Feeder

Rick 0920

Senior Member
A friend just called me with a question that I wasn't sure about. There is a tap box coming from the secondary side of the single phase transformer supplying (4) 125A services for apartment homes. The feeders are 3/0 AL. in two parallel sets. I know the secondaries from the power company don't use our ampacity table for conductor size so I'm not questioning this entirely. He wants to pull in single conductors instead of parallel. I was thinking of using Table 8. Determining the circ. mils of a 3/0 conductor (167,800) x 2 = 335,600 circ. mils A 400 MCM AL conductor is 400,000 circ. mils I suggested this as being more than equivalent to what was installed. But is it enough for the (4) 125A services? How do we determine this? Thanks in advance.

Dennis Alwon

Moderator
Staff member
Look up the amps of the parallel wires and find the one conductor that matches

don_resqcapt19

Moderator
Staff member
Are these conductors service conductors or feeder conductors?

Rick 0920

Senior Member
They are coming from the secondary side of the transformer terminating in a "J" box. From there, (4) sets of conductors go to individual 125A main breakers for (4) separate apartment units. I'm not sure if that constitutes feeder conductors or service conductors?

tom baker

First Chief Moderator
Staff member
Where is the meter(s)? Can you attach a one line drawing.

Rick 0920

Senior Member
That I don't have. The information was via phone call. I'm thinking it's a multi-bank meter/main combo installed on the outside of the building beside the tap box.

don_resqcapt19

Moderator
Staff member
They are coming from the secondary side of the transformer terminating in a "J" box. From there, (4) sets of conductors go to individual 125A main breakers for (4) separate apartment units. I'm not sure if that constitutes feeder conductors or service conductors?
Assuming the transformer is the utility transformer, these conductors are all service conductors. If the conductors between the transformer and the j-box are customer supplied, as they would be in my area, they would be sized to the total calculated load of the four units. The conductors between the j-box and the 125 amp service disconnects must have an ampacity of at least 125 amps.

Edit based on post #6.

If you are installing conductors between the j-box and the meter center bus, they need to be sized to carry the total calculated load of the four units. If you don't want to do those calculations, you need to size the conductors for 500 amps.
It appears that the original installation the parallel 3/0 aluminum must have been based on the load calculation as you have 310 amps of conductor.

If you want to match that using a single conductor, you need 500 kcmil aluminum or 350 kcmil copper.

Last edited:

infinity

Moderator
Staff member
He wants to pull in single conductors instead of parallel. I was thinking of using Table 8. Determining the circ. mils of a 3/0 conductor (167,800) x 2 = 335,600 circ. mils A 400 MCM AL conductor is 400,000 circ. mils I suggested this as being more than equivalent to what was installed.
Just as a rule of thumb if you want to provide something equivalent then you should be using the conductor ampacity not the circular mil area. As conductors get larger their amps/circular mil go down.

Rick 0920

Senior Member
Assuming the transformer is the utility transformer, these conductors are all service conductors. If the conductors between the transformer and the j-box are customer supplied, as they would be in my area, they would be sized to the total calculated load of the four units. The conductors between the j-box and the 125 amp service disconnects must have an ampacity of at least 125 amps.

Edit based on post #6.

If you are installing conductors between the j-box and the meter center bus, they need to be sized to carry the total calculated load of the four units. If you don't want to do those calculations, you need to size the conductors for 500 amps.
It appears that the original installation the parallel 3/0 aluminum must have been based on the load calculation as you have 310 amps of conductor.

If you want to match that using a single conductor, you need 500 kcmil aluminum or 350 kcmil copper.
Since the original installation was done in parallel, wouldn't those conductors be subject to an adjustment factor? 310 A x 80% = 248 A
In this case, would a 400 kcmil AL with an ampacity of 270 A be sufficient? Thank you for the response.

Dennis Alwon

Moderator
Staff member
Since the original installation was done in parallel, wouldn't those conductors be subject to an adjustment factor? 310 A x 80% = 248 A
In this case, would a 400 kcmil AL with an ampacity of 270 A be sufficient? Thank you for the response.

Rick 400 kcm AL is rated 305 amps at 90C. 305 * .8 = 244 amps. If the load is 270 amps then this won't work. So I am confused.. If these are parallel 3/0 then you have 225 amps @90C assuming the 90C insulation. 225 x 2 = 450 amps

450 * .8 = 360 amps. which is fine for a 270 amp load but I am confused .. Is the 270 a calculated load?

Rick 0920

Senior Member
Rick 400 kcm AL is rated 305 amps at 90C. 305 * .8 = 244 amps. If the load is 270 amps then this won't work. So I am confused.. If these are parallel 3/0 then you have 225 amps @90C assuming the 90C insulation. 225 x 2 = 450 amps

450 * .8 = 360 amps. which is fine for a 270 amp load but I am confused .. Is the 270 a calculated load?
Thank you Dennis. I see my error.

infinity

Moderator
Staff member
Rick 400 kcm AL is rated 305 amps at 90C. 305 * .8 = 244 amps. If the load is 270 amps then this won't work. So I am confused.. If these are parallel 3/0 then you have 225 amps @90C assuming the 90C insulation. 225 x 2 = 450 amps

450 * .8 = 360 amps. which is fine for a 270 amp load but I am confused .. Is the 270 a calculated load?

Sounds like he wants to replace the parallel sets with a single set. First you determine the ampacity of the parallel set and then chose an equivalent single set based on the same ampacity.

don_resqcapt19

Moderator
Staff member
Since the original installation was done in parallel, wouldn't those conductors be subject to an adjustment factor? 310 A x 80% = 248 A
In this case, would a 400 kcmil AL with an ampacity of 270 A be sufficient? Thank you for the response.
Yes, I skipped the ampacity adjustment. 3/0 aluminum has an 90°C ampacity of 175, so the ampacity of the parallel sets would be (2)(175)(0.8) or 280 amps. (this does assume the original conductors are rated at 90°C)

However, basing the new conductors on the ampacity of the old conductors assumes that the original conductors were based on the load calculation.

If you are confident that they were, then replace with the equivalent ampacity of 280 amps. Since your 400 kcmil is only rated at 270 amps, you will need to use 500 kcmil to get to the original ampacity of 280 amps.