EGC increased in size per 250.122(B)

[sandsnow]

Anyone know of a web based or anything type of calculator for this? Such as plug in wire sizes and you get an answer in wire size.

Gets kind of tedious with a lot of feeders to check.

 

[coulter]

... web based or anything type of calculator ...

Larry -

JAO - the problem with "anything type of calculator" web based or not, is knowing if the algolrithm is correct. I've seen a few that weren't. Could be they are worth what you pay for them.

carl

 

[George Stolz]

That's a heck of an idea. I think I'll whip one together for my PDA, I was going to get a conduit-fill guide cooking anyway.

 

[stickboy1375]

I use this quite a bit, very easy and you can upgrade with future code changes... Look on e-bay I got mine for about 30 dollars, they wanted 90 dollars at the supply house...

 

[sandsnow]

coulter
True, though the math isn't hard, just time consuming. People tell me it could be done with an Excel macro.

george
Make one for me please!

stickboy
I must have missed something in the instructions. I'll have to check mine tomorrow.

 

[sandsnow]

You can get the CM of a conductor by entering the wire size such as 4/0 and then pressing the Grnd key 3 times. I don't see how this makes it any faster to get the increased EGC size.

Are there some shortcuts I'm missing?

Thanks

 

[Bob NH]

Larry -

JAO - the problem with "anything type of calculator" web based or not, is knowing if the algolrithm is correct. I've seen a few that weren't. Could be they are worth what you pay for them.

carl

The algorithm is not obvious from the paragraph and Table 250.122. For example:

A 10 AWG (10.38 kcmil) copper EGC is permitted for 30, 40, 50, and 60 Amp overcurrent devices.

I can use 10 AWG EGC with a 60 amp breaker with 6 AWG (26.24 kcmil) ungrounded conductor. If I use 2 AWG (66.36 kcmil) for the ungrounded conductor, then I need to increase the EGC proportionaltely to:

(10.38 kcmil)(66.36 kcmil/26.24 kcmil) = 26.25 kcmil ==> 6 AWG
(I assume that the difference of 0.01 kcmil is within the tolerance and 6 AWG would be acceptable)

Now what do I do if my original circuit was 30 Amps with 10 AWG and I want to increase it to 2 AWG to reduce voltage drop? If I increase it proportionately to the original ungrounded conductor, then I would have to increase the EGC to 2 AWG.

So it would be more economical to put a 60 amp breaker on the circuit and allow use of the smaller wire.

What is the correct algorithm, and how does it apply to the two cases in the illustration?

And if the answer is that a 30 Amp breaker circuit with 2 AWG ungrounded conductor requires a 2 AWG EGC, while the same ungrounded conductor with a 60 Amp breaker requires only a 6 AWG EGC; then what is the technical rationale for that requirement?

 

[coulter]

Bob -

Yikes! You picked an interesting one.

... The algorithm is not obvious from the paragraph and Table 250.122. ...

Yes, I'd agree with that. Although I probably would have said, "not always obvious".

... What is the correct algorithm, and how does it apply to the two cases in the illustration?...

I would say you nailed it.

...And if the answer is that a 30 Amp breaker circuit with 2 AWG ungrounded conductor requires a 2 AWG EGC, while the same ungrounded conductor with a 60 Amp breaker requires only a 6 AWG EGC; then what is the technical rationale for that requirement?

Hummm. One might think that if the 30A circuit conductors needed to be upsized for VD, then likely the 60A ckt would also need to be upsized. So it isn't exacly an apples to apples comparison. (But you knew that already

... then what is the technical rationale for that requirement?

As you laid it out, none I know. File this one under, "The code isn't a design guide. We can dig holes that make a strict interpretation near ridculous." (This one also falls under the "You already knew this catagory.")

carl

 

[don_resqcapt19]

Bob,

And if the answer is that a 30 Amp breaker circuit with 2 AWG ungrounded conductor requires a 2 AWG EGC, while the same ungrounded conductor with a 60 Amp breaker requires only a 6 AWG EGC; then what is the technical rationale for that requirement?

There is no technical reason for this. How would you write the rule to avoid this problem?
Don

 

[iwire]

How would you write the rule to avoid this problem?
Don

I think therein lies the problem, to make the rule perfect would make for a long and complicated section of code.

But I also think that is what Don is thinking.

 

[steve66]

I think it is pretty obvious that if you upsize the breaker to avoid upsizing the GEC, you are cheating.

And many 30A loads wouldn't be permitted on a 60A breaker per other sections of the code.

I think this rule is better than the old "if the wire is increased in size due to voltage drop....." That really didn't ever give the inspectors a chance to enforce that paragraph. I doubt anyone would ever admit they upsized the wire "for voltage drop". It was easy to say "Well, 20% voltage drop would have been fine, but I just happened to have this larger wire on my truck...."

Steve

 

[charlie b]

How would you write the rule to avoid this problem?

How about this (new text is in bold):

"Where ungrounded conductors are increased in size beyond the smallest size that satisfies the required ampacity . . . ."

 

[don_resqcapt19]

Charlie,
How does that change anything for OCPDs 30 amps and below? We still have a one to one ratio between the circuit conductor and the EGC.
Don

 

[sandsnow]

I was hoping for a calculator of some sort to be used as follows:

Enter min. size circuit conductor required by Code such as #2 used for 100A feeder
Enter increased circuit conductor size say 1/0
Enter min. EGC in this case #8
Push Button and receive answer in this case #4

That's all I want it to do. I was just wondering if one existed.

