Let say you have 8 current caring conductors in a conduit on a roof 500 feet away. I would do my voltage drop calc first to come up with a conductor size. Then based on the load I would do my conductor derating to see if the upsized conductors due to voltage drop will be sufficient. Is this correct?
Applying voltage drop and concuctor derating.
- Thread starter JdoubleU
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- Location
- Massachusetts
I would find the code minimum sizes based on derating and once that was done I would see what the voltage drop was using those conductors. If the voltage drop was too much I would just increase the wire size.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Since derating these 8 CCC's is required by the NEC I would do that first to make sure that the conductors are the correct size. Then if you choose you could do the VD calculation which is not required by the NEC.
If you do the VD calc first, you could end up needing a larger ground. Ground is upsized when phases are upsized for VD. However, if you upsize for ampacity adjustment first, the upsized conductors could meet the needed VD and the ground would not need upsized because the upsized phase conductors are the starting ampacity (min. ampacity to meet the code). If that makes sense.
rlundsrud
Senior Member
- Location
- chicago, il, USA
Don't forget to account for the temperature as it is run across a roof.
Carultch
Senior Member
- Location
- Massachusetts
I recommend calculating the MINIMUM LOCAL SIZE first. That is to say, the size required if the circuit length is so short, that voltage drop doesn't matter.
First start with the termination rule, usually with the 75C column of the NEC.
Then apply your derate factors to the wire rating, usually the 90C column of the NEC.
Now, confirm the wire at conditions of use, is protected by the OCPD if required. Applying the "next size up rule" if applicable.
All of the above is how you determine the "minimum local size". And you need to use this as your starting point, if you do upsize to curtail voltage drop. Because you also may need to upsize the ground wire proportionally to the minimum local ground wire size. This is also useful to know, in case you have to adapt to the terminals of your equipment, such that you know the smallest size that the short local section of your wire can be, that connects from the "sweetheart connector" to the equipment terminal.
Finally, you upsize as needed to curtail voltage drop, considering both the individual circuit voltage drop, and the full system voltage drops, making sure you don't exceed the NEC recommendations.
If you need to upsize for voltage drop, you also upsize the EGC proportional to the KCMIL ratio between the actual size and the minimum local size. In general, if you upsize by three sizes on the chart, you will also upsize your ground by three sizes on the chart.
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