Sizing EGC when feeder is increased for VD

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bob

bob

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Alabama
This question was asked on another forum. I would like to hear your response.

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, what is the minimum size for the equipment grounding conductor based on the proportional-increase requirement?

The caculations were made correctly and #2 copper was given as the proper size.

There was nothing said about the 500 ft and its impact on the EGC.
You comments please.
 
Re: Sizing EGC when feeder is increased for VD

There are specific calculations that can be performed if you are concerned that the EGC will not perform correctly during a fault.

I am assuming from the post that the phase conductors were increased in size due to the long run, and as should be so was the EGC. Exactly what are you looking for?
 
Re: Sizing EGC when feeder is increased for VD

Originally posted by bob:The caculations were made correctly and #2 copper was given as the proper size.
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I did the calculation, and I agree with using a #2 EGC.
Originally posted by bob:There was nothing said about the 500 ft and its impact on the EGC.
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The impact is that it will be expensive, counting both material and labor costs. What other impact were you expecting?
 
Re: Sizing EGC when feeder is increased for VD

This example is exactly the same as the example in the 2005 NEC Handbook on page 231.
 
Re: Sizing EGC when feeder is increased for VD

I read something about this on another site. What I got was second hand. I am trying to get a link to the original forum where this question was posted.

The assertion made to me was that the new ecg size at that distance was not large enough to effectively clear a line to ground fault. Don't shoot the messenger, I am not an engineer, and I don't have access to the original forum where this topic was being discussed. I would like to hear from you engineers here. I would also like to hear from the OP about what exactly the original problem was.

I was sent a link to a site that has some software available to determine egc size. They had some case study done and it seems that they are asserting that the new ecg size is great as long as the conductors are run in steel raceway, but not if they are run in another type raceway. The study done was funded by an orginization of steel conduit manufacturers.

This is a link to the site that I am refering to: http://www.steelconduit.org/gemi.htm

[ December 01, 2005, 05:12 PM: Message edited by: jbwhite ]
 
Re: Sizing EGC when feeder is increased for VD

I wouldn't be too quick to accept any assertion that there is a problem in your installation, and that it can only be solved by buying their product.

In general, every 100 feet of length of a conductor will require upsizing the phase conductors one step, in order to keep voltage drop within reasonable limits. Each of those steps up in phase conductor size must be accompanied by a proportional change in EGC size. I think that that particular NEC rule has already taken into account the need for a larger EGC to help clear a ground fault. In other words, the NEC does not tell us to upsize the EGC by 150% of the amount by which the phase conductors is increased. So you don't have to go larger than a proportional increase, and the EGC should be able to clear a fault.
 
Re: Sizing EGC when feeder is increased for VD

Why doesn't the code just say that for this size feeder you need this size ECG. If you increase the conductors for any reason, then you go to this chart and find the correct ECG.
 
Re: Sizing EGC when feeder is increased for VD

There was nothing said about the 500 ft and its impact on the EGC
Click to expand...
The impact is that distance directly relates to the size of the EGC.
As Charlie says
In general, every 100 feet of length of a conductor will require upsizing the phase conductors one step, in order to keep voltage drop within reasonable limits.
Click to expand...
In the 1996 code there was a table listed that showed the maximum length of the EGC, conduit or conductor, that was a result of a study done by Georgia Tech. The table results were based on
40 arc voltage and a fault current equal to 400% of the OC device. Using these figures the maximum touch potential at the fault would be about 40 volts.
The point is that table 250.122 gives the minimum
conductor and it may not always be the correct size. In the example posted IMO the #2 is not the correct size for a 500 ft circuit.
There is a software program that was developed from this study and it is located at this site
http://www.steelconduit.org/gemi.htm.
I wouldn't be too quick to accept any assertion that there is a problem in your installation, and that it can only be solved by buying their product.
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Charlie in most cases I would agree. This product is not for sale. Its free. It allows you to caculate the proper EGC using many variables.
Check it out.
So you don't have to go larger than a proportional increase, and the EGC should be able to clear a fault.
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If you have a bolted fault that is true. If it is an arcing fault it may not happen fast enough.
If you suscribe to the Soares Book on Grounding this information is presented in the book.

[ December 01, 2005, 07:58 PM: Message edited by: bob ]
 
Re: Sizing EGC when feeder is increased for VD

The software is free. But it was writen based on studies paid for by the same companies that are trying to sell thier product as a cure for the problem.

I would like to see some independent studies on these issues.

The web site you link suggest that EMI is the cause of the problem. I dont see the relationship between EMI and fault current explained anywhere on that site. Just a link to the software that produces the need for a larger EGC or use of steel conduit.
 
Re: Sizing EGC when feeder is increased for VD

But it was writen based on studies paid for by the same companies that are trying to sell thier product as a cure for the problem
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What is the problem? What is being sold as the cure? We already know the conduit is a better EGC that a copper wire.
That has been common knowledge for a while.
 
