Question on 250.122(B), increasing the ground size.

[GerryB]

I believe this section means if you run larger conductors for voltage drop, like # 10's for a 20 amp circuit, then your ground wire would also have to be #10 because it is the same circular mill as the ungrounded conductors. (the wording about being increased in size "proportionately" is what confuses me, since the egc is often smaller.) Do I have this right?

In relation to this, a question came up about grounding/bonding the gas "tract" pipe. Around here a #6 is run from the corrugated gas pipe knuckle to the ground rods, water meter, or into the panel. (The plumber is allowed to do either of the first two methods.) But in this case it is new construction with temporary heat and a temp service about a hundred feet away. The question was could a ground rod be driven at the building and be done with it.

The answer from a state BO was "no, it has to go to the rods at the temp service." He also added that you should have an electrician do it because of the distance involved, which got me considering 250.122(B). Since there are no conductors here I am thinking there is no reason to increase the size of this ground wire. Any thoughts?

 

[electricalist]

My thought would be to run it in EMT and that would be my EGC no matter how much I increase my conductors it would be an acceptable ground.


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[Carultch]

I believe this section means if you run larger conductors for voltage drop, like # 10's for a 20 amp circuit, then your ground wire would also have to be #10 because it is the same circular mill as the ungrounded conductors. (the wording about being increased in size "proportionately" is what confuses me, since the egc is often smaller.) Do I have this right?

In relation to this, a question came up about grounding/bonding the gas "tract" pipe. Around here a #6 is run from the corrugated gas pipe knuckle to the ground rods, water meter, or into the panel. (The plumber is allowed to do either of the first two methods.) But in this case it is new construction with temporary heat and a temp service about a hundred feet away. The question was could a ground rod be driven at the building and be done with it.

The answer from a state BO was "no, it has to go to the rods at the temp service." He also added that you should have an electrician do it because of the distance involved, which got me considering 250.122(B). Since there are no conductors here I am thinking there is no reason to increase the size of this ground wire. Any thoughts?

It is true that you can use the conduit as a replacement for the EGC, provided that it has electrical continuity throughout the path from source to load. If you use a plastic conduit, obviously you have to use a wire EGC. Some job specs may call for a wire EGC even when conduit alone suffices per code.

Step 1: calculate the minimum size that has sufficient ampacity for the intended installation, of current-carrying conductors. I call this the minimum local size.
Step 2: calculate the corresponding equipment grounding conductor based off the OCPD per the table in 250.122, or other method that applies if OCPDs aren't part of the circuit.
Step 3: calculate the size of circuit conductors you need for curtailing voltage drop to your desired percentage. If this calculation is smaller than step 1, obviously step 1's calculation overrides it.
Step 4: take the ratio from steps 3 & 1. Multiply this ratio by what you determined in part 2. It is the KCMIL that you use in this calculation, which you have to cross reference with the gauge and aught sizes.

Your results from steps 3 and 4 are what become your resulting conductors. Observe that steps 1 and 2, are the calculation that you do assuming length is not a factor. Suppose it is only a 3-foot circuit, what sizes do you need? All factors that determine ampacity apply for step 1, such as terminations, temperature corrections, and bundling adjustment factors.

For 15A, 20A, and 30A circuits, the default ground and wire sizes are identical to one another. I.e. #14Cu, #12Cu, and #10 Cu respectively. So increasing these in size will result in also matching the EGC size to the current carrying conductors. You get more interesting calculation examples with 40A and 60A circuits. In general, if you increase the circuit conductors by N gauge size increments, you will increase the EGC by N gauge size increments as well. So a step from #6 to #2 for circuit conductors, would mean an EGC upsize from #10 to #6.

A 60A circuit with 75C terminations uses #6Cu wire and a #10Cu ground by default. Suppose we upsize to #2Cu.

