Conductor impedance

Besoeker

Senior Member
Location
UK
Why would (2) sets of 500 kcmil have a lower impedance than (1) set of 1000 kcmil according to Table 9
The conductor impedance comprises two parts, R and Z.
The R part reduces approximately in line with the increase in cross sectional area. The X does not.

We use metric cable sizes and and express the voltage drop in mV/A/m. I compared 150mm[SUP]2[/SUP] with 300mm[SUP]2[/SUP]
.......................... 150mm[SUP]2[/SUP] .......................... 300mm[SUP]2[/SUP]
R .......................... 0.31 .......................... 0.16
X .......................... 0.19 .......................... 0.18
 
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GoldDigger

Moderator
Staff member
Some, but not a lot.
It goes from about 1.10 to 1.03 as a ratio of Ra/Rc (alternating to DC).
Source is Emmett (Alternating Current Wiring and Distribution).
Looking at table 9, for PVC so that magnetic induction effects of the conduit are not important, 500kcmil = .027 ohm/1000ft. 1000kcmil - .015 ohm/1000ft.

Simple parallel of two 500s would be .0135, compared to .015 for a ratio of .9, compared to 1.10 to 1.03 for a ratio of .94. That is close enough for me to say the main contributor to the resistance difference is the skin effect.

Now for aluminum or steel conduit, the mismatch is much greater, and most of it is explained by the contribution of magnetic coupling to the conduit and the induced voltage that the eddy currents cause in the conductor.
 

jumper

Senior Member
cppoly, look at the ampacity values in 310.16 for 500 kcmil and 1000 kcmil.

What conclusion would you draw from this data?

Are 2 500s equal to a 1000 in ampacity?

Edit: Some day I will learn to read the actual OP question closer.:slaphead:
 
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Besoeker

Senior Member
Location
UK
Looking at table 9, for PVC so that magnetic induction effects of the conduit are not important, 500kcmil = .027 ohm/1000ft. 1000kcmil - .015 ohm/1000ft.
Is that resistance or impedance?
My figures for the paralled conductors make their impedance about 0.75 of the single conductor.
The ratio of their resistances is 0.95.
 

GoldDigger

Moderator
Staff member
Is that resistance or impedance?
My figures for the paralled conductors make their impedance about 0.75 of the single conductor.
The ratio of their resistances is 0.95.
That is just resistance, and in a non-magnetic conduit. The OP asked about resistance, and so that was what I concentrated on.
However, the resistance and the reactive components when in conduit have a greater percent difference.

And, FWIW, skin effect will also cause the resistance of two large conductors running very close to each other to be slightly higher than the resistance of the two in parallel but separated.
And the proximity effect (http://en.wikipedia.org/wiki/Proximity_effect_(electromagnetism)) can cause the resistance to increase when the outbound and the return wires are close to each other. But because of the spacing provided by the insulation on power conductors this is usually not as important as the skin effect.
 

texie

Senior Member
Which is all but negligible at 60Hz and below 250kcmil. Even at 500kcmil it is rarely as high as a 1.1X correction ratio.But, for higher frequencies it can have a profound effect.
Jim,
That said, why is there so much more ampacity (per the NEC Tables) per circular mil when using most parallel combinations vs 1conductor (at 60 Hz)? Is it due to more surface area for cooling? I always assumed it was due to skin effect. I may be getting an education here.
 

Besoeker

Senior Member
Location
UK
That is just resistance, and in a non-magnetic conduit. The OP asked about resistance, and so that was what I concentrated on.
But from the OP, post #1:
Why would (2) sets of 500 kcmil have a lower impedance
However, the resistance and the reactive components when in conduit have a greater percent difference.
We don't use conduit. I did figures for non-armoured and armoured (steel wire armoured. The figures were the same. You might expect since the magnetic fields from the conductors ought to balance.
 

Besoeker

Senior Member
Location
UK
Jim,
That said, why is there so much more ampacity (per the NEC Tables) per circular mil when using most parallel combinations vs 1conductor (at 60 Hz)? Is it due to more surface area for cooling? I always assumed it was due to skin effect. I may be getting an education here.
Yes, surface area for the most part.
If you look at copper bus bars their criss section is rectangular, much taller than they are thick. Square or round bars would give you a much lower rating.
 
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