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

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
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Electrical Contractor
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.
Fuses as in opens, or becomes one?
 

romex jockey

Senior Member
Location
Vermont
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~
 

romex jockey

Senior Member
Location
Vermont
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~
 

mbrooke

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United States
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Electricity
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~

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.
 

nmonaco

Member
Location
Baltimore
Occupation
Civil Engineer
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.
 

mbrooke

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Location
United States
Occupation
Electricity
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.

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. 😇
 

kwired

Electron manager
Location
NE Nebraska
Typically you won't unless the run is very long and the fault is at the far end.
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
 

mbrooke

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Electricity
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

It does not lesson anything, the EGC will melt regardless should current persist for to long.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
It does not lesson anything, the EGC will melt regardless should current persist for to long.
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
 

kwired

Electron manager
Location
NE Nebraska
It does not lesson anything, the EGC will melt regardless should current persist for to long.
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.
 

mbrooke

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United States
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Electricity
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

Correct. Table 250.122 allows for conductors less than 1/10th the size of current carrying conductors.
 

mbrooke

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Electricity
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.

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.
 

kwired

Electron manager
Location
NE Nebraska
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.
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.
 

mbrooke

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United States
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Electricity
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.


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.
 

kwired

Electron manager
Location
NE Nebraska
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.
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.
 

mbrooke

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Location
United States
Occupation
Electricity
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.

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.
 
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