Looks like I'm going to be taking an Excel class. Maybe I'll be able to write my own.

 

[coulter]

And if the answer is that a 30 Amp breaker circuit with 2 AWG ungrounded conductor requires a 2 AWG EGC, while the same ungrounded conductor with a 60 Amp breaker requires only a 6 AWG EGC; then what is the technical rationale for that requirement?

There is no technical reason for this. ...Don

Maybe there is a reason.

For a start, #2 on a 30ACB is likely because it is an 800' circuit. As we all knew, the bonding impedance needs to be low to trip the CB quickly. Hence the oversize EBC. No news here.

Now, a 60ACB with #2 it is likely a 400' circuit. With only half the wire, smaller wire is okay for the EBC. Aarggg! Still no news.

Now, I need some help with an application where this could matter: Short Circuit Engineering in conjunction with Runnin Hot Electric Contractors have equipment to install normally covered by 30ACB and #10. But it is 800 feet away and it has to work at least once before hey get paid. So, so they decide to install the bob NH circuit.

Here is the question: For $3.40, paid at the yuppie coffie stand of my choice, "What could that piece of equipment be?"

carl

 

[Bob NH]

Bob,

There is no technical reason for this. How would you write the rule to avoid this problem?
Don

Don,

Here is a suggested amendment to 250.122(B).

250.122(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, but shall not be required to be larger than the minimum size equipment grounding conductor required by Table 250.122 that corresponds to the ungrounded conductor ampacity specified in 310.15.

Table 250.122 is based on overcurrent protection ratings and 240.4 uses the phrase ". . . shall be protected against overcurrent in accordance with their ampacities specified in 310.15, . . . ", so I used the "ampacity specified in 310.15" on the presumption that it is an understood and accepted usage.

I need to do more analysis, but at a first cut it appears that the proposed language would provide about the same ratio of fault current/OCPD for the 30 amp case with larger ungrounded conductors as it does for the 60 amp case with minimum conductors.

The "small conductor rule" for 10, 12, and 14 AWG conductors causes severe distortion when the 250.122(B) requirement is applied. For larger conductors the minimum equipment grounding conductor is less than 15 percent of the area of the ungrounded conductor. The rationale supporting that result is not apparent and it may just be accidental or the result of an attempt to "smooth" the requirement over the range of OCPD ratings.

Bob

 

[winnie]

Enter min. size circuit conductor required by Code such as #2 used for 100A feeder
Enter increased circuit conductor size say 1/0
Enter min. EGC in this case #8
Push Button and receive answer in this case #4

If you stay in the range of wire sizes where AWG is used, then there is a very quick and easy way to answer the above sizing question.

Because of the way AWG sizes are defined, a given numeric difference in AWG size will always have the same proportion of circular mil area, perhaps with negligible differences created by rounding. The area ratio of a #1/0 conductor to a #2 conductor is the same as the area ratio of a #6 conductor to a #8 conductor.

This provides a simple method for doing the proportional area calculation.

upsized EGC gauge = {standard EGC gauge} - ( {standard circuit conductor gauge} - { upsized circuit conductor gauge} )

If you make the circuit conductors 4 gauges bigger, then you make the EGC 4 gauges bigger. Unfortunately this falls apart when you hit the kcmil sized conductors.

The equation for AWG size is referenced here:
http://www.unc.edu/~rowlett/units/dictG.html#gauge

-Jon

 

[ptonsparky]

I was hoping for a calculator of some sort to be used as follows:

Enter min. size circuit conductor required by Code such as #2 used for 100A feeder
Enter increased circuit conductor size say 1/0
Enter min. EGC in this case #8
Push Button and receive answer in this case #4

That's all I want it to do. I was just wondering if one existed.

Looks like I'm going to be taking an Excel class. Maybe I'll be able to write my own.

I have not worked this particular problem, but have done it for conduit fill, voltage drop, box fill, fault current calculations, and others, You can do it with a spreadsheet, just takes time.

 

[charlie b]

Charlie, How does that change anything for OCPDs 30 amps and below? We still have a one to one ratio between the circuit conductor and the EGC.

It doesn?t, and it doesn?t have to. If for example you calculate a load of 17 amps, and if you planned to use copper THHN, then that load would require a minimum of #12. Now if you use a #10, or anything larger, then you have met the description I suggested above. You are using a size that is larger than the size needed to support the load. Therefore, you must upsize the EGC.

My suggested wording has nothing to do with figuring out what new size EGC you need, nor does it impact the relative size of the new EGC and the new ungrounded conductors. My suggested wording was only a way to clarify whether or not you need to upsize the EGC.

 

[charlie b]

Looks like I'm going to be taking an Excel class. Maybe I'll be able to write my own.

I once created a Excel spreadsheet that did this very job. But it was designed to work in conjuction with a "company standard" table for feeder sizes. Many engineering companies have such tables, created by employees for their own use. I noticed that if you starting with a known load, and picked the feeder type from the table, and then wanted to upsize the feeder (e.g., for voltage drop), and just picked the next size feeder from the table, you might fall into the trap of using an EGC that was not correctly upsized.

So I came up with a simple spreadsheet that was intended to save time and effort, in that it would tell you what size EGC you needed, if you upsized from one feeder type to another. It's actually a simple concept. But you need to input the actual cross-sectional areas of each conductor size, so there is an opportunity for a typing error to invalidate the spreadsheet. You need to have it checked in detail by another person, after you finish typing it all in.