Conductor Voltage Drop Calculations

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Hello,

In doing voltage drop calculations with the use of Table 9 in NEC 2008 (effective Z at 0.85 PF for copper wire), I've encountered a question asking if this calculation can be done for 90 deg. C as opposed to 75 deg. C which NEC Table 9 specifies. It is evident that NEC 2008 does not publish a table with resistance/reactance values for 90 deg. C. Could the effective Z listed in the table be adjusted using the appropriate temperature coefficient of copper? Any feedback or past experience with this issue would be greatly appreciated. Thanks in advance. :)
 

charlie b

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I don't quite follow the question. When the NEC uses 75C and 90C, it is talking about the rating of insulation system, not the copper itself. Nor do either of those two temperature values refer to the ambient temperature. So I don't see how they would come into play for a VD calculation. May I ask what situation you are trying to model?

Welcome to the forum.
 
I don't quite follow the question. When the NEC uses 75C and 90C, it is talking about the rating of insulation system, not the copper itself. Nor do either of those two temperature values refer to the ambient temperature. So I don't see how they would come into play for a VD calculation. May I ask what situation you are trying to model?

Welcome to the forum.

Hi charlie b,

Thanks for the reply. Let me clarify a bit. In using 90 deg. C rated wire (such as XHHW), Table 9 isn't applicable since it indicates that all values in the table are valid only at 75 deg. C (refer to note #1). My question then is how to adjust the values listed in the table when using 90 deg. C rated wire. Does that help? Thanks again.
 

charlie b

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My question then is how to adjust the values listed in the table when using 90 deg. C rated wire.
You don't. I mis-spoke (mis-wrote? :roll:) earlier, when I said that 75C and 90C don't refer to ambient temperature. That was true in the context of Table 310.16, the place we most often go for ampacity values. But Table 9 is using the 75C as an ambient temperature. The insulation system of a conductor does not have an influence on voltage drop.

 
You don't. I mis-spoke (mis-wrote? :roll:) earlier, when I said that 75C and 90C don't refer to ambient temperature. That was true in the context of Table 310.16, the place we most often go for ampacity values. But Table 9 is using the 75C as an ambient temperature. The insulation system of a conductor does not have an influence on voltage drop.

Ah ok. That definitely clears it up then. The 75 deg. C that is referenced in Table 9 pertains to ambient temperature rather than temperature rating of the conductor insulation. So Table 9 can be used in voltage drop calculations for any application that has an ambient temperature of 75 deg. C and below. Thanks for the information charlie b. :)
 

mivey

Senior Member
Ah ok. That definitely clears it up then. The 75 deg. C that is referenced in Table 9 pertains to ambient temperature rather than temperature rating of the conductor insulation. So Table 9 can be used in voltage drop calculations for any application that has an ambient temperature of 75 deg. C and below. Thanks for the information charlie b. :)
Well, for any application using an ambient of 75 deg C anyway.
 

winnie

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As I understand it, the 75C value in table 9 is the _conductor_ temperature, caused by the combination of self heating and ambient temperature.

Ampacity of a conductor is set by the current which, if carried on a continuous basis in the specified ambient conditions, would cause the conductor to self heat to its temperature limit. In theory, if you had exactly the conditions used to calculate table 310.16, and you ran say 100 A through each of a set of 3 #3 conductors bundled together, then the copper would heat up to 75C. For what its worth, the calculations of table 310.16 are _very_ conservative with respect to thermal conductivity to ambient, and conductors are rarely continuously 100% loaded if they are sized per NEC calculations, so it is rare to actually find conductors that self heat to the temperatures limits.

The notes to table 9 show how the 'effective Z' is calculated, allowing you to calculate Ze from wire resistance and wire reactance it for any power factor. The notes to table _8_ give the calculation used to adjust resistance for different conductor temperatures.

IMHO even if you are using 90C rated conductors, it is _very_ likely that the conductor temperature will be well below 75C, and that the resistance of the conductor will be below that given in table 9.

-Jon
 

Smart $

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As I understand it, the 75C value in table 9 is the _conductor_ temperature, caused by the combination of self heating and ambient temperature.
I agree.

...

The notes to table 9 show how the 'effective Z' is calculated, allowing you to calculate Ze from wire resistance and wire reactance it for any power factor. The notes to table _8_ give the calculation used to adjust resistance for different conductor temperatures.
However, no means to calculate conductor temperature is provided. You need to know the conductor temperature (T2) to use the formula provided in Table 8 Notes.

IMHO even if you are using 90C rated conductors, it is _very_ likely that the conductor temperature will be well below 75C, and that the resistance of the conductor will be below that given in table 9.
In most cases, the conductor temperature will be limited to 75?C because of 110.14(C) termination temperature-rating restrictions.
 

mivey

Senior Member
As I understand it, the 75C value in table 9 is the _conductor_ temperature, caused by the combination of self heating and ambient temperature.
You are correct of course, I should not have said ambient.
 
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