Voltage Drop to Determine Wire Size on a 480/277 Line-to-Neutral Lighting Run?

Status
Not open for further replies.
mbrooke said:
Just to put things into perspective for those reading:

10 gauge copper wire fuses at 8900 amps of current in 32 milli seconds (this would trip a 30 amp breaker instantly (and larger handle ratings on IM) before fusing can take place) where as that same 10 gauge wire fuses at 333 amps in 10 seconds.


https://en.wikipedia.org/wiki/American_wire_gauge#Tables_of_AWG_wire_sizes

Increasing time results in a steep decrease in a conductor's ability to carry short circuit current.

Ideally the final temprature of the EGC after the breaker opens should not exceed 150*C for THHN/THWN-2 installations and 250*C for XHHW (XLPE) conductor.
Click to expand...
Fuses as in opens, or becomes one?
 
So on one extreme, we've ocpd's that could grenade , on the other ecg's that are functionally ineffective , the ~Z~ being the devil in the details for both here.

~RJ~
 
well ok...... correct me if i'm wrong , but i do not believe the NEC addresses VD for EGC's , or perhaps i should say they do poorly?

~RJ~
 
romex jockey said:
well ok...... correct me if i'm wrong , but i do not believe the NEC addresses VD for EGC's , or perhaps i should say they do poorly?

~RJ~
Click to expand...


Voltage drop is not mandated by the NEC. Simply if you upsize the current carrying condcutors for voltage drop you must do the same for the EGC. But its not mandated that you upsize the CCCs in the first place.
 
the EGC in my design is required to be the same as the largest conductor in the conduit, so it shouldn't be undersized. Now these would lights on the side of a limited access highway, so their access to public would minimal.
 
nmonaco said:
the EGC in my design is required to be the same as the largest conductor in the conduit, so it shouldn't be undersized. Now these would lights on the side of a limited access highway, so their access to public would minimal.
Click to expand...


Understood, you have this thought out well.

But also consider how many young kids tend to fool around in these places when adults aren't watching. 😇
 
mbrooke said:
Typically you won't unless the run is very long and the fault is at the far end.
Click to expand...
In that case the current very well is less because of resistance of conductor. May still present voltage rise issues in the EGC but lessens the chance of melting the conductor.

Not exactly the same thing as making a fuse link out of 6 inches of small conductor and placing it in series with much larger conductors
 
kwired said:
In that case the current very well is less because of resistance of conductor. May still present voltage rise issues in the EGC but lessens the chance of melting the conductor.

Not exactly the same thing as making a fuse link out of 6 inches of small conductor and placing it in series with much larger conductors
Click to expand...


It does not lesson anything, the EGC will melt regardless should current persist for to long.
 
mbrooke said:
It does not lesson anything, the EGC will melt regardless should current persist for to long.
Click to expand...

If the EGC is the same size as the conductors, and the OCPD is properly sized to protect the conductors, then the EGC will not melt even if it takes a long time for the breaker to open.

The issue with EGCs fusing happens when they are smaller than the protected conductors and the fault current path is not 'effective' enough.

-Jon
 
mbrooke said:
It does not lesson anything, the EGC will melt regardless should current persist for to long.
Click to expand...
May not lessen enough in some instances, but Ohm's law still applies. Current in a conductor is limited by resistance of the conductor. Increase conductor size and you lessen conductor resistance, maybe enough to allow OCPD operation in desirable time.

Voltage drop at "normal load" is one thing, voltage drop at fault current levels is going to be much more, though fault current will still be limited by impedance of the circuit.

And as I said before making a fuse link out of a short length of small conductor is not the same thing as having several hundred feet of same size conductor - the resistance of those several hundred feet is going to be current limiting and you will not get the same fault current. Depending on circumstances may still melt the conductor but won't be in the same amount of time. Good connections is important, a connection is likely to be what has complete failure first in this situation.
 
winnie said:
If the EGC is the same size as the conductors, and the OCPD is properly sized to protect the conductors, then the EGC will not melt even if it takes a long time for the breaker to open.

The issue with EGCs fusing happens when they are smaller than the protected conductors and the fault current path is not 'effective' enough.

-Jon
Click to expand...


Correct. Table 250.122 allows for conductors less than 1/10th the size of current carrying conductors.
 
kwired said:
May not lessen enough in some instances, but Ohm's law still applies. Current in a conductor is limited by resistance of the conductor. Increase conductor size and you lessen conductor resistance, maybe enough to allow OCPD operation in desirable time.

Voltage drop at "normal load" is one thing, voltage drop at fault current levels is going to be much more, though fault current will still be limited by impedance of the circuit.

And as I said before making a fuse link out of a short length of small conductor is not the same thing as having several hundred feet of same size conductor - the resistance of those several hundred feet is going to be current limiting and you will not get the same fault current. Depending on circumstances may still melt the conductor but won't be in the same amount of time. Good connections is important, a connection is likely to be what has complete failure first in this situation.
Click to expand...


Right fault current will go down with length, increasing breaker opening time and in turn EGC heating.

Connections should not be allowed to get hot enough to fail. in fact this is why the code forbids soldier connections.
 
mbrooke said:
Right fault current will go down with length, increasing breaker opening time and in turn EGC heating.

Connections should not be allowed to get hot enough to fail. in fact this is why the code forbids soldier connections.
Click to expand...
Soldiers are not allowed to make connections? :unsure:

Connections are still typically the weakest link, that junction point can develop resistance easier than a point within the conductor itself, presuming c metal properties remains the same throughout.
 
kwired said:
Soldiers are not allowed to make connections? :unsure:

Connections are still typically the weakest link, that junction point can develop resistance easier than a point within the conductor itself, presuming c metal properties remains the same throughout.
Click to expand...



Here is an example of a 60 amp breaker taking its time to open from high loop impedance:




At 20 seconds in the insulation starts smoking, breaker trip at about 46 seconds.
 
mbrooke said:
Here is an example of a 60 amp breaker taking its time to open from high loop impedance:




At 20 seconds in the insulation starts smoking, breaker trip at about 46 seconds.
Click to expand...
The loop on the load side of the breaker is short enough it won't add a lot of impedance. Stick a ferrous rod though the "coils" and you maybe end up with enough impedance that current is low enough it won't trip the breaker? Probably need more turns though. What we don't know is what is the capacity of the source, or size and length of conductors going back to source, or the system voltage for that matter.
 
kwired said:
The loop on the load side of the breaker is short enough it won't add a lot of impedance. Stick a ferrous rod though the "coils" and you maybe end up with enough impedance that current is low enough it won't trip the breaker? Probably need more turns though. What we don't know is what is the capacity of the source, or size and length of conductors going back to source, or the system voltage for that matter.
Click to expand...


Right, however notice the thicker primary conductors do not smoke let alone glow red.

Given about 40 seconds of opening time for a 60 amp breaker I'm guesstimating about 150 amps of fault current.
 
Status
Not open for further replies.
Top