8-2 w/G wire (how many amps?)

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Sometimes I hate coming home from work at least 3 hours after you guy's do. I was going to say something similar to Roger, but scrolling down showed me he beats me to it one more time. One of these days Rog, I'm coming home early.......
 
m73214 said:
According to my calcs, you would need 3/0 CU to supply a 120V,20A load with a max voltage drop of 3.6V @1000'.
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1793 said:
With this answer I come up with 1/0 for EGC.
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To meet the requirements of 250.122(B) you would need a 3/0 CU EGC.

In fact using 8/2 NM on a 20 amp circuit will create a 250.122(B) violation.
 
Dnkldorf said:
Bob, how?

The egc is already upsized?
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It is but not enough to comply with the requirements of 250.122(B)

Table 250.122 shows that 15, 20 and 30 amp circuits require an EGC that is sized to a 1:1 ratio with the circuit conductors.

250.122(B) requires the ratio to be maintained.....no matter how strange and costly the results are.
 
iwire said:
It is but not enough to comply with the requirements of 250.122(B)

Table 250.122 shows that 15, 20 and 30 amp circuits require an EGC that is sized to a 1:1 ratio with the circuit conductors.

250.122(B) requires the ratio to be maintained.....no matter how strange and costly the results are.
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I don't see this.

Tbl 250.122 is showing me that a 20a circuit requires a #12 min EGC. It says nothing about a ratio.

Where did you see this ratio being required?
 
Dnkldorf said:
I don't see this.

Tbl 250.122 is showing me that a 20a circuit requires a #12 min EGC. It says nothing about a ratio.

Where did you see this ratio being required?
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250.122(B)

I can't post it from where I am.
 
Here you go Bob. ;)

(B) Increased in Size Where ungrounded conductors are increased in size, equipment grounding conductors, where installed, shall be increased in size proportionately according to circular mil area of the ungrounded conductors.
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Roger
 
Here is the proportional relationship that Bob was talking about:

250.122 Size of Equipment Grounding Conductors.
(A) General. Copper, aluminum, or copper-clad aluminum equipment grounding conductors of the wire type shall not be smaller than shown in Table 250.122 but shall not be required to be larger than the circuit conductors supplying the equipment. Where a raceway or a cable armor or sheath is used as the equipment grounding conductor, as provided in 250.118 and 250.134(A), it shall comply with 250.4(A)(5) or (B)(4).
(B) Increased in Size. Where ungrounded conductors are increased in size, equipment grounding conductors, where installed, shall be increased in size proportionately according to circular mil area of the ungrounded conductors.
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Good morning gentlemen and thank you both. :)

I did some searching and found the handbook commentary.

Equipment grounding conductors on the load side of the service disconnecting means and overcurrent devices are sized based on the size of the feeder or branch circuit overcurrent devices ahead of them. If the ungrounded conductors are increased in size to compensate for voltage drop, the equipment grounding conductors must also be increased proportionately.

Example
A 240-volt, single-phase, 250-ampere load is supplied from a 300-ampere breaker located in a panelboard 500 ft away. The conductors are 250 kcmil copper, installed in rigid nonmetallic conduit, with a 4 AWG copper equipment grounding conductor. If the conductors are increased to 350 kcmil, to what size must the equipment grounding conductor be increased?
Solution

Step 1.
Calculate the size ratio of the new conductors to the existing conductors:

Step 2.
Calculate the cross-sectional area of the new equipment grounding conductor. According to Chapter 9, Table 8, 4 AWG, the size of the existing grounding conductor has a cross-sectional area of 41,740 circular mils.

Step 3.
Determine the size of the new equipment grounding conductor. Again, referring to Chapter 9, Table 8, we find that 58,436 circular mils is larger than 3 AWG. The next larger size is 66,360 circular mils, which converts to a 2 AWG copper equipment grounding conductor.
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Mike and Trevor, I guess I'm on a roll here. ;)

George, I have done a number of installations where we have increased voltage to compensate for distance and to reduce material cost. My 7200v was a little bit of an exageration for this though. :)

Roger
 
I was thinking that 250.122b was just for upsizing the EGC for voltage drop.

Hypotheticaly here.

A piece of 8-2NM 20' long feeding a piece of equipment, on a 20a breaker, is a violation of 250.122b?

The EGC in this case would satisfy 250.4(a)(5) wouldn't it?
 
Dnkldorf said:
Hypotheticaly here.

A piece of 8-2NM 20' long feeding a piece of equipment, on a 20a breaker, is a violation of 250.122b?
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Nothing hypothetical here...it would be a direct violation of 250.122(B)

The EGC in this case would satisfy 250.4(a)(5) wouldn't it?
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I would say yes but as you know we have to satisfy all requirements.

In your example the EGC will need to be an 8 AWG.

Change the breaker to a 40 amp and the EGC could be 10 AWG.

Strange but true.
 
iwire said:
Nothing hypothetical here...it would be a direct violation of 250.122(B)


Strange but true.
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Bob, why do you do this to me on a saturday morning?

This will be on my mind all day now.

UGH..
 
iwire said:
Good morning gentlemen and thank you both. :)

I did some searching and found the handbook commentary.
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I thought this what I did in my post. Just looking at the numbers. If we took the #8 up to 3/0, that 916%. then the #10, 10380, would increase by 916% to 105,460.8, to 1/0 @ 105,600?
 
1793 said:
I thought this what I did in my post. Just looking at the numbers. If we took the #8 up to 3/0, that 916%. then the #10, 10380, would increase by 916% to 105,460.8, to 1/0 @ 105,600?
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If you look at 250.122 you'll see that a 20 amp OCPD requires the same size EGC as current carrying conductors. Therefore the ratio required ratio is 1 to 1.
 
How can a #10 EGC provide a low enough impeadance to clear a 40A fault, and not be sufficient enough to clear a 20A fault?
 
Dnkldorf said:
How can a #10 EGC provide a low enough impeadance to clear a 40A fault, and not be sufficient enough to clear a 20A fault?
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I am just going to give you my untrained opinion.

It could in your example.

The issue IMO is only a by product of the CMP wanting to fix a real problem in some circuits but the wording they chose does that and much more.

FWIW if we are talking about a bolted ground fault condition the current will be equal regardless of a 15 or a 100 amp breaker.

I believe the instantaneous trip of breakers from 15 to 100 is pretty much the same.

It takes a possible but odd / unusual condition (IMO) to put a slow overload condition on the EGC that would be tripping the thermals in any breaker.
 
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