Long RMC run and short circuit.

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darekelec

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Hi. My co workers will have to make 600' (yes, six hundred) feet long run to feed lights and receptacles. They can manage voltage drop but they don't think of possibility if ground fault would trip breaker at end of run.
What do you do with long runs in aluminum RMC. They thread instead of compression couplings.
 
Have no idea what you mean. There is always a chance to have a ground fault in a circuit. Does the system on which this circuit is derived have ground fault protection? Not the circuit itself but the main switchgear. If you make sure your circuit is properly grounded i.e. equipment grounding conductor provided, all metal parts of the raceway system and equipment used grounded, you've done all you can do. The ground fault protection for the system will do it's work.
 
hbendillo said:
Have no idea what you mean. There is always a chance to have a ground fault in a circuit. Does the system on which this circuit is derived have ground fault protection? Not the circuit itself but the main switchgear. If you make sure your circuit is properly grounded i.e. equipment grounding conductor provided, all metal parts of the raceway system and equipment used grounded, you've done all you can do. The ground fault protection for the system will do it's work.
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I believe he's saying conduit is the only EGC. No wire EGC.
 
When you look at the resistance of a properly installed conduit system, 1000 ft of 1/2" aluminum has approximately the same resistance as a 2/0 Cu conductor* so there should be no problem with a 600 ft run. That said, I have always felt the addition of an actual conductor for equipment grounding is a good idea.

*based on "Electrical Grounding" by R P O'Riley
 
hbendillo said:
Have no idea what you mean. There is always a chance to have a ground fault in a circuit. Does the system on which this circuit is derived have ground fault protection? Not the circuit itself but the main switchgear. If you make sure your circuit is properly grounded i.e. equipment grounding conductor provided, all metal parts of the raceway system and equipment used grounded, you've done all you can do. The ground fault protection for the system will do it's work.
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I believe don_resqcapt19 once posted a link to an EGC effectiveness calculator for conduit. May have only been for GRC, though... I don't recall. Perhaps he'll reply...
 
augie47 said:
When you look at the resistance of a properly installed conduit system, 1000 ft of 1/2" aluminum has approximately the same resistance as a 2/0 Cu conductor* so there should be no problem with a 600 ft run. That said, I have always felt the addition of an actual conductor for equipment grounding is a good idea.

*based on "Electrical Grounding" by R P O'Riley
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Bingo! Thank you.
 
Do you guys do ground loop impedance calculations as part of the design process?

Here in the UK, we have to calculate the fault current which would flow for a bolted ground fault at the end of a proposed run.

Putting this calculated fault current against the tripping curve for the proposed OCPD will give you the tripping time.

Checking this trip time against a table of trip times v standard circuit types will tell you if the proposed conductor size/length of run/OCPD combination is acceptable.

If this doesn't work out then you have to change something or consider GFCI

Is there not a similar requirement/procedure in the NEC?
 
No. The NEC generally deals not in specified outcomes but in wiring procedures that if followed will assure those outcomes (with a large safety factor to make up for lack of specific calculations.
 
Smart $ said:
I believe don_resqcapt19 once posted a link to an EGC effectiveness calculator for conduit. May have only been for GRC, though... I don't recall. Perhaps he'll reply...
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It is for conduit used as the EGC. It does does calcs for EMT, IMC and Rigid steel conduit and rigid aluminum conduit.

In general, steel conduit is an effective EGC in lengths longer than a code sized EGC of the wire type.

http://steeltubeinstitute.org/steel-conduit/resources/gemi-analysis-research/
 
augie47 said:
When you look at the resistance of a properly installed conduit system, 1000 ft of 1/2" aluminum has approximately the same resistance as a 2/0 Cu conductor* so there should be no problem with a 600 ft run. That said, I have always felt the addition of an actual conductor for equipment grounding is a good idea.

*based on "Electrical Grounding" by R P O'Riley
Click to expand...

There is a little more to it than what is posted. You need to know the conductor size,
the breaker size and the study includes a 40 volt drop across the arc. This arc adds
additional impedance to the circuit. You also need the avilable fault at the panel.
 
As mentioned metal raceways will typically have less resistance then the contained conductors.

Long runs still will introduce enough impedance that the response time of overcurrent devices will be slower because actual fault current will be less, this is complicated even more when the available fault current from the source is low to start with.

I see this all the time with irrigation equipment on the farms, we are talking many times circuits over 1000 feet in length. Fuses will blow - but it takes more time then for similar circuits with a much shorter run, if you want to lessen that time you need to select overcurrent devices with a different trip curve.
 
augie47 said:
When you look at the resistance of a properly installed conduit system, 1000 ft of 1/2" aluminum has approximately the same resistance as a 2/0 Cu conductor* so there should be no problem with a 600 ft run. That said, I have always felt the addition of an actual conductor for equipment grounding is a good idea.

*based on "Electrical Grounding" by R P O'Riley
Click to expand...

Where are you getting the resistance for the different types and sized of metallic conduits? Thanks.
 
AdrianWint said:
Do you guys do ground loop impedance calculations as part of the design process?

Here in the UK, we have to calculate the fault current which would flow for a bolted ground fault at the end of a proposed run.

Putting this calculated fault current against the tripping curve for the proposed OCPD will give you the tripping time.

Checking this trip time against a table of trip times v standard circuit types will tell you if the proposed conductor size/length of run/OCPD combination is acceptable.

If this doesn't work out then you have to change something or consider GFCI

Is there not a similar requirement/procedure in the NEC?
Click to expand...
In general the NEC is a prescriptive code and does not get into things like that. Rules like that are most often in performance codes, not prescriptive codes. The prescriptive rule in the NEC that addresses this issue is found in 250.122(B) that requires that you increase the size of the Equipment Grounding Conductor proportionally where the size of the circuit conductors has been increased because of voltage drop, however that rule only applies to EGCs of the wire type. It does not apply to cases where the metallic raceway is used as the EGC
 
AdrianWint said:
Do you guys do ground loop impedance calculations as part of the design process?

Here in the UK, we have to calculate the fault current which would flow for a bolted ground fault at the end of a proposed run.

Putting this calculated fault current against the tripping curve for the proposed OCPD will give you the tripping time.

Checking this trip time against a table of trip times v standard circuit types will tell you if the proposed conductor size/length of run/OCPD combination is acceptable.

If this doesn't work out then you have to change something or consider GFCI

Is there not a similar requirement/procedure in the NEC?
Click to expand...
I have read that 90% of ground faults are arcing faults so checking the curves would not work.
 
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