Too small ground wire
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The purpose of ground wire is just to carry current in milliseconds enough to trip the circuit breaker, it won't even get hot. So what is wrong with using very small ground wire? For example. You are using #4 AWG in the Hot and Neutral wires (70 ampere capacity) and you use #14 AWG as ground (15 ampere capacity). If the ground is connected to an enclosure and the Hot wire touches it. It would take very fast for the breaker to trip, not enough to heat the #14 AWG. What is the reasons this is not allowed?
grounding
asked 1 hour ago
Samzun
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migrated from electronics.stackexchange.com 1 hour ago
This question came from our site for electronics and electrical engineering professionals, students, and enthusiasts.
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up vote
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The purpose of ground wire is just to carry current in milliseconds enough to trip the circuit breaker, it won't even get hot. So what is wrong with using very small ground wire? For example. You are using #4 AWG in the Hot and Neutral wires (70 ampere capacity) and you use #14 AWG as ground (15 ampere capacity). If the ground is connected to an enclosure and the Hot wire touches it. It would take very fast for the breaker to trip, not enough to heat the #14 AWG. What is the reasons this is not allowed?
grounding
asked 1 hour ago
Samzun
61
migrated from electronics.stackexchange.com 1 hour ago
This question came from our site for electronics and electrical engineering professionals, students, and enthusiasts.
It is a bit more complicated than that. I am not an expert (someone else who is will answer more completely). But a couple of things: 1 - With a GFCI, even a little current going to ground will result in a very fast trip as the difference between neutral & hot is detected, but not all circuits have a GFCI; 2 - With a regular breaker, it is the current on the Hot wire that trips the breaker, and depending on the breaker (some allow for more time for motor startup, etc.) it can take a while. A regular breaker only trips really fast with a very high current overload.
– manassehkatz
1 hour ago
@manassehkatz interesting notion with the GFCI ground only having to carry 8-30 milliamps. However that relies on a key assumption: that this ground Is monogamous to this circuit. Given the retrofit grounding rules (grounds are allowed and encouraged to be promiscuous), that ground could easily be grounding other stuff that may not have GFCI protection. The GFCI would never know since GFCI's do not interact with ground.
– Harper
32 mins ago
@Harper I'm not actually suggesting using a small ground wire. I'm trying to say that the ONLY time where it could possibly make sense is with a GFCI - i.e., the same place where you might end up not using a ground wire at all.
– manassehkatz
29 mins ago
@manassehkatz heh, yes, true.
– Harper
29 mins ago
add a comment |Â
up vote
1
down vote
favorite
up vote
1
down vote
favorite
The purpose of ground wire is just to carry current in milliseconds enough to trip the circuit breaker, it won't even get hot. So what is wrong with using very small ground wire? For example. You are using #4 AWG in the Hot and Neutral wires (70 ampere capacity) and you use #14 AWG as ground (15 ampere capacity). If the ground is connected to an enclosure and the Hot wire touches it. It would take very fast for the breaker to trip, not enough to heat the #14 AWG. What is the reasons this is not allowed?
grounding
asked 1 hour ago
Samzun
61
The purpose of ground wire is just to carry current in milliseconds enough to trip the circuit breaker, it won't even get hot. So what is wrong with using very small ground wire? For example. You are using #4 AWG in the Hot and Neutral wires (70 ampere capacity) and you use #14 AWG as ground (15 ampere capacity). If the ground is connected to an enclosure and the Hot wire touches it. It would take very fast for the breaker to trip, not enough to heat the #14 AWG. What is the reasons this is not allowed?
grounding
grounding
asked 1 hour ago
Samzun
61
asked 1 hour ago
Samzun
61
asked 1 hour ago
Samzun
61
asked 1 hour ago
Samzun
61
asked 1 hour ago
Samzun
61
61
migrated from electronics.stackexchange.com 1 hour ago
This question came from our site for electronics and electrical engineering professionals, students, and enthusiasts.
migrated from electronics.stackexchange.com 1 hour ago
This question came from our site for electronics and electrical engineering professionals, students, and enthusiasts.
