VD wire Calc?

Status
Not open for further replies.
O

olly

Senior Member
Location
Berthoud, Colorado
Occupation
Master Electrician
I am upsizing my wire to 350kcmil AL for voltage drop. Therefore I have to proportionally upsize the EGC.

It has to be at least 12.5% of the ungrounded conductor correct?

So 350 kcmil = .364 found in conductor properties page 705

my EGC must be at least .0455 = #3 AL

Can someone tell me if this is correct?
 
olly said:
I am upsizing my wire to 350kcmil AL for voltage drop. Therefore I have to proportionally upsize the EGC.

It has to be at least 12.5% of the ungrounded conductor correct?

So 350 kcmil = .364 found in conductor properties page 705

my EGC must be at least .0455 = #3 AL

Can someone tell me if this is correct?
Click to expand...
What was original size conductor before VD factor came into play?
 
olly said:
It has to be at least 12.5% of the ungrounded conductor correct?
Click to expand...

No, you are thinking of 250.102(C) calculations for SSBJ's, rather than 250.122 calculations. Once behind the disconnect and ocpd, 250.102(C) and 250.66 no longer apply for sizing your green wire. Instead, 250.122 applies for how you size your green or bare wire.

Just to have some numbers to work with, I'll assume this is a 150A circuit that could be built with #3/0 AL wire if length were insignificant. Table 250.122 indicates #4 AL as the EGC that would accompany the #3/0AL feeder wires.

#4 = 41.7 kcmil
#3/0 = 168 kcmil
Actual feeder size = 350 kcmil

Upsize ratio = 350/168 = 2.08
41.7 * 2.08 = 86.875 kcmil

That corresponds to #1/0 AL, for your upsized EGC.

You will notice a pattern that the incremental change in the gauge size of the main conductors corresponds to the same incremental change in the gauge size of the EGC. This is not just a coincidence. This is what you should expect from the way the AWG sizes are defined as a logarithmic scale of the kcmil. Incremental changes in AWG correspond to multiplicative changes in kcmil. This reasoning no longer applies once you are beyond 4/0, but for sizes that we name within the AWG scale, this pattern will occur.
 
Carultch said:
No, you are thinking of 250.102(C) calculations for SSBJ's, rather than 250.122 calculations. Once behind the disconnect and ocpd, 250.102(C) and 250.66 no longer apply for sizing your green wire. Instead, 250.122 applies for how you size your green or bare wire.

Just to have some numbers to work with, I'll assume this is a 150A circuit that could be built with #3/0 AL wire if length were insignificant. Table 250.122 indicates #4 AL as the EGC that would accompany the #3/0AL feeder wires.

#4 = 41.7 kcmil
#3/0 = 168 kcmil
Actual feeder size = 350 kcmil

Upsize ratio = 350/168 = 2.08
41.7 * 2.08 = 86.875 kcmil

That corresponds to #1/0 AL, for your upsized EGC.

You will notice a pattern that the incremental change in the gauge size of the main conductors corresponds to the same incremental change in the gauge size of the EGC. This is not just a coincidence. This is what you should expect from the way the AWG sizes are defined as a logarithmic scale of the kcmil. Incremental changes in AWG correspond to multiplicative changes in kcmil. This reasoning no longer applies once you are beyond 4/0, but for sizes that we name within the AWG scale, this pattern will occur.
Click to expand...
Are you looking at page 705 Conductor properties - Overall Area inch? When I look at that column I see 3/0 is .173? Where are you looking to get 168 kcmil
 
olly said:
Are you looking at page 705 Conductor properties - Overall Area inch? When I look at that column I see 3/0 is .173? Where are you looking to get 168 kcmil
Click to expand...

This is where I'm getting my information. Chapter 9 Table 8 has the same information in the 3rd column, in prefixless circular mils.

American wire gauge - Wikipedia

en.wikipedia.org en.wikipedia.org

KCMIL is a measure of metal-only area. It doesn't count the airspace between the strands, or the insulation. You can either use the circular mil areas, kcmil, or mm^2 values, provided they apply to the metal-only area, and not the area of the entire wire. The entire wire area applies to raceway sizing.

KCMIL means thousand circular mils. Mil means thousandth of an inch. Circular mil is the area of a circle with a mil as its diameter.
 
Status
Not open for further replies.
Top