Parallel Feeders

[eager2learn]

Whats the purpose for parrallel feeders? Does it have to do with amperage?

 

[nafis]

A lot of reasons amperage, labor, cost ?

 

[charlie b]

It has to do with ampacity (i.e., the ability to carry current without exceeding the insulation system's temperature limits), not amperage (i.e., the amount of current actually flowing at any given time). There is a general presumption, though I cannot tell you where to find this information in the NEC, that if you put two or more conductors in parallel, the ampacity of the combined circuit becomes the sum of the ampacities of the individual conductors. That has, as nafis pointed out, possible advantages in material and labor cost. In addition, there are situations in which you just can't land a single large conductor on a termination point, but you can land a pair of smaller conductors on that same point.

 

[kingpb]

It could be simply for installation purposes. Maybe you have two 2' conduits in an existing duct bank, and the single conductor would require a 2-1/2" or larger. You could run parallel cables cause that's what will fit.

Or, the contractor just doesn't want to work with 750KCMIL cable because it's 28 Deg outside. That big stuff can be rather stiff to work with regardless of the temp!

 

[G._S._Ohm]

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.

???

 

[tkb]

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.

???

I have never heard this, I you find the table please post it.

 

[charlie b]

You won't find that table in the NEC. You also won't find a table or other statement that a set of "n" cables each having an ampacity of "I" will have a combined ampacity of "nI." We all take that for granted, but I've never seen it in writing.

 

[G._S._Ohm]

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

 

[steve66]

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

The Nehr McGrath calculation is for two sets of conductors burried together in the ground, or burried in common insulation. You will get less than nI due to the mutual heating between the two sets of conducotrs.

But if you have 2 conduits running in air, Nehr McGrath doesn't apply.

 

[G._S._Ohm]

The example at the bottom of this link
http://www.electriciancalculators.com/ampacity/understanding_nm.pdf
uses a conduit in air.
The "Re" thermal resistance value, between the cable/conduit and its environment, is adjusted for air or concrete or soil. The TA value for soil is probably more like 10 C.

 

[LarryFine]

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'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.

For many conductors in parallel the effect of varying contact resistance levels out, for statistical reasons.

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.