Extreme distance voltage drops

winnie

Senior Member
If the voltage drop at load current is 5%, then a short circuit at the load should give 20X the normal load current.

OCPD sized to the load should have no problem tripping on short circuit.

If you have a huge long run, with a small load and wire sized for voltage drop at the expected load current, but then you size your OCPD for the _wire_, then it might not trip on short circuit... but the current is so limited that it is much less of a safety hazard.

I think in such case ground fault detection rather than overcurrent protection _is_ the best approach.

IMHO the goal is _not_ to have high short circuit currents; the goal is to reliably shut off faulted circuits. High short circuit currents are the means to the end of reliably shutting off faulted circuits, by giving the OCPD a reliable trigger.

For extreme long circuits where you won't reliably have high short circuit currents, other means of shutting off faulted circuits start to make more sense.

-Jon
 

kwired

Electron manager
Location
NE Nebraska
Was the the question Short Circuit and not Ground Fault?
Op was using 480. Not sure about L-N worse case SCA with Y xfmr.
L-N maybe worse case SCA for short runs of center tap 120/240v
Yes OP said 480/277 was available at the source end, probably much more than a 1 kVA transformer as well so source impedance is less consideration than circuit length resistance. Line to neutral/ground fault in this situation should have less fault current than line to line fault and would mean longer clearing time for overcurrent protection. 41 amps on a 20 amp breaker will seem like it takes an eternity to trip compared to having same fault near the source where a few thousand amps might flow during the incident.
 

ramsy

Senior Member
Location
LA basin, CA
..If you have a huge long run, with a small load and wire sized for voltage drop at the expected load current.. ..ground fault detection rather than overcurrent protection _is_ the best approach.
Does GFPE circuit breaker instructions inform us of distance limits from capacitance, or is this published elsewhere?
 

tortuga

Senior Member
Location
(44.057116, -123.103394)
Occupation
field supervisor

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
If the OP installed 6500' of #4 AL and it was on a 2 pole 60 a L-G fault would never trip.
Even at 86 amps it might take a while.
The 60A breaker would prevent damage on a #4 AL, but a ground wire could be over-stressed in a sustained L-G fault if it's a smaller gauge than this. That's where the ground fault protection Jon mentioned could help. It would need to have a relatively high trip level to tolerate the capacitance of a 6500 ft. run, as was brought up earlier.

The impedance of a 6500 ft. wire run would act as a ballast that would limit the current and stabilize an arc if it was present at the end of the run. The voltage vs. current curve of an arc typically has a negative resistance region which can cause the arc to be unstable and blow out. A series ballast resistance which is larger in magnitude than the negative resistance will make the arc more sustainable. Which is good if you want an arc welder but not in most other situations. :rolleyes:
 

ramsy

Senior Member
Location
LA basin, CA
using this http://www.cooperindustries.com/content/public/en/bussmann/electrical/resources/fc2.html
I used 2% for the Z gives me:
Itotal s.c. (L-L) 86 AMPS
Is.c. (L-N) 43 AMPS
V (L-L) 480 V
for #4 AL.
If the OP installed 6500' of #4 AL and it was on a 2 pole 60 a L-G fault would never trip.
Even at 86 amps it might take a while.
I guess this is where 110.10 comes into play enforcing voltage drop.
Now your the SCA master.
Since IEEE standard for Short Ckt. Analysis uses 0.85 pf, all you need is a Voltage Drop calculator that can adjust Power Factor.
 

ramsy

Senior Member
Location
LA basin, CA
Why not use the R and X values for the circuit run added to that of the source?
NEC Chap. 9. Table 9 shows R and X conductor values, and a formula for effective Z in Tbl.9, based on those table properties, assuming pf=0.85.

NEC Table 9 selections between PVC and Steel raceway Reactance (X) appear to affect impedance (Z) much less than changes in power factors. Yet no verified and validated calculator has been found for free, beyond those based on NEC tables fixed on pf=0.85, or worse K factors with unity pf=1.
 

mbrooke

Senior Member
Location
United States
NEC Chap. 9. Table 9 shows R and X conductor values, and a formula for effective Z in Tbl.9, based on those table properties, assuming pf=0.85.

NEC Table 9 selections between PVC and Steel raceway Reactance (X) appear to affect impedance (Z) much less than changes in power factors. Yet no verified and validated calculator has been found for free, beyond those based on NEC tables fixed on pf=0.85, or worse K factors with unity pf=1.

Thats the place (y) But still don't know the source Z- at least not without a perspective fault current tester.

BS7671 has the tables with everything you need- wish the NEC was like that.


Article 250 is light years away from actual electrical theory.
 

RD35

Member
I'd be curious to know if the equipment being fed is all multi-voltage capable. Sometimes these types of equipment can be fed any 50-60HZ voltage between 120 and 277 with no ill effects. Voltage drop is a FPN in the code anyway...so 5% is not a hard requirement....just a suggestion. So if the load is, say, all electronic equipment and LED lighting with the multi-volt type power requirements...then a 20-30% drop may still get the job done. Am I missing something here?
 

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
I'd be curious to know if the equipment being fed is all multi-voltage capable. Sometimes these types of equipment can be fed any 50-60HZ voltage between 120 and 277 with no ill effects. Voltage drop is a FPN in the code anyway...so 5% is not a hard requirement....just a suggestion. So if the load is, say, all electronic equipment and LED lighting with the multi-volt type power requirements...then a 20-30% drop may still get the job done. Am I missing something here?
No, you’re not.
 
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