volt drop and upsized ground

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flatlander

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Hi yall,
As a question from another thread. Given that a conductors termination at both ends are good for 75c. And since ampacity varies with temp rating. Who choose if a conductor is upsized or using the lowest temp connection.
 
flatlander said:
Hi yall,
As a question from another thread. Given that a conductors termination at both ends are good for 75c. And since ampacity varies with temp rating. Who choose if a conductor is upsized or using the lowest temp connection.
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If I understand you correctly, this relates to who determines the "normal" conductor size so as to not require an EGC upsize. In the 2014 NEC there was some rewording in 250.122(B) to make this a little more clear.
 
texie said:
If I understand you correctly, this relates to who determines the "normal" conductor size so as to not require an EGC upsize. In the 2014 NEC there was some rewording in 250.122(B) to make this a little more clear.
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Yes that was my concern. I don't have a copy of the 2014. Our State won't be on that until 2017 or so.
Can you provide that please it sound interesting.

Anyway what do you do under the 2011.
 
flatlander said:
Yes that was my concern. I don't have a copy of the 2014. Our State won't be on that until 2017 or so.
Can you provide that please it sound interesting.

Anyway what do you do under the 2011.
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I just upsize the EGC, I am too lazy to figure out a way around it.

PS. I hope someone posts the 2014, I would like to see what it says. I haven't bought a copy yet either.
 
jumper said:
I just upsize the EGC, I am too lazy to figure out a way around it.

PS. I hope someone posts the 2014, I would like to see what it says. I haven't bought a copy yet either.
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Here ya go....NEC 2014, 250.122(B). I bolded the new text.
(B) Increased in Size.
Where ungrounded conductors are
increased in size from the minimum size that has sufficient
ampacity for the intended installation, wire-type equipment
grounding conductors, where installed, shall be increased in
size proportionately according to the circular mil area of
the ungrounded conductors.
 
texie said:
Here ya go....NEC 2014, 250.122(B). I bolded the new text.
(B) Increased in Size.
Where ungrounded conductors are
increased in size from the minimum size that has sufficient
ampacity for the intended installation, wire-type equipment
grounding conductors, where installed, shall be increased in
size proportionately according to the circular mil area of
the ungrounded conductors.
Click to expand...

Dumbest rule in the NEC. No engineering reasoning behind this rule.
 
Would it not stand to reason that if there is so much impedance on a circuit that the wire size must be increased to achieve suitable operation under normal conditions, then the fault path must also be upsized if the ocpd is going to clear the fault in a timely manner?
 
arcsnsparks98 said:
Would it not stand to reason that if there is so much impedance on a circuit that the wire size must be increased to achieve suitable operation under normal conditions, then the fault path must also be upsized if the ocpd is going to clear the fault in a timely manner?
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Yes. What does not stand to reason is that the larger the over current protection the smaller the EGC.
 
arcsnsparks98 said:
Would it not stand to reason that if there is so much impedance on a circuit that the wire size must be increased to achieve suitable operation under normal conditions, then the fault path must also be upsized if the ocpd is going to clear the fault in a timely manner?
Click to expand...
What does not stand to my reasoning is that when (for whatever reason) you upsize the ungrounded conductor, that alone will reduce the impedance of the fault path. Why, then, must we further reduce the impedance of the fault path by also upsizing the EGC?

 
charlie b said:
What does not stand to my reasoning is that when (for whatever reason) you upsize the ungrounded conductor, that alone will reduce the impedance of the fault path. Why, then, must we further reduce the impedance of the fault path by also upsizing the EGC?
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Two possible concerns:
1. You reduce the resistance of only 1/2 of the circuit, which may not be enough.
2. FWIW, the voltage divider effect would put closer to the full line voltage rather than 1/2 the line voltage on exposed bonded metal.
Another interesting observation is that at some (extreme) distance the earth path between two good grounding electrodes will become a better path than the EGC, and the whole safety question becomes moot.
 
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arcsnsparks98 said:
Would it not stand to reason that if there is so much impedance on a circuit that the wire size must be increased to achieve suitable operation under normal conditions, then the fault path must also be up-sized if the ocpd is going to clear the fault in a timely manner?
Click to expand...

That is a good reason to increase the size of the circuit and EGC. The problem there is no science behind the rule of up-sizing. A poster a while back had a 20 amp circuit to a pump on a golf course. Because of the distance he installed 2/0 copper conductors and due to the rule he had to install a 2/0 EGC. According to the table, if he had used a 200 amp breaker, he would have needed a #6 EGC. There is no science behind the rule for the EGC when up-sizing a circuit. On long circuits, the designer needs to consider the size of the EGC using some engineering methods to ensure the conductor is properly sized.

Table 250.122?Minimum Size Equipment Grounding (Bonding) Conductor

Protection Rating.......CopperConductor d
15A..........................14 AWG
20A...........................12 AWG
30?60A..................10 AWG
70?100A..................8 AWG
110?200A................6 AWG
225?300A................4 AWG
350?400A................3 AWG
450?500A................2 AWG
600A...........................1 AWG
700?800A.............1/0 AWG
1,000A.....................2/0 AWG
1,200A.....................3/0 AWG​
 
GoldDigger said:
Two possible concerns:
1. You reduce the resistance of only 1/2 of the circuit, which may not be enough.
2. FWIW, the voltage divider effect would put closer to the full line voltage rather than 1/2 the line voltage on exposed bonded metal.
Another interesting observation is that at some (extreme) distance the earth path between two good grounding electrodes will become a better path than the EGC, and the whole safety question becomes moot.
Click to expand...
Yes, but the final upsized circuit conductor resistance is roughly the same as the resistance of the minimum size circuit conductor at a shorter length. This also results in less heat generated per linear distance by the upsized conductors. Change in wire resistance is ~ directly proportional to temperature change. A normal sized EGC would not be subjected to as high a temperature as a result of upsizing the circuit conductors, and thus have a lower resistance at the shorter length, roughly equivalent at the longer length.
 
My understanding is its not only about clearing an OCPD but providing equal potential while the fault is happening. Larger phases have the potential to give more current into the fault, but a smaller EGC would mean more voltage drop across the grounding conductor thus more potential between the frame experiencing the fault and another grounded object.
 
mbrooke said:
My understanding is its not only about clearing an OCPD but providing equal potential while the fault is happening. Larger phases have the potential to give more current into the fault, but a smaller EGC would mean more voltage drop across the grounding conductor thus more potential between the frame experiencing the fault and another grounded object.
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The sizing of the EGC is to provide a low impedance path for the fault current. Studies have shown that a 40 volt drop is a safe
level if you happen to be touching the the panel at the fault point. It is not an equal potential that is desired but a low voltage level.
 
bob said:
The sizing of the EGC is to provide a low impedance path for the fault current. Studies have shown that a 40 volt drop is a safe
level if you happen to be touching the the panel at the fault point. It is not an equal potential that is desired but a low voltage level.
Click to expand...

That low voltage level comes from the equal potential design, or more specifically bonding. Perhaps not a perfect zero equal potential under a fault (its physically impossible) but one that's at least below 100 volts. Its a good enough equal potential that's relatively safe, not perfect a perfect zero volts, that was my point.

Theoretically we could get away with a number 10 ground on a 100amp feeder, but in longer runs the impedance is just to high.
 
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