Rigid Aluminum conduit & voltage drop

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JJWalecka

JJWalecka

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New England
I was reading about Rigid Aluminum conduit. It was stated that being a nonmagnetic metal, it reduced ?voltage drop in installed copper or aluminum wire up to twenty percent of a corresponding steel conduit installation when AC circuits are involved.?

How does being a nonmagnetic conduit involve voltage drop? PVC is nonmagnetic also?
Is there a correction factor in determining voltage drop in this aspect?

Pardon my ignorance

JJ
 
Just read that PVC being nonmagnetic provides less voltage drop and fewer reactance losses than are possible with metallic conduits with corresponding conductors used on AC.

I'm still curious about a potential correction factor?
 
Found some info

"Eddy currents are induced currents in surrounding magnetic or non-magnetic metal. These currents create heating in the metal and therefore act as an energy loss that translates into an increase in resistance of the circuit. NEC? Chapter 9, Table 9 segregates conduits into three groups, PVC, Aluminum and Steel to account for the added resistance with each of the three conduit group types. PVC, being non-metallic, does not produce any eddy currents and therefore has the least resistance value of the three. Steel and Aluminum conduit, being metallic, do produce eddy currents and their respective resistance values are reflective of this."

JJ
 
I am having trouble with this concept, it sounds like snake oil. Consider running a single current carrying conductor through a steel conduit or tubing, you have inductive heating and voltage drop due to the load of the inductive heating. Now, run two current carrying conductors through a steel conduit or tubing (out and back), you do not have inductive heating. Do you have voltage drop due to the load of the inductive heating? I think not. Is my thinking wrong? :-?
 
All I can add to this is that 400 Hz wiring is frequently done in aluminum conduit. From what I've read, the inductive reactance goes up with magnetic conduit as opposed to non magnetic, especially as frequency rises. I think the heating here is in the wires due to voltage drop and the warmer wires then warm the conduit.

At 60 Hz, I'm not so sure I'd bother, and the aluminum conduit is not cheap. PVC would be the way to go, but in many commercial inside applications you can't run it unless you have a fire suppression system in that room (and sprinklers usually aren't enough -- need CO2 or Halon).
 
This is clearly a snake oil pitch, but there is a large kernel of truth to it.

Just look at table 9 in the NEC.

Both the resistance and the reactance of conductors will change depending upon the raceway that they run through.

Charlie's point about the magnetic fields canceling out when all of the circuit conductors are in the same raceway is only partially true. Over any sort of extended distance, the net current is zero, and the magnetic fields cancel. But right up next to the conductors, you find that there is space between them, and magnetic fields created in this space. Large feeders are known to rattle because of these fields.

These fields extend far enough to interact with the raceway. Put conductors in a magnetic raceway, and they will have greater inductive reactance. If conductors induce current in the raceway walls, then the conductor will show greater AC resistance.

The snake oil part comes into play when you consider the sort of conditions needed to get 20% greater voltage drop with conductors in a steel conduit rather than an aluminium conduit. I think that you could get there if you have large conductors (where the reactance can be larger than the resistance), and where the load power factor is extremely low. For purely resistive loads, the voltage drop with aluminium conduit could greater than the voltage drop in steel conduit.

-Jon
 
JJWalecka said:
Iwire Could you please tell me where to reasearch these formulas?
Click to expand...

No, I can not.

These where software programs I have used and along with the wire size and type you had to enter the conduit type. If you changed the conduit type the voltage drop results would change. If I recall the results where not changed that much.
 
winnie said:
This is clearly a snake oil pitch, but there is a large kernel of truth to it.

Just look at table 9 in the NEC.

Both the resistance and the reactance of conductors will change depending upon the raceway that they run through.

Charlie's point about the magnetic fields canceling out when all of the circuit conductors are in the same raceway is only partially true. Over any sort of extended distance, the net current is zero, and the magnetic fields cancel. But right up next to the conductors, you find that there is space between them, and magnetic fields created in this space. Large feeders are known to rattle because of these fields.

These fields extend far enough to interact with the raceway. Put conductors in a magnetic raceway, and they will have greater inductive reactance. If conductors induce current in the raceway walls, then the conductor will show greater AC resistance.

The snake oil part comes into play when you consider the sort of conditions needed to get 20% greater voltage drop with conductors in a steel conduit rather than an aluminium conduit. I think that you could get there if you have large conductors (where the reactance can be larger than the resistance), and where the load power factor is extremely low. For purely resistive loads, the voltage drop with aluminium conduit could greater than the voltage drop in steel conduit.

-Jon
Click to expand...

250kCMil/480V/1000'/200A

http://www.csgnetwork.com/voltagedropcalc.html Al:5.2% Cu:3.6%

http://www.southwire.com/voltagedropcalculator.jsp Al:7.14% Cu: 4.67% this is at 0.9 pf.

http://www.jhlarson.com/ind_tables/volt/calc_voltdrop.htm AL: 5.2% Cu:3.6%

http://www.powerstream.com/Wire_Size.htm Al: 5.99% Cu: 3.8%

http://mypq.epri.com/tools/voltagedrop.asp Al: 6.1% Cu: 3.7%

http://www.menninga.com/voltcalc.htm Al: 6.12% Cu:3.72%

My calculator:

p.f.=1.0 Al: 7.07% Cu: 4.3%

p.f.:0.9 Al:6.36% Cu:3.87% (Seems pretty darn close to the Soutwire numbers.)

p.f.: .85 Al:6.1% Cu:3.66% (seems pretty close to the otehr numbers.) So it seesm like the majority of the fols are using values for an average power factor of 0.85.
 
weressl said:
p.f.: .85 Al:6.1% Cu:3.66% (seems pretty close to the otehr numbers.) So it seesm like the majority of the fols are using values for an average power factor of 0.85.
Click to expand...