Re: Sizing EGC when feeder is increased for VD

Originally posted by bob:
We already know the conduit is a better EGC that a copper wire.
That has been common knowledge for a while.
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Pssssst... over here bob. .... shhhhh...
how much do you want for a bag of what you are smoking.
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Re: Sizing EGC when feeder is increased for VD

I dont know the resistance of a stick of conduit, but i do know that running a "full size" copper ground inside the conduit is a much better installation.

with conduit, the fittings are not alwasy tightened as much as they should be, they come loose over time, they rust, espectially if underground. locknuts prove to be a poor way of guarnteeing continuity. ect.

i will admit that most of the above could happen with the copper wire, but since i have reduced the number of connection points on a 500 foot run from 60 to 70 for conduit, to 2 for a full size copper wire.

i guess you can see my point.

[ December 01, 2005, 11:12 PM: Message edited by: jbwhite ]
 
Re: Sizing EGC when feeder is increased for VD

The voltage drop for the installation with 350Kcmil Cu conductors is approx 3.3 volts.

Performing a calculation for 350Kcmil, cu conductors protected by a 300 ampere OCD, with a No. 2awg, cu equipment ground for a 500 ft PVC conduit, run results in an approximate maximum allowable length of 685 ft with an arc voltage drop of 50 volts. Where the arc voltage drop is reduced to 40 volts the maximum length is approx. 720 feet. Remember the arc voltage may vary. The greater the arc voltage the more damage can occur and take longer to actuate the overcurrent device.

In my opinion, the installation conforms to the Code.
 
Re: Sizing EGC when feeder is increased for VD

John
In your caculations what value of current did you use.

JB

I dont know the resistance of a stick of conduit, but i do know that running a "full size" copper ground inside the conduit is a much better installation.
Click to expand...
It may be slightly better. Studies have shown that when the EGC consists of a copper conductor and conduit, over 90% of the fault returns over the conduit.

And by the way, you still haven't asswered the question as to "what the problem is" and "what is being sold"

[ December 02, 2005, 11:53 AM: Message edited by: bob ]
 
Re: Sizing EGC when feeder is increased for VD

I used 250 amperes for the voltage drop calc.

The question was based on an install in PVC conduit, not steel. Where steel conduit is used with a copper EGC, it is true most of the fault current returns on the metal conduit.
 
Re: Sizing EGC when feeder is increased for VD

Performing a calculation for 350Kcmil, cu conductors protected by a 300 ampere OCD, with a No. 2awg, cu equipment ground for a 500 ft PVC conduit, run results in an approximate maximum allowable length of 685 ft with an arc voltage drop of 50 volts. Where the arc voltage drop is reduced to 40 volts the maximum length is approx. 720 feet.
Click to expand...
John
I'm not understanding your values of max length of 685 ft and 720 ft. What criteria are you using to determine these distances? VD?

[ December 03, 2005, 10:28 AM: Message edited by: bob ]
 
Re: Sizing EGC when feeder is increased for VD

Originally posted by bob:
And by the way, you still haven't asswered the question as to "what the problem is" and "what is being sold"
Click to expand...
I dont know what is being sold.
the link i got was to software created by a group of steel conduit mfgs.
 
Re: Sizing EGC when feeder is increased for VD

Bob, There are a few variables that if you change them the results will change, for example changing the ambient temp, arc voltage, or trip time for the overcurrent device will change the end result for the max distance.
 
Re: Sizing EGC when feeder is increased for VD

I agree John. I used 25C, 40v arc and inst for the breaker trip. These are the values used in the Soares Book.
At the time this section was entered into the code I suspect there was not a great deal of information regarding the effects of electrical shock on the human body. We now have information that has provided the use of GFI devices to limit the effects on the human body. In the Soares Book there is a chart that list voltage level and the human body response.

For body resistance of 500, 1500 and 3000 ohms, the let go voltage is 4.5, 13.5 and 27 volts. For fibrillation the voltages were 11.5, 34.5 and 69 volts. Soares is using 40 volts to ground as the limit. This voltage is the result of the fault current x the impedance of the EGC.
Soares was using 5 times the OC rating to get an instant trip. However later test showed that 4 times the rating was adequate.
Table 250.122 provides the MINIMUM SIZE EGC. I think everyone realizes that distance plays a part in selecting the EGC. Charlie pointed out that a general rule is that over 100 ft the EGC selected from table 122 is increased one size for each 100 ft. The given EGC size for a 300 amp breaker is #4 copper which is good for a circuit of 100 ft. We need to increase the EGC 4 times to meet the general rule. That gives us 3/0 copper. This has nothing to do with the increasing of the feeder for voltage drop. We will need 4 time the OD device rating to insure an instant opening. Given a 300 amp breaker we need a 1200 amp fault. In order to check the voltage to ground we multiply the 3/0 impedance by the 1200 amps. 3/0 at 25C x 1200 = Voltage to ground = 1200 amps x .037 = 45 volts. That's very close to the suggested value. Using #2 EGC
the voltage to ground is 0.08 x 1200 = 96 volts.
While #2 meets the requirements of the NEC, the longer distance requires that we consider a larger conductor.
 
Re: Sizing EGC when feeder is increased for VD

There is a great free software program from the Steel Tube Institute called GEMI that will calculate the size of the EGC for any length of circuit based on the arc voltage, conduit size and conductor size. its available as a free download from this site.
 
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