Raceway as EGC

[bonding jumper]

Re: Raceway as EGC

Ok Scott, I'm gonna take a stab at it. These equations make the following assumptions, Infinate short circuit available feeding xfmr and 5hp motors are single phase 240V.

After XFMR

37.5KVA / .02% imp = 1875KVA

At PanelBoard Entrance

[200A * 120V(V per leg, each conductor)] / .2% (Voltage drop based on 126A load, 6-5hp motors @ 90% = 28A) = 120000KVA

Rating of 40A 2P breaker is 10KAIC, so:
10KAIC * 240V = 2400KVA

At Fault

#6 = 65A * 120V(Per conductor)/.1(% Voltage drop based on 28A) = 78000KVA

These parts must be combined to get our Fault current rating at fault. They must be used in a recripocal equation as follows,
1 / [(1/1875KVA) + (1/120000KVA) + (1/78000KVA)] = 1803KVA

Since we have available 2400 KVA at the 40A breaker, we have adequate short circuit protection for our fault.

If we swap #6's with #10's we will be dividing by a larger percentage for the branch circuit calculation, thus giving us a smaller KVA rating, thus a smaller short circuit current.

If we increase the size of our xfmr, to 75 kva, or 112.5kva, we will increase the short circuit fault rating. Without doing calculations, I would guess our rating would be too high for our 40A breaker which is rated at 2400KVA.

If we move the xfmr, or use smaller feeders, we again decrease the short circuit rating from the calculated figure because we are increasing our voltage drop percentage.

If the ground fault occurs 100 feet away from the panelboard, we are decreasing our short circuit again cuz of a higher volage drop.

If all the motors were starting we would be decreasing again cuz of a higher amperage being pulled through the conductors, thus a higher voltage drop.

If all the motors shut off, we would have a higher short circuit current because we would have a lower voltage drop due to 0 current being drawn on conductors.

Both ungrounded conductors fault to ground, these calculations should be fore worst case fault, which is line to line fault, haveing both conductors short to ground has the same effect of a line to line fault.


P.S., this is my first time doing short circuit calculations, questions i had, were ratings for KVA I used for conductors. I used 200A for the 3/0 and 65A for the #6. These are from table 310, and I know they are Allowable Ampacities of insulated conductors, and they do not represent the maximum current that these conductors can tollerate or deliver, so I am not sure if my ratings of 200 or 65 are applicable. Lemme know how I screwed up. I'm sure everybody will find a different problem. Thanks.

My methods came from the following web page web page

[ March 29, 2004, 11:59 AM: Message edited by: bonding jumper ]

 

[bonding jumper]

Re: Raceway as EGC

Bump

 

[bonding jumper]

Re: Raceway as EGC

Can anyone shed some light on my answers to the proposed question? Am I off?

 

[scott thompson]

Re: Raceway as EGC

Bonding Jumper (or can I call you - "Bond,... Jumper Bond ...)
Sorry for the extreme latency of my reply!

Scott

 

[ryan_618]

Re: Raceway as EGC

I ran across this thread by trying to link to it for another similiar thread. I noticed that while reading it, there was concern regerding the fact that 250.4(A)(5) doesn't address the opening of an OCPD. I just thought I would throw it out here that in the 2005 there are two changes to 250.4(A)(5), both of which I think are great.

Electrical equipment and wiring and other electrically conductove material likely to become energized shall be installed in a manner that creates a permanant, low-impedance circuit facilitating the operation of the overcurrent device ot ground detector for high-impedance grounded systems. It shall be capable of safely carrying the maximum ground fault current likely to be impsoed on it from any point on the wiring system where a ground fault may occur to the electrical supply source. The earth shall not be considered as an effective ground fault current path

.

So, the two changes clarify that:
1) the circuit must be able to trip the OCPD, and
2) the earth can't be considered to acheive the requirement. Remember the 2002 simply said that the earth can't be the sole EGC.

I like both changes, and in fact, I think panel 5 did a lot of nice work in this cycle.

 

[charlie]

Re: Raceway as EGC

Write an in depth report on something that is say 25 pages long. Lay it down for a couple of weeks and go back to it. You will find errors to be corrected and changes that need to be made. Do it again and you will have the same result.

The Code is much more involved than that. Add to it the changing technology, and people trying to use the Code to sell their pet product. You now have a very dynamic document. I think a very good job is done by all the panels every cycle. There is only one perfect book and it is not one but 66 books in one cover (just my opinion).

