perplexed with an exam question....

[starbright28]

I have this one test question that I'm wondering if I answered correctly or not.

the test question that I had is as follows: you have 2 paralleled 500 kcmil copper conductors going to the service. I don't think they said what the overcurrent protection was, or even if it was in the problem. anyways they were asking the size of the equipment ground needed. (copper)


Here is how I answered. I'm looking for the correct way of figuring this out because i am not sure if I was correct, or not. And if I am not correct then our engineer is not correctly sizing the equipment ground when we have paralleled sets.

My answer: 1/0 from table 250.66. I took the 500kcmil copper condutors to get my size from the chart.

Comments??????

 

[James@CHA]

That looks correct.

 

[Dennis Alwon]

Equipment ground is based on Table 250.122 and uses the OCPD

 

[starbright28]

so..i'm still wondering....

so..i'm still wondering....

I'm still relying on memoery from teh test last Wednesday - and I thought I wrote part of the question down, but at the moment I can't find it.

I went to Table 250.66 for the grounding electrode conductor because it said it was 500 kcmil copper conductors for a SERVICE.

Hopefulyl I really didn't screw up and use the wrong table now. I know I didn't use T250.122 because it wasn't looking for that answer. But I could be wrong, and there is no way of finding out if I was correct or not.

As soon as I find my test questions written down, i'll double confirm if this is indeed correct.

I even read the notes about the multiple sets of service entrance conductros are used as permitted in 230.40, exception No. 2 that the equivalent size of the largest service entracne conductor shall be determined by the largest sum of the areas of the corresponding conductors of each set.

 

[Dennis Alwon]

GEC is different from a egc. If they wanted a GEC then you were correct to use 250.66 but your post said equipment ground. If you have parallel 500 KCM then you must use 1000 kcm which would say you need 2/0 copper

 

[starbright28]

i know in my post i said equipment ground, but at this moment I can't rememeber exactly the question - forgive me.

i still think i have it wrong, because as you say.... parallel you add them up and then find the conductor size... oops.


now.. i want something verified that my engineer puts on the drawings:

for a 400a feeder
(2) sets of
(4) 3/0 awg thhn copper conductors (l1, l2, l3, n)
(1) set of
(1) #3 awg copper equipment ground

isn't sets like paralleled conductors, and if so, then is the copper equipment ground wrong???

 

[Dennis Alwon]

i know in my post i said equipment ground, but at this moment I can't rememeber exactly the question - forgive me.

i still think i have it wrong, because as you say.... parallel you add them up and then find the conductor size... oops.


now.. i want something verified that my engineer puts on the drawings:

for a 400a feeder
(2) sets of
(4) 3/0 awg thhn copper conductors (l1, l2, l3, n)
(1) set of
(1) #3 awg copper equipment ground

isn't sets like paralleled conductors, and if so, then is the copper equipment ground wrong???

With egc you would use Table 250.122. This tables works with the overcurrent protection. Wire size does not come into play here.

400 amps needs a #3 copper ground (EGC)

 

[crossman]

If indeed there is an EGC on the line side of the service disconnect, we aren't going to use 250.122 for the size.

 

[starbright28]

okay - boy do i get confused with this grounding stuff!!!

so even though it says feeder and the feeder size - most of the time it is the conductors coming from the transformer to the main distribution panel, and then from the main distributioin panel to the other panelboards.

that is why I wonder - the table 250.52 is for services - would what I just asked be also considered a service??

Confusing!

 

[charlie b]

isn't sets like paralleled conductors, and if so, then is the copper equipment ground wrong???

I don't know what your engineer meant by saying (1) set of (1) EGC. But it sounds wrong to me.

If the installation is going into separate conduits, then each conduit needs to have its own #3 EGC. Reference 250.122(F).

If the installation is going into a single conduit, then you need to derate the conductor ampacity, and a pair of 3/0 copper will not have a total ampacity of 400 amps. Reference Table 310.15(B)(2)(a).

 

[crossman]

okay - boy do i get confused with this grounding stuff!!!

so even though it says feeder and the feeder size - most of the time it is the conductors coming from the transformer to the main distribution panel, and then from the main distributioin panel to the other panelboards.

that is why I wonder - the table 250.52 is for services - would what I just asked be also considered a service??

Confusing!

Now you have me confused. EGCs on the load side of the service go by 250.122. Equipment Bonding Jumpers on the line side of the service go by 250.102 which sends you to 250.66.

So are these service entrance conductors or feeders?

 

[starbright28]

okay here's an example:

From the main transformer to the main distribution we put this:
(this is from a job we have)
(4) sets of:
(4) - #3/0 thhn copper conductors [l1, l2, l3, n]
(1) set of:
(1) #4 awg copper grounding electride conductor
in (4) 2" pvc schedule 40 conduits


okay now from the main distribution to a panelboard:
(1) set of:
(4) #3/0 awg thhn copper conductors [l1, l2, l3, n]
(1) set of:
(1) #6 awg copper equipment ground
in (1) 2" emt conduit

we've been doing this for a few months now.... should something be worded differently, or should my explanations be a bit more clearer?

 

[crossman]

What do you mean by "main transformer"?

 

[starbright28]

What do you mean by "main transformer"?

Main transformer as the where the power comes from the power company into the transformer and then out of the transformer to the main distribution panel.

 

[Dennis Alwon]

From the transformer you would only need the neutral and phase conductors in the conduit. Is that correct. There should be no EGC. The #4 GEC you are talking about is connected to the ground rod or water pipes--correct?

If the 3/0 copper feeder is a 200 amp main then a #6 EGC is correct.

 

[starbright28]

see attached one line - this is from the job that i posted the information on.

now comment....

 

[Smart $]

see attached one line - this is from the job that i posted the information on.

now comment....

The #4 grounding electrode conductor is superflous. The only thing it can do is ground the transformer case and or pick up another ground rod (but not the one required and connected to the neutral). Better to eliminate it and bond the neutral and rod to the case.

This drawing looks like it is software generated including the annotations... so I don't know if the format can be altered. I familiar with annotations such as, for the service: 4 SETS 4 #3/0 THWN 2"C PVC. Another example, for the feeder to the left: 4 #3/0 THHN 1 #6 THHN EGC 2"C EMT. Many times the wire and conduit type are not even annoted as it is covered in job spec's as an "unless otherwise noted".

 

[starbright28]

thought i woud re-comment about the one line diagram pdf that i posted.

believe it or not, I drew this diagram. There is no software program that we use. The information listed for wire stuff and what not is what was written by the electrical engineer, and he did the calculations. we were told to start showing size of wire, conduit size and type, etc. versus we had absolutely nothign before.

and since eventually I'll start doing this type of design, it is critical to know if everything is correct.

from the one line i posted, along with the stupid test question - i'm getting confused... here i thought i understood the code a bit and then it turns around on me and i'm smacked with it again...

 

[starbright28]

here's another project showing the whole one line diagram. now I'm looking for comments.... and what table should be used, etc.

i know it's getting away from the original question... but I don't mind...

 

[starbright28]

in regards back to my original question that was on the MN journeyman exam... well i got my results today - did not pass. this is like my 6th time around.

65 my score. 70 passing.