It is a bit more complicated than that. I am not an expert (someone else who is will answer more completely). But a couple of things: 1 - With a GFCI, even a little current going to ground will result in a very fast trip as the difference between neutral & hot is detected, but not all circuits have a GFCI; 2 - With a regular breaker, it is the current on the Hot wire that trips the breaker, and depending on the breaker (some allow for more time for motor startup, etc.) it can take a while. A regular breaker only trips really fast with a very high current overload.
– manassehkatz
1 hour ago
@manassehkatz interesting notion with the GFCI ground only having to carry 8-30 milliamps. However that relies on a key assumption: that this ground Is monogamous to this circuit. Given the retrofit grounding rules (grounds are allowed and encouraged to be promiscuous), that ground could easily be grounding other stuff that may not have GFCI protection. The GFCI would never know since GFCI's do not interact with ground.
– Harper
32 mins ago
@Harper I'm not actually suggesting using a small ground wire. I'm trying to say that the ONLY time where it could possibly make sense is with a GFCI - i.e., the same place where you might end up not using a ground wire at all.
– manassehkatz
29 mins ago
@manassehkatz heh, yes, true.
– Harper
29 mins ago
add a comment |Â
It is a bit more complicated than that. I am not an expert (someone else who is will answer more completely). But a couple of things: 1 - With a GFCI, even a little current going to ground will result in a very fast trip as the difference between neutral & hot is detected, but not all circuits have a GFCI; 2 - With a regular breaker, it is the current on the Hot wire that trips the breaker, and depending on the breaker (some allow for more time for motor startup, etc.) it can take a while. A regular breaker only trips really fast with a very high current overload.
– manassehkatz
1 hour ago
@manassehkatz interesting notion with the GFCI ground only having to carry 8-30 milliamps. However that relies on a key assumption: that this ground Is monogamous to this circuit. Given the retrofit grounding rules (grounds are allowed and encouraged to be promiscuous), that ground could easily be grounding other stuff that may not have GFCI protection. The GFCI would never know since GFCI's do not interact with ground.
– Harper
32 mins ago
@Harper I'm not actually suggesting using a small ground wire. I'm trying to say that the ONLY time where it could possibly make sense is with a GFCI - i.e., the same place where you might end up not using a ground wire at all.
– manassehkatz
29 mins ago
@manassehkatz heh, yes, true.
– Harper
29 mins ago
It is a bit more complicated than that. I am not an expert (someone else who is will answer more completely). But a couple of things: 1 - With a GFCI, even a little current going to ground will result in a very fast trip as the difference between neutral & hot is detected, but not all circuits have a GFCI; 2 - With a regular breaker, it is the current on the Hot wire that trips the breaker, and depending on the breaker (some allow for more time for motor startup, etc.) it can take a while. A regular breaker only trips really fast with a very high current overload.
– manassehkatz
1 hour ago
It is a bit more complicated than that. I am not an expert (someone else who is will answer more completely). But a couple of things: 1 - With a GFCI, even a little current going to ground will result in a very fast trip as the difference between neutral & hot is detected, but not all circuits have a GFCI; 2 - With a regular breaker, it is the current on the Hot wire that trips the breaker, and depending on the breaker (some allow for more time for motor startup, etc.) it can take a while. A regular breaker only trips really fast with a very high current overload.
– manassehkatz
1 hour ago
@manassehkatz interesting notion with the GFCI ground only having to carry 8-30 milliamps. However that relies on a key assumption: that this ground Is monogamous to this circuit. Given the retrofit grounding rules (grounds are allowed and encouraged to be promiscuous), that ground could easily be grounding other stuff that may not have GFCI protection. The GFCI would never know since GFCI's do not interact with ground.
– Harper
32 mins ago
@manassehkatz interesting notion with the GFCI ground only having to carry 8-30 milliamps. However that relies on a key assumption: that this ground Is monogamous to this circuit. Given the retrofit grounding rules (grounds are allowed and encouraged to be promiscuous), that ground could easily be grounding other stuff that may not have GFCI protection. The GFCI would never know since GFCI's do not interact with ground.
– Harper
32 mins ago
@Harper I'm not actually suggesting using a small ground wire. I'm trying to say that the ONLY time where it could possibly make sense is with a GFCI - i.e., the same place where you might end up not using a ground wire at all.
– manassehkatz
29 mins ago
@Harper I'm not actually suggesting using a small ground wire. I'm trying to say that the ONLY time where it could possibly make sense is with a GFCI - i.e., the same place where you might end up not using a ground wire at all.