 

[jlam]

Re: Raceway as EGC

I believe that grounding is a very important issue,with all the sensitive equiptment to feed I believe 500 ft has a lot of teminating points (couplings)than just 2 (ground wire in conduit)
many times in my career have serviced many situations where a lost ground due to corrusionbut did not lose power (conductors in the conduit) so ground in conduit should be a must

 

[petersonra]

Re: Raceway as EGC

I am not a fan of using raceways as an EGC. I like to see a nice green wire.

Having said that, I recognise that in many cases, using the conduit as an EGC is perfectly acceptable and safe.

One thing I don't like is when people wire up big motors with flex conduit and no green wire. They always seem to use the cheap stuff and don't put the external bonding wire on it, or one inside the flex. One electrician told me he does not do the external wire route because "it looks bad".

 

[paul]

Re: Raceway as EGC

Originally posted by petersonra:
I am not a fan of using raceways as an EGC. I like to see a nice green wire.

I was thinking the same thing. Four hundred feet of diecast set screw connectors hanging off of ceiling wires just doesn't cut it for me. I ALWAYS pull a ground wire and ground every j-box...case closed.

 

[friebel]

Re: Raceway as EGC

I have responded to this very important topic before, and I think, no I know that it still is very important for the companies and/or electricians that are still using the Raceway for their equipment grounding conductor,(EGC).
I worked for the DuPont company for 45 years and when I first started, we always used the conduit as our ground conductor. So, when we would take a branch circuit of three wires to a 575 volt motor,three-phase we would rely on the conduit for carrying our ground continuity.
At our plant we had a program called Fault & Static Inspection. What that meant was the following: We would go with an ohmmeter and check the continuity of ground through the conduits. We did this on an annual basis. I can tell you from experience, that we would always find a lack of continuity through some of the conduit couplings due to corrosion. We then would have to install a bonding jumper around the corroded coupling. A very expensive maintenance item. Also another instance of losing your ground is a broken conduit. Now this could be a very dangerous situation, if a person comes in contact with a broken conduit that has a fault to ground and he touches the conduit that has the fault and also a good ground, the result could be a fatality.
But, about 15 years ago, our leader in the Electrical Design Group, a man by the name of John Branscome,an outstanding Electrical Engineer, John put out the edict and the design, that we would always carry an internal ground wire in the raceway. Today, they mainly have Tray cable, so we would purchase cable, and depending upon the size of the motor, would dictate the size of the ground conductor in that cable.

 

[iwire]

Re: Raceway as EGC

friebel It is clear you have a lot of experience in industrial work.

But not every place needs the same design. For example it is my understanding that in Chicago where they wire houses in EMT many do not pull separate EGCs. I doubt you could find a need for it in a house.

The runs are short and the fittings are not exposed to corrosive influences.

My point here is this, the NEC recognizes a blanket statement such as 'all raceways shall have a EGC pulled in them' would be overkill.

Facilities that have special needs like Dupont, Data centers, refineries, etc. will have the engineers on board to write specific job specs.

JMO, Bob

 

[friebel]

Re: Raceway as EGC

To: iwire, Bob Badger,
Bob, I do appreciate your views on the subject of the EGC, although we do not completely agree.
Yes, I do have a great deal of experience in the industrial field. I am now retired, and teaching three subjects, National Electrical Code for electricians taking their Masters exam, etc.
Programmable Logic Controllers, and Electrical Controls for HVAC/R equipment. I teach at Delaware Technical & Community College in Newark, Delaware. I realize that you are correct in what you are saying about the EGC and the NEC.
But, again on any job that I would do, if it be residential, commercial, or industrial, I would not rely upon the raceway soley for my grounding conductor. I would still use the raceway in parallel with a ground conductor inside of the raceway. But, we have some very intelligent people on our Code Panels, and allow using the raceway, solely for the EGC. End of Story. Have a very Merry Christmas, Bob.

 

[iwire]

Re: Raceway as EGC

Originally posted by friebel:
Have a very Merry Christmas, Bob.

And you as well sir.

By the way, I read the posts and have a good memory, I know where you worked and what you teach.

You have a lot to offer please do not let our differences of opinion turn you off to posting.

Happy Holidays

Bob

 

[friebel]

Re: Raceway as EGC

To iwire, Bob, I enjoy it when my students challenge me, it shows that they are paying attention and are interested in learning. That is the way that I perceive you. I always like a good electrical discussion, especially if we disagree, but keep it on a professional level.
There have been times when I have been in error and my students have helped me out, again I appreciate that type of support.
I remember a time in San Diego at a seminar, when Mike Holt and I did not agree, and boy, Mike is sure a strong personality. But, again I enjoy that. I will sign off now, Bob. Merry Christmas.