paullmullen
Senior Member
- Location
- Wisconsin
- Occupation
- Electrical Engineer & Master Electrician
A 400A feeder requires 500kcmil @75°C. Would you rather work with the 500 kcmil conductors or manage two 4/0s in parallel?
Single Conductor or Two in Parallel: 3/0 or 500 kcmil for 400A
- Thread starter paullmullen
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LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
The latter. It may even cost less.
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
CU or AL? It seems your sizes are a little off.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Single 500's. I eat that stuff for breakfast. 
wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
500 kcmil copper has a 75C ampacity of 380A, so that works for a 380A feeder protected 400A.
3/0 Cu (per the subject line) has a 75C ampacity of 200A, and a 90C ampacity of 225A. So two of them in separate conduits will give you a 400A ampacity. Two sets of them in the same conduit will give you a 80%*225*2 = 360A ampacity (assuming no temperature correction), which may be protected at 400A.
4/0 Cu (per the OP) has a 75C ampacity of 230A, and a 90C ampacity of 260A. So two sets of them in the same conduit will give you 80% * 260 * 2 = 416A ampacity (assuming no temperature correction).
So definitely Cu. [Mostly just working out the math for myself but out loud.]
Cheers, Wayne
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
Ok, I didn't catch 3/0 in the title, saw 4/0 in the body and 2 sets of 4/0 AL in separate raceways would next size up to 400.
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
WE are doing a 500' run soon, only 100 amps but upsizing for VD, and I decided to go with 2 sets of 250 AL vs one set of 500. The voltage drop is lower for the 2x250 and its easier for me to terminate.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
There is a little more to it than just base material cost.
500 cost more than 3/0, but it is one run vs two.
3 inch cost more than 2 inch but is one run vs two. However price difference seems to be significantly higher for raceway over 2 inch than for 2 inch and less. Should you need to run 2.5 inch instead of 2 inch for the parallel 3/0 which becomes even more likely if you are running three phase 5 wires then maybe that evens things out more.
Larger raceway will take more labor to handle and install, but at same time you are doing 1 run instead of 2.
It likely comes down to every installation has it's advantages and disadvantages either way.
500 cost more than 3/0, but it is one run vs two.
3 inch cost more than 2 inch but is one run vs two. However price difference seems to be significantly higher for raceway over 2 inch than for 2 inch and less. Should you need to run 2.5 inch instead of 2 inch for the parallel 3/0 which becomes even more likely if you are running three phase 5 wires then maybe that evens things out more.
Larger raceway will take more labor to handle and install, but at same time you are doing 1 run instead of 2.
It likely comes down to every installation has it's advantages and disadvantages either way.
wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
Hmm, that was news to me, but looking at Chapter 9 Table 9, while the resistance of the 500 Al is basically half (in ohms/km in PVC conduit, it's 0.141 vs 0.279), the reactance is not much lower (0.128 vs 0.135), and the resistances are low enough that the reactance is significant.
Cheers, Wayne
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
I believe it is due to skin effect no?
wwhitney
Senior Member
- Location
- Berkeley, CA
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- Retired
I don't think so, isn't skin effect the reason that the resistance is not exactly half (0.141 vs 0.279)? That's a very small effect.
Maybe some physicist will come by and explain why the reactance drops so slowly with increasing conductor size.
Cheers, Wayne
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Skin effect is pretty much negligible at frequencies below 100Hz (isn't it only around 1.06 for 500kcmil?). I don't think it would show up in the Table 9 values.
Skin effect is one of those topics I think they waste time teaching to electricians who only work on nominal 60Hz systems.
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
Skin effect starts to come into play above around 300-350MCM sizes. But I am unsure of the net effect in a 500 MCM conductor. There does not seem to be a readily available online calculator that provides a result. One online voltage drop calculator shows nearly a 1% less voltage drop for the parallel 250 case over the 500. What is the reason if not skin effect?
- Location
- New Jersey
- Occupation
- Journeyman Electrician
I don't think that it's the skin effect at 60 hz but who knows. One thing for sure is that for the same amount of copper 2*250=500 kcmil or 1-500 kcmil the ampacity is 510 amps versus only 380.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
Heat dissipation. Two smaller conductors have better heat transfer to the air than one large one.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Have you considered the effect of paralleling resistors?
Proximity effect could potentially come into play.
Other than by electricians, I don't think I have ever seen skin effect brought up during any of the power systems studies I have performed. The NEC tables are so conservative, I don't usually consider adding more 'resistance'.
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
Wouldn't proximity effect INCREASE the impedance in parallel conductors?
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Yes, typically that would be the concern. But does it overcome the effect of having resistors in parallel?Wouldn't proximity effect INCREASE the impedance in parallel conductors?
My real point was, electricians seem to be taught about skin effect, while other areas of concern are ignored.
Why not just use the NEC values?
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
In this case I prefer to use the two sets of 250 for other reasons. It's a bit of a bonus that the voltage drop appears to be a little lower than the single 500. I think we were all just discussing the curiosity of why the voltage drop appears to be lower for the parallel sets half the size.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Exactly. The total resistance decreases in proportion to the cross sectional area (proportional to square of diameter) while the thermal resistance between conductor surface and free air (ignoring insulation for the moment) decreases in proportion to the surface area (proportional to the diameter only).
This is the reason that doubling the size of a conductor does not double the allowed ampacity (for small number of conductors. before the adjustment kicks in.)
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