– manassehkatz
29 mins ago
@manassehkatz heh, yes, true.
– Harper
29 mins ago
@manassehkatz heh, yes, true.
– Harper
29 mins ago
add a comment |Â
2 Answers
2
active
oldest
votes
up vote
3
down vote
Total heat notwithstanding, wires also have resistance. As in "ohm" resistance.
What you may not be understanding is breaker trip curves. Breakers have a thermal trip mode from 1.0x up to about 10.0x breaker rating, where the action is delayed as a bimetal strip warms up. This is to be permissive of motor starts, inrush current, or "toaster and microwave at the same time" short term overloads. The thermal trip is sized to warm at about the same speed as the wire in the walls, so it trips somewhat before the wire warms enough to be dangerous. That requires matching between breaker and wire size.
Breakers also have a magnetic trip mode that instantly trips at around 10x breaker rating.
With too-small a ground wire, the breaker cannot magnetic-trip because it cannot flow enough current to hit 10x, because of the too-high resistance of the wire. And as you can guess, that's trouble.
Now we're in thermal trip mode. We have a 70A breaker's bimetal strip, in a race with a #14 wire to see if the breaker trips before the #14 wire hits Fahrenheit 451. Obviously, the breaker is going to lose.
Now as a stupidly practical thing, one way you can solve this is do the big stuff in EMT or other fixed metal conduit. In those cases, the steel conduit and boxes are the ground path. And steel conduit has such good conductance (1/resistance) that it can ground pretty much any wire that will fit in it.
answered 42 mins ago
Harper
57.3k335116
One point this glosses over is that ground wires are undersized relative to the circuit conductors on circuits that size -- a 70A or 80A circuit using 4AWG for the CCCs will use an 8AWG ground wire as per NEC table 250.122 (in fact, that 8AWG EGC is good all the way out to 100A).
– ThreePhaseEel
10 mins ago
add a comment |Â
up vote
0
down vote
A fuse, especially a slow-blow could allow enough current to pass for enough time to completely blow off a small ground wire, leaving the device connected to the mains through a nice fat wire, ready to cause death and mayhem.
The fuse should be specified to protect the mains wiring (AWG4 in your case) so it will also protect an AWG4 ground wire, should a ground fault occur.
answered 17 mins ago
Spehro Pefhany
40127
add a comment |Â
2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
3
down vote
Total heat notwithstanding, wires also have resistance. As in "ohm" resistance.
What you may not be understanding is breaker trip curves. Breakers have a thermal trip mode from 1.0x up to about 10.0x breaker rating, where the action is delayed as a bimetal strip warms up. This is to be permissive of motor starts, inrush current, or "toaster and microwave at the same time" short term overloads. The thermal trip is sized to warm at about the same speed as the wire in the walls, so it trips somewhat before the wire warms enough to be dangerous. That requires matching between breaker and wire size.
Breakers also have a magnetic trip mode that instantly trips at around 10x breaker rating.
With too-small a ground wire, the breaker cannot magnetic-trip because it cannot flow enough current to hit 10x, because of the too-high resistance of the wire. And as you can guess, that's trouble.
Now we're in thermal trip mode. We have a 70A breaker's bimetal strip, in a race with a #14 wire to see if the breaker trips before the #14 wire hits Fahrenheit 451. Obviously, the breaker is going to lose.
Now as a stupidly practical thing, one way you can solve this is do the big stuff in EMT or other fixed metal conduit. In those cases, the steel conduit and boxes are the ground path. And steel conduit has such good conductance (1/resistance) that it can ground pretty much any wire that will fit in it.
answered 42 mins ago
Harper
57.3k335116
One point this glosses over is that ground wires are undersized relative to the circuit conductors on circuits that size -- a 70A or 80A circuit using 4AWG for the CCCs will use an 8AWG ground wire as per NEC table 250.122 (in fact, that 8AWG EGC is good all the way out to 100A).
– ThreePhaseEel
10 mins ago
add a comment |Â
up vote
3
down vote
Total heat notwithstanding, wires also have resistance. As in "ohm" resistance.
