eager2learn
Senior Member
- Location
- Mennifee,Ca
Whats the purpose for parrallel feeders? Does it have to do with amperage?
if you put two or more conductors in parallel but insulated from each other along their lengths, the ampacity of the combined circuit becomes the sum of the ampacities of the individual conductors.
I can't find the table right now but I could swear that for n conductors in parallel each of which can carry I amps, they can only carry something less than nI, and this was probably due to contact resistance variation at each end.
For many conductors in parallel the effect of varying contact resistance levels out, for statistical reasons.
???
Neher McGrath doesn't take contact resistance into account. Here's what they say when I only change the number of conductors.
2 >enter number of conductors
0.09 > enter insulation thickness in inches x 2
^0.09" for #2 from table 310-13
0.292 > enter conductor diameter [DC] in inches
^from '99 NEC chap 9, table 8
400 > enter R-value of insulation on a per cm basis
^in C-cm/Watt
^400 or 450 for polyethylene
3.2 >enter A' value from table
0.19 >enter B' value from table
^from table 7 in the link
2.15 >enter factor from table
^from table 7 in the link
^x diameter for 3 cables
0.95 >enter emissivity value
^look this up on the Net.
1.315 >enter conduit OD
^from table 7 in the link
194 >enter Rdc at 75 C in uOhms
^from '99 NEC chap 9, table 8
90 > enter TC, conductor temp in C
40 > enter TA, ambient temp in C
56 =calc'd conduit surface temp in C
157 =calc'd I value in amps
3 >enter number of conductors
0.09 > enter insulation thickness in inches x 2
^0.09" for #2 from table 310-13
0.292 > enter conductor diameter [DC] in inches
^from '99 NEC chap 9, table 8
400 > enter R-value of insulation on a per cm basis
^in C-cm/Watt
^400 or 450 for polyethylene
3.2 >enter A' value from table
0.19 >enter B' value from table
^from table 7 in the link
2.15 >enter factor from table
^from table 7 in the link
^x diameter for 3 cables
0.95 >enter emissivity value
^look this up on the Net.
1.315 >enter conduit OD
^from table 7 in the link
194 >enter Rdc at 75 C in uOhms
^from '99 NEC chap 9, table 8
90 > enter TC, conductor temp in C
40 > enter TA, ambient temp in C
120 =calc'd I value in amps
61 =calc'd conduit surface temp in C
That's certainly true when multiple sets occupy the same raceway. It also depends on how close to the maximum ampacity you calculate and actually load them.I can't find the table right now but I could swear that for n conductors in parallel each of which can carry I amps, they can only carry something less than nI, and this was probably due to contact resistance variation at each end.
That has a lot to do with my point. If paralleled sets are loaded to their ampacities, variations in impedance will cause the 'weakest' set to overheat first.For many conductors in parallel the effect of varying contact resistance levels out, for statistical reasons.