#6 = 26.3 kcmil
#10 = 10.4 kcmil
#2 = 66.4 kcmil

EGC req'd = (kcmil of #10) * (kcmil of #2)/(kcmil of #6)
EGC req'd = 10.4 * 66.4/26.3
EGC req'd = 26.25 kcmil
EGC req'd = #6 Cu

 

[JFletcher]

I believe this section means if you run larger conductors for voltage drop, like # 10's for a 20 amp circuit, then your ground wire would also have to be #10 because it is the same circular mill as the ungrounded conductors. (the wording about being increased in size "proportionately" is what confuses me, since the egc is often smaller.) Do I have this right?

In relation to this, a question came up about grounding/bonding the gas "tract" pipe. Around here a #6 is run from the corrugated gas pipe knuckle to the ground rods, water meter, or into the panel. (The plumber is allowed to do either of the first two methods.) But in this case it is new construction with temporary heat and a temp service about a hundred feet away. The question was could a ground rod be driven at the building and be done with it.

The answer from a state BO was "no, it has to go to the rods at the temp service." He also added that you should have an electrician do it because of the distance involved, which got me considering 250.122(B). Since there are no conductors here I am thinking there is no reason to increase the size of this ground wire. Any thoughts?

250.122(B) is n/a here; there are no ungrounded conductors. The gas line pipe would need a GEC to the GES under 250.52, yes? Just looked, 250.52 FPN re: gas pipes goes to 250.104(B).

 

[don_resqcapt19]

I believe this section means if you run larger conductors for voltage drop, like # 10's for a 20 amp circuit, then your ground wire would also have to be #10 because it is the same circular mill as the ungrounded conductors. (the wording about being increased in size "proportionately" is what confuses me, since the egc is often smaller.) Do I have this right?

In relation to this, a question came up about grounding/bonding the gas "tract" pipe. Around here a #6 is run from the corrugated gas pipe knuckle to the ground rods, water meter, or into the panel. (The plumber is allowed to do either of the first two methods.) But in this case it is new construction with temporary heat and a temp service about a hundred feet away. The question was could a ground rod be driven at the building and be done with it.

The answer from a state BO was "no, it has to go to the rods at the temp service." He also added that you should have an electrician do it because of the distance involved, which got me considering 250.122(B). Since there are no conductors here I am thinking there is no reason to increase the size of this ground wire. Any thoughts?

There are no rules in the NEC that apply to that conductor.

 

[310 BLAZE IT]

How do I calculate the minimum size for something like a 1600A feeder? I can reach this with different combinations including both copper and aluminum. Also, what if it doesn't reach the next size? So say for this 1600A feeder I use 600kcmil Aluminum. I meet the ampacity with 5 sets, but I am using 6 in this case. By this logic I have 6*600(1600/(5*600)) which equals 1920A... does not even go up one rating in 250.122. This rule should be clarified for larger feeders. It is much different than the bonding jumper requirement but I think some people like to think it works similarly

 

[Carultch]

How do I calculate the minimum size for something like a 1600A feeder? I can reach this with different combinations including both copper and aluminum. Also, what if it doesn't reach the next size? So say for this 1600A feeder I use 600kcmil Aluminum. I meet the ampacity with 5 sets, but I am using 6 in this case. By this logic I have 6*600(1600/(5*600)) which equals 1920A... does not even go up one rating in 250.122. This rule should be clarified for larger feeders. It is much different than the bonding jumper requirement but I think some people like to think it works similarly

Good question, and I have the same question myself. The way I understand it, is that it is the minimum size in the same number of sets and configuration that has sufficient ampacity.

So if you are using 6 sets of 600 kcmil AL, though 5 sets would be sufficient, forget that you ever thought about 5 sets. Now revisit it, and determine what size it would be, if you were using 6 sets. That would be 400 kcmil. So your upsize ratio is 600/400.

Note that 1920A does go up one rating from 1600A. If 1920A were required for your circuit, you'd have to use 2000A worth of wire.


I agree that the way the rules are written, it leaves all sorts of solutions that feel like loopholes. Like instead of installing a 40A circuit with #8 wire upsized to #6 & #8 ground, I can install a 60A circuit with a #6 wire and a #10 ground. The circuit would not even be considered upsized for voltage drop, but instead for larger than necessary OCPD.