calc the lowest wire size current (and applicable code) allows, then check voltage drop.

 

[mbrooke]

Don't forget you may get a break on ampacity up the wall, but on the roof you still have the "Temperature Adder" of 310.15(B)(3)(c). If conduit is on the roof or up to 0.5" spacing above, you get to add 60°F... followed by 40° for 0.5" to 3", 30° for 3.5" to 12", then 25° for 12" to 36". So your 50°F outside ambient amounts to correction factors of, respectively:
110°F@0.87
90°F@0.96
80°F@1.00
75°F@1.04

I will look into that. Also, is this temperature adder new? Ive seen tons of old specs that never adjust for conduit on roofs using that particular correction factor.

 

[Smart $]

I will look into that. Also, is this temperature adder new? Ive seen tons of old specs that never adjust for conduit on roofs using that particular correction factor.

Added in 2008 Edition [310.15(B)(2)(c)... 310.15(B)(3)(c) in 2011, 2014 Editions].

 

[mbrooke]

Added in 2008 Edition [310.15(B)(2)(c)].

Makes sense now. Although I will add that prior I dont think their were any problems without de-rating.

 

[kwired]

(You mean 50F. If outside temperatures were 50C, it would be hot as Hades.)

That is a good point. When the natural temperatures are below 30C, by the purpose of the circuit in use, I would agree that you could take credit for the temperature correction that increases ampacity.

It is when you rely upon artificial means to get it cold, for which I would disagree.

But isn't a boiler room artificially hot? Ambient temp is what we normally expect as far as I am concerned. Can a freezer fail and be warm, yes, but that that is usually temporary. Quite often any wiring inside a freezer will have minimal loading on it as well if the freezer is warm. No need for defrost heaters door frame heaters or things of that nature in fact if the freezer has broken down those items are going to not be desired when it comes to keeping what cold you can in the freezer.

 

[Carultch]

But isn't a boiler room artificially hot? Ambient temp is what we normally expect as far as I am concerned. Can a freezer fail and be warm, yes, but that that is usually temporary. Quite often any wiring inside a freezer will have minimal loading on it as well if the freezer is warm. No need for defrost heaters door frame heaters or things of that nature in fact if the freezer has broken down those items are going to not be desired when it comes to keeping what cold you can in the freezer.

A boiler room is artificially hot, and we plan our electrical calculations according to the maximum temperature in the room we expect, whether artificial or natural.

A freezer room is artificially cold, and a conservative electrical wiring calculation is to consider the case with the freezer room not being cold yet. I know it is temporary, but it could be long enough for the heat to build up in the wire. And it would be important to take this conservative calculation for the active circuits required during the transient cool-down period (such as the compressor motor). For other circuits that have no purpose operating when it isn't cold (like defrost heaters), then I'll admit that it is reasonable to take credit for the lower ambient temperature.

A similar discussion happens with rooftop structural loading. There is a calculation that allows you to take credit for a normally heated building. If the boiler fails when it is 20 below, and the repair crew is backed up for weeks, you seriously want to cut corners for the roof strength under 3' of snow? And that's not an unrealistic Murphy's law situation. A lot of boilers do fail in the middle of winter.

 

[mbrooke]

This also leads me to ask, are most installations outside of conditioned (people) environments against code? Few I know actually adjust when they should as in a boiler room for example.

 

[Smart $]

This also leads me to ask, are most installations outside of conditioned (people) environments against code? Few I know actually adjust when they should as in a boiler room for example.

I believe you have answered your own question.

Then as, you know, whether there actually is or was a Code that applies. For instance, roof-top installations prior to 2008 NEC and later purview, no compensation was required. Whether temperature compensation of any sort was used is unknown, but I surmise many did not. As a result, many are likely non-compliant with the more recent NEC editions.

Then you simply have electricians which don't even bother with temperature correction no matter what the case may be... :happyyes:

 

[mbrooke]

I believe you have answered your own question.

Then as, you know, whether there actually is or was a Code that applies. For instance, roof-top installations prior to 2008 NEC and later purview, no compensation was required. Whether temperature compensation of any sort was used is unknown, but I surmise many did not. As a result, many are likely non-compliant with the more recent NEC editions.

Then you simply have electricians which don't even bother with temperature correction no matter what the case may be... :happyyes:

My thoughts exactly. Many read table 310.15 without knowing you have to adjust for temps over 30*C.

 

[kwired]

If you keep number of conductors below 4-6 current carrying, you may still be compliant in many cases even if you were not thinking about ambient temp.