How to calculate the maximum length of EMT/IMC/RSC that can be used as equipment grou

[clint2000]

I was wondering if any knows a formula or link to an online calculator to calculate conduit loss when used as an equipment grounding conductor per 250-118.

635’ of 2.5” EMT conduit run with 4/0 copper conductors, 120/208 volt, fused at 100 amps
325’ of 3” IMC/RSC conduit run with 300mcm copper conductors, 120/208 volt, fused at 150 amps
425’ of 4” IMC/RSC conduit with 500mcm copper conductors, 120/208 volt, fused at 350 amps.

The intention is to calculate the loss to indentify if an equipment grounding conductor will need to be installed to supplement the conduits loss.

Thank you

 

[LarryFine]

The general concensus is that the conduit size that's required to enclose any given conductor size would have a lower impedance than the otherwise-required grounding conductor would have.

If the conductors have to be upsized for VD, the resultant conduit size increase would lower the impedance even more. In other words, the run length would never require a conductor instead.

 

[mcclary's electrical]

I agree with Larry, A properly sized conduit, will always be a superior ground compared to a properly sized (wire) EGC.

 

[gar]

090913-0640 EST

See my post #20 at
http://forums.mikeholt.com/showthread.php?t=110537&highlight=resistance+measurement+conduit
for resistance measurements I made on EMT.

See my 4-terminal resistance measurement method in photos P20 and P21 at
http://beta-a2.com/EE-photos.html

Note: each coupling will add a small amount of resistance to the total path length resistance of conduit. For a specific cross-sectional area steel has about 10 times the resistance of copper. If a coupling connection is poor, then the added resistance could be high. Rigid conduit will have a much lower resistance because of the greater wall thickness.

The 1/2 EMT had about 17.5 times the area of #12wire and with a resistivity ratio of 10 to 1 this would mean the resistance of the conduit should be about 10/17.5 times the #12 wire resistance. My ratio of conduit resistance to #12 copper was 0.7/1.63 = 0.43 and that compares well with 10/17.5 = 0.57 .

Also see
http://en.wikipedia.org/wiki/Four-terminal_sensing

.

 

[bob]

I was wondering if any knows a formula or link to an online calculator to calculate conduit loss when used as an equipment grounding conductor per 250-118.

1. 635? of 2.5? EMT conduit run with 4/0 copper conductors, 120/208 volt, fused at 100 amps
2. 325? of 3? IMC/RSC conduit run with 300mcm copper conductors, 120/208 volt, fused at 150 amps
3. 425? of 4? IMC/RSC conduit with 500mcm copper conductors, 120/208 volt, fused at 350 amps.

The intention is to calculate the loss to indentify if an equipment grounding conductor will need to be installed to supplement the conduits loss.

Thank you

GEMI Calculator would calculate
1. 664 ft
2. 582 ft
3. 387 ft.

 

[bob]

GEMI Calculator would calculate
1. 664 ft
2. 582 ft
3. 387 ft.

These figures were with out an added EGC conductor. For #3 if an EGC conductor of size #4 were added, the distance would be about 450 ft.

 

[chris kennedy]

What is the GEMI calculator?

 

[don_resqcapt19]

What is the GEMI calculator?

Chris,
That is a free software program from the steel tube institute.
http://www.steelconduit.org/gemi.htm

 

[chris kennedy]

Chris,
That is a free software program from the steel tube institute.
http://www.steelconduit.org/gemi.htm

Thanks Don, downloading.

 

[weressl]

I was wondering if any knows a formula or link to an online calculator to calculate conduit loss when used as an equipment grounding conductor per 250-118.

635? of 2.5? EMT conduit run with 4/0 copper conductors, 120/208 volt, fused at 100 amps
325? of 3? IMC/RSC conduit run with 300mcm copper conductors, 120/208 volt, fused at 150 amps
425? of 4? IMC/RSC conduit with 500mcm copper conductors, 120/208 volt, fused at 350 amps.

The intention is to calculate the loss to indentify if an equipment grounding conductor will need to be installed to supplement the conduits loss.

Thank you

The conduit's UL listing contains information on the maximum allowable length of the conduits for grounding conductor use. Contact the conduit manufacturer or website(s) for the information..