What you may not be understanding is breaker trip curves. Breakers have a thermal trip mode from 1.0x up to about 10.0x breaker rating, where the action is delayed as a bimetal strip warms up. This is to be permissive of motor starts, inrush current, or "toaster and microwave at the same time" short term overloads. The thermal trip is sized to warm at about the same speed as the wire in the walls, so it trips somewhat before the wire warms enough to be dangerous. That requires matching between breaker and wire size.
Breakers also have a magnetic trip mode that instantly trips at around 10x breaker rating.
With too-small a ground wire, the breaker cannot magnetic-trip because it cannot flow enough current to hit 10x, because of the too-high resistance of the wire. And as you can guess, that's trouble.
Now we're in thermal trip mode. We have a 70A breaker's bimetal strip, in a race with a #14 wire to see if the breaker trips before the #14 wire hits Fahrenheit 451. Obviously, the breaker is going to lose.
Now as a stupidly practical thing, one way you can solve this is do the big stuff in EMT or other fixed metal conduit. In those cases, the steel conduit and boxes are the ground path. And steel conduit has such good conductance (1/resistance) that it can ground pretty much any wire that will fit in it.
answered 42 mins ago
Harper
57.3k335116
One point this glosses over is that ground wires are undersized relative to the circuit conductors on circuits that size -- a 70A or 80A circuit using 4AWG for the CCCs will use an 8AWG ground wire as per NEC table 250.122 (in fact, that 8AWG EGC is good all the way out to 100A).
– ThreePhaseEel
10 mins ago
add a comment |Â
up vote
3
down vote
up vote
3
down vote
Total heat notwithstanding, wires also have resistance. As in "ohm" resistance.
What you may not be understanding is breaker trip curves. Breakers have a thermal trip mode from 1.0x up to about 10.0x breaker rating, where the action is delayed as a bimetal strip warms up. This is to be permissive of motor starts, inrush current, or "toaster and microwave at the same time" short term overloads. The thermal trip is sized to warm at about the same speed as the wire in the walls, so it trips somewhat before the wire warms enough to be dangerous. That requires matching between breaker and wire size.
Breakers also have a magnetic trip mode that instantly trips at around 10x breaker rating.
With too-small a ground wire, the breaker cannot magnetic-trip because it cannot flow enough current to hit 10x, because of the too-high resistance of the wire. And as you can guess, that's trouble.
Now we're in thermal trip mode. We have a 70A breaker's bimetal strip, in a race with a #14 wire to see if the breaker trips before the #14 wire hits Fahrenheit 451. Obviously, the breaker is going to lose.
Now as a stupidly practical thing, one way you can solve this is do the big stuff in EMT or other fixed metal conduit. In those cases, the steel conduit and boxes are the ground path. And steel conduit has such good conductance (1/resistance) that it can ground pretty much any wire that will fit in it.
answered 42 mins ago
Harper
57.3k335116
Total heat notwithstanding, wires also have resistance. As in "ohm" resistance.
What you may not be understanding is breaker trip curves. Breakers have a thermal trip mode from 1.0x up to about 10.0x breaker rating, where the action is delayed as a bimetal strip warms up. This is to be permissive of motor starts, inrush current, or "toaster and microwave at the same time" short term overloads. The thermal trip is sized to warm at about the same speed as the wire in the walls, so it trips somewhat before the wire warms enough to be dangerous. That requires matching between breaker and wire size.
Breakers also have a magnetic trip mode that instantly trips at around 10x breaker rating.
With too-small a ground wire, the breaker cannot magnetic-trip because it cannot flow enough current to hit 10x, because of the too-high resistance of the wire. And as you can guess, that's trouble.
Now we're in thermal trip mode. We have a 70A breaker's bimetal strip, in a race with a #14 wire to see if the breaker trips before the #14 wire hits Fahrenheit 451. Obviously, the breaker is going to lose.
Now as a stupidly practical thing, one way you can solve this is do the big stuff in EMT or other fixed metal conduit. In those cases, the steel conduit and boxes are the ground path. And steel conduit has such good conductance (1/resistance) that it can ground pretty much any wire that will fit in it.
answered 42 mins ago
Harper
57.3k335116
answered 42 mins ago
Harper
57.3k335116
answered 42 mins ago
Harper
57.3k335116
answered 42 mins ago
Harper
57.3k335116
57.3k335116
One point this glosses over is that ground wires are undersized relative to the circuit conductors on circuits that size -- a 70A or 80A circuit using 4AWG for the CCCs will use an 8AWG ground wire as per NEC table 250.122 (in fact, that 8AWG EGC is good all the way out to 100A).