Agreed; for voltage drop calculations, if the p.f. is not known, a value of 0.85 is often assumed.

Going back to the original post in the thread, JJ was asking about a claim that he had read, that for the _same_ conductor type, installing those conductors in Aluminum rigid conduit would reduce the voltage drop relative to an installation in Steel rigid conduit, by "up to 20%".

Using your calculator, and your parameters (250kCMil/480V/1000'/200A) and adding the following additional parameters: Cu conductors and 0.85pf, what is the voltage drop for an installation in Aluminium rigid versus Steel rigid?

I believe that to get the claimed "up to 20%" difference in voltage drop between the two different conduit types, the power factor needs to be very low, so as to emphasize the difference in inductive reactance.

-Jon
 
winnie said:
Agreed; for voltage drop calculations, if the p.f. is not known, a value of 0.85 is often assumed.

Going back to the original post in the thread, JJ was asking about a claim that he had read, that for the _same_ conductor type, installing those conductors in Aluminum rigid conduit would reduce the voltage drop relative to an installation in Steel rigid conduit, by "up to 20%".

Using your calculator, and your parameters (250kCMil/480V/1000'/200A) and adding the following additional parameters: Cu conductors and 0.85pf, what is the voltage drop for an installation in Aluminium rigid versus Steel rigid?

I believe that to get the claimed "up to 20%" difference in voltage drop between the two different conduit types, the power factor needs to be very low, so as to emphasize the difference in inductive reactance.

-Jon
Click to expand...

Well, let me dissemble:

(Just the first example) Al:5.2% Cu:3.6%

3.6% is 31% smaller than 5.2%

5.2% is 44% greater than 3.6%.

In other words they were talking about the change in the percentages, not that the VALUE of the voltage drop will be 20% greater.

So 20% was not an overstatement, actually it was a very conservative average. It happens - albeit seldom - that when we cry 'snake-oil' salesman we understood the claim incorrectly.

This is a common feature of the devices that are marketed as 'energy-savers" for the household market. While it is TRUE that capacitors improve the power factor and that improvement can be significant, it has no VALUE in your electric bill.
 
winnie said:
Using your calculator, and your parameters (250kCMil/480V/1000'/200A) and adding the following additional parameters: Cu conductors and 0.85pf, what is the voltage drop for an installation in Aluminium rigid versus Steel rigid?
Click to expand...

I use an old "Book of Electrical Information" from Ferraz.
Their table is generates a multiplier to use to determine the Line-line voltage drop for 600V single conductor cable per 10,000 A-ft.

For your 250kcmil parameters the multipliers are:
CU in magnetic = 1.2
CU in non-magnetic = 1.1
for a reduction of 8.3% (.1/1.2)

However for 750kcmil @ .80 PF the numbers are
CU in magnetic = .73
CU in non-magnetic = .59
for a reduction of 19.2% (.14/.73)
 
weressl said:
250kCMil/480V/1000'/200A

http://www.csgnetwork.com/voltagedropcalc.html Al:5.2% Cu:3.6%

http://www.southwire.com/voltagedropcalculator.jsp Al:7.14% Cu: 4.67% this is at 0.9 pf.

http://www.jhlarson.com/ind_tables/volt/calc_voltdrop.htm AL: 5.2% Cu:3.6%

http://www.powerstream.com/Wire_Size.htm Al: 5.99% Cu: 3.8%

http://mypq.epri.com/tools/voltagedrop.asp Al: 6.1% Cu: 3.7%

http://www.menninga.com/voltcalc.htm Al: 6.12% Cu:3.72%

My calculator:

p.f.=1.0 Al: 7.07% Cu: 4.3%

p.f.:0.9 Al:6.36% Cu:3.87% (Seems pretty darn close to the Soutwire numbers.)

p.f.: .85 Al:6.1% Cu:3.66% (seems pretty close to the otehr numbers.) So it seesm like the majority of the fols are using values for an average power factor of 0.85.
Click to expand...

Do you guys realize what a blockhead I am?

I was comparing aluminum vs. copper voltage drop, NOT voltage drop in steel vs. al. CONDUIT!

Where is a solid block wall so I can run into it....head first?:confused:
 
weressl said:
Where is a solid block wall so I can run into it....head first?:confused:
Click to expand...


It's right here:
banghead.gif
 
winnie said:
I believe that to get the claimed "up to 20%" difference in voltage drop between the two different conduit types, the power factor needs to be very low, so as to emphasize the difference in inductive reactance.

-Jon
Click to expand...
Any reduction of the voltage drop when you change to aluminum conduit from steel conduit meets the claim of "up to 20%". This is very common type of wording in sales documents. They imply a 20% reduction of voltage drop, but don't actually claim that.
 
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