Voltage Drop Calculator

[ron]

Does anyone know of a voltage drop calculator that also shows the required upsize for the equipment ground conductor?

It is unclear if it increases based on the cmil ratio increase of the phase conductors or is the the ratio of ampacity increase that could be determined by 310.15(B)(16)

 

[charlie b]

The increase in the EGC is based on the increase in the cross sectional area of the phase conductors. 250.122(B). I don't know of any calculation tool that does this determination for you. However, I have seen some feeder schedules that can lead a person into a trap. I am talking about schedules that show the current rating of various feeders, and give you the conduit and conductor sizes (phase, neutral, and EGC). If you start in one row of that table, and wish to select a larger feeder due to voltage drop (or other reason), and if you just use the information in the next row, you might not be getting the proper upsizing of the EGC.

 

[Smart $]

Does anyone know of a voltage drop calculator that also shows the required upsize for the equipment ground conductor?

It is unclear if it increases based on the cmil ratio increase of the phase conductors or is the the ratio of ampacity increase that could be determined by 310.15(B)(16)

250.122 clearly indicates...

(B) Increased in Size. Where ungrounded conductors areincreased in size, equipment grounding conductors, where
installed, shall be increased in size proportionately according
to the circular mil area of the ungrounded conductors.

What is not clear, is whether upsizing can be based directly on the relative difference in AWG sizes. FWIW, I am a proponent of such.

 

[charlie b]

What is not clear, is whether upsizing can be based directly on the relative difference in AWG sizes. FWIW, I am a proponent of such.

I do not understand what you are suggesting here. Do you mean (and I hope you do not) that if I were to upsize the ungrounded from a #10 to a #8, you would call that a change of a factor of 125% (i.e., 10/8 = 1.25), and therefore upsize the EGC by a factor of 125%? If that is not what you meant, then may I ask what you did mean? :?


To me the rule does seem clear. For example, a #10 has an area of 10380 cmil, and a #8 has an area of 16,510 cmil. To upsize from a #10 to a #8 is therefore a factor of 16510/10380, or 159%.

 

[GoldDigger]

What is not clear, is whether upsizing can be based directly on the relative difference in AWG sizes. FWIW, I am a proponent of such.

It would be much simpler all right, provided you correctly figure the AWG size progression for #N/0 sizes and do not go so large that you have to use wires denominated in MCM.
What you are suggesting is that is you upsize the ungrounded from #6 to #8 you would upsize the EGC by two AWG sizes also. It makes perfect sense, but that is not what the Code specifies.
Not that I can give you an example of a specific calculation where the two methods would give different results unless you are into really large sizes.

 

[Smart $]

I do not understand what you are suggesting here. Do you mean (and I hope you do not) that if I were to upsize the ungrounded from a #10 to a #8, you would call that a change of a factor of 125% (i.e., 10/8 = 1.25), and therefore upsize the EGC by a factor of 125%? If that is not what you meant, then may I ask what you did mean? :?


To me the rule does seem clear. For example, a #10 has an area of 10380 cmil, and a #8 has an area of 16,510 cmil. To upsize from a #10 to a #8 is therefore a factor of 16510/10380, or 159%.

See GoldDigger's response...

 

[winnie]

If you go by the _definition_ of AWG, than a 1 AWG difference always represents a constant circular mil % difference. The ratio 8AWG to 10AWG is the same as the ratio of 1AWG to 3AWG by circular mil area.

Where problems come in with the wording of the code is when you use AWG tables (such as those which are part of the code), that have rounding and production tolerances built in.

If you look at table 8 of chapter 9, the ratio of a #6 to a #8 is ever so slightly different than the ratio of a #8 to a #10. These small ratio difference lead to different interpretations of what is required by code.

-Jon

 

[GoldDigger]

One clear advantage of the NEC wording is that it tells you exactly what you have to do if you increase the size of the ungrounded conductors from #4/0 to 250kcmil.
It just makes all of the other calculations just a little bit harder.

 

[Smart $]

One clear advantage of the NEC wording is that it tells you exactly what you have to do if you increase the size of the ungrounded conductors from #4/0 to 250kcmil.
It just makes all of the other calculations just a little bit harder.

I still say the best way to do it is to just size the EGC per Table 250.122 based on the ocpd rating or the ampacity of the ungrounded conductors, whichever is greater.