– ThreePhaseEel
10 mins ago
add a comment |Â
One point this glosses over is that ground wires are undersized relative to the circuit conductors on circuits that size -- a 70A or 80A circuit using 4AWG for the CCCs will use an 8AWG ground wire as per NEC table 250.122 (in fact, that 8AWG EGC is good all the way out to 100A).
– ThreePhaseEel
10 mins ago
One point this glosses over is that ground wires are undersized relative to the circuit conductors on circuits that size -- a 70A or 80A circuit using 4AWG for the CCCs will use an 8AWG ground wire as per NEC table 250.122 (in fact, that 8AWG EGC is good all the way out to 100A).
– ThreePhaseEel
10 mins ago
One point this glosses over is that ground wires are undersized relative to the circuit conductors on circuits that size -- a 70A or 80A circuit using 4AWG for the CCCs will use an 8AWG ground wire as per NEC table 250.122 (in fact, that 8AWG EGC is good all the way out to 100A).
– ThreePhaseEel
10 mins ago
add a comment |Â
up vote
0
down vote
A fuse, especially a slow-blow could allow enough current to pass for enough time to completely blow off a small ground wire, leaving the device connected to the mains through a nice fat wire, ready to cause death and mayhem.
The fuse should be specified to protect the mains wiring (AWG4 in your case) so it will also protect an AWG4 ground wire, should a ground fault occur.
answered 17 mins ago
Spehro Pefhany
40127
add a comment |Â
up vote
0
down vote
A fuse, especially a slow-blow could allow enough current to pass for enough time to completely blow off a small ground wire, leaving the device connected to the mains through a nice fat wire, ready to cause death and mayhem.
The fuse should be specified to protect the mains wiring (AWG4 in your case) so it will also protect an AWG4 ground wire, should a ground fault occur.
answered 17 mins ago
Spehro Pefhany
40127
add a comment |Â
up vote
0
down vote
up vote
0
down vote
A fuse, especially a slow-blow could allow enough current to pass for enough time to completely blow off a small ground wire, leaving the device connected to the mains through a nice fat wire, ready to cause death and mayhem.
The fuse should be specified to protect the mains wiring (AWG4 in your case) so it will also protect an AWG4 ground wire, should a ground fault occur.
answered 17 mins ago
Spehro Pefhany
40127
A fuse, especially a slow-blow could allow enough current to pass for enough time to completely blow off a small ground wire, leaving the device connected to the mains through a nice fat wire, ready to cause death and mayhem.
The fuse should be specified to protect the mains wiring (AWG4 in your case) so it will also protect an AWG4 ground wire, should a ground fault occur.
answered 17 mins ago
Spehro Pefhany
40127
answered 17 mins ago
Spehro Pefhany
40127
answered 17 mins ago
Spehro Pefhany
40127
answered 17 mins ago
Spehro Pefhany
40127
40127
add a comment |Â
add a comment |Â
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It is a bit more complicated than that. I am not an expert (someone else who is will answer more completely). But a couple of things: 1 - With a GFCI, even a little current going to ground will result in a very fast trip as the difference between neutral & hot is detected, but not all circuits have a GFCI; 2 - With a regular breaker, it is the current on the Hot wire that trips the breaker, and depending on the breaker (some allow for more time for motor startup, etc.) it can take a while. A regular breaker only trips really fast with a very high current overload.
– manassehkatz
1 hour ago
@manassehkatz interesting notion with the GFCI ground only having to carry 8-30 milliamps. However that relies on a key assumption: that this ground Is monogamous to this circuit. Given the retrofit grounding rules (grounds are allowed and encouraged to be promiscuous), that ground could easily be grounding other stuff that may not have GFCI protection. The GFCI would never know since GFCI's do not interact with ground.
– Harper
32 mins ago
@Harper I'm not actually suggesting using a small ground wire. I'm trying to say that the ONLY time where it could possibly make sense is with a GFCI - i.e., the same place where you might end up not using a ground wire at all.
– manassehkatz
29 mins ago
@manassehkatz heh, yes, true.
– Harper
29 mins ago