Equipment Ground Conductors on Parallel Feeders

Status
Not open for further replies.
M

mull982

Senior Member
I have (6) parallel feeds of 3/c 500MCM per phase coming off of a three phase 480V 2000A rated breaker (off secondary of xfmr) feeding a 480V MCC. My question concerns what size equpment ground conductors need to be run with each of the (6) parallel feeds?

Per 250.122(F) I understand that as saying that each equipment ground conductor run with each of the (6) 3/c feeders needs to be rated for the rating of the overcurrent device which in this case is 2000A. Looking at table 250.11 the size of the EGC corrosponding to 2000A is 250MCM. I am understanding this to mean that each of the (6) ECG in this parallel feed needs to be 250MCM or greater. Is this correct?

What if I had a similar situation where I had the same circuit going between the low side of a transformer over to a piece of switchgear which contained a main breaker (15ft) ? There is no overcurrent device directly on the transformer so what would I use as the overcurrent device rating in this case to corrospond to 250.122?
 
mull982 said:
I have (6) parallel feeds of 3/c 500MCM per phase coming off of a three phase 480V 2000A rated breaker (off secondary of xfmr) feeding a 480V MCC. My question concerns what size equpment ground conductors need to be run with each of the (6) parallel feeds?

Per 250.122(F) I understand that as saying that each equipment ground conductor run with each of the (6) 3/c feeders needs to be rated for the rating of the overcurrent device which in this case is 2000A. Looking at table 250.11 the size of the EGC corrosponding to 2000A is 250MCM. I am understanding this to mean that each of the (6) ECG in this parallel feed needs to be 250MCM or greater. Is this correct?

What if I had a similar situation where I had the same circuit going between the low side of a transformer over to a piece of switchgear which contained a main breaker (15ft) ? There is no overcurrent device directly on the transformer so what would I use as the overcurrent device rating in this case to corrospond to 250.122?
Click to expand...

You are correct you will have to size the ground wire for the full 2000amps in each conduit, unless you are using EMT, Rigid, or IMC. Then you can omit the ground wire (unless your specs call for it).

What is the rating of the MCCs OCP? I would guess it to be 4000amps. You would then be looking at using 500mcm copper. In each raceway, of course.
 
Go to 250.30 for the second part of your question. The transformer supplying a panel or switchboard is considered a "Separatly Derived System". Also look at 240.21 for the requirements of the cable/conduit for this feeder.
 
mull982 said:
I have (6) parallel feeds of 3/c 500MCM per phase coming off of a three phase 480V 2000A rated breaker (off secondary of xfmr) feeding a 480V MCC. My question concerns what size equpment ground conductors need to be run with each of the (6) parallel feeds?

Per 250.122(F) I understand that as saying that each equipment ground conductor run with each of the (6) 3/c feeders needs to be rated for the rating of the overcurrent device which in this case is 2000A. Looking at table 250.11 the size of the EGC corrosponding to 2000A is 250MCM. I am understanding this to mean that each of the (6) ECG in this parallel feed needs to be 250MCM or greater. Is this correct?

What if I had a similar situation where I had the same circuit going between the low side of a transformer over to a piece of switchgear which contained a main breaker (15ft) ? There is no overcurrent device directly on the transformer so what would I use as the overcurrent device rating in this case to corrospond to 250.122?
Click to expand...



You have six sets of 3c-500 kcmil?

Is this a delta or wye secondary from the load side of the transformer?
 
Mull982,
What if I had a similar situation where I had the same circuit going between the low side of a transformer over to a piece of switchgear which contained a main breaker (15ft) ? There is no overcurrent device directly on the transformer so what would I use as the overcurrent device rating in this case to corrospond to 250.122?
Click to expand...
You then start with 250.30(A)(2) and get sent to 250.102(C) and then to 250.66 to find out that in that application you need a 1/0 bonding jumper in each raceway.
Don
 
don_resqcapt19 said:
Mull982,

You then start with 250.30(A)(2) and get sent to 250.102(C) and then to 250.66 to find out that in that application you need a 1/0 bonding jumper in each raceway.
Don
Click to expand...
Seems to me you are thinking of the XO connection to the building steel ( not less than 12.5% of the conductor area), not the equipment ground per the table sized for the OCP.
 
jrannis said:
Seems to me you are thinking of the XO connection to the building steel ( not less than 12.5% of the conductor area), not the equipment ground per the table sized for the OCP.
Click to expand...


Actually that's not what Don meant. The bonding jumper in each raceway is based on the size of the conductors in that raceway. 250.30(A)(2) sends you to 250.120(C) which tells you this:

250.102(C) Size ? Equipment Bonding Jumper on Supply Side of Service. The bonding jumper shall not be smaller than the sizes shown in Table 250.66 for grounding electrode conductors. Where the service-entrance phase conductors are larger than 1100 kcmil copper or 1750 kcmil aluminum, the bonding jumper shall have an area not less than 12? percent of the area of the largest phase conductor except that, where the phase conductors and the bonding jumper are of different materials (copper or aluminum), the minimum size of the bonding jumper shall be based on the assumed use of phase conductors of the same material as the bonding jumper and with an ampacity equivalent to that of the installed phase conductors. Where the service-entrance conductors are paralleled in two or more raceways or cables, the equipment bonding jumper, where routed with the raceways or cables, shall be run in parallel. The size of the bonding jumper for each raceway or cable shall be based on the size of the service-entrance conductors in each raceway or cable.
Click to expand...
 
jrannis said:
Seems to me you are thinking of the XO connection to the building steel ( not less than 12.5% of the conductor area), not the equipment ground per the table sized for the OCP.
Click to expand...
My response was based on the second part of the question where the feeders are run from the transformer to the the switchgear with the OCPD in the switchgear. In that case the conductors in question are not EGCs, they are equipment bonding jumpers and the sizing rules are not the same.
Don
 
don_resqcapt19 said:
My response was based on the second part of the question where the feeders are run from the transformer to the the switchgear with the OCPD in the switchgear. In that case the conductors in question are not EGCs, they are equipment bonding jumpers and the sizing rules are not the same.
Don
Click to expand...

Don

With the six sets of 500MCM going from the secondary of the transformer to the 2000A breaker in the switchgear, I have the equivilent of 250MCM EGC run with each circuit. These are actually a 3/c Type GGC mining cable so I am using the grounding conductors inside this cable. I am landing these EGC's on the ground bus of the switchgear, and back on the transformer I am landing these on a ground pad that is on the outside of the secondary side of the transformer. ( Ground pad is bonded to transformer frame. Outdoor transformer).

Are you saying that there is some other sort of bonding that I need to run with these EGC's in the parallel circuit, or are you saying that I need to bond these EGC's to another place on the transformer such as the primary side ground?

The secondary of the transformer is High Resistance grounded so we are taking the XO bushing through a HRG package on the switchgear.
 
iwire said:
IMO the NEC does not accept 'the equivalent of' for the bonding jumpers or EGCs.
Click to expand...

The 3/c cable that I have includes (2) 2/0 ground conductors. The combined CMIL for these two conductors is 266 CMIL which is larger than the 250MCM required by table 250.122. Theoryatically I would think this would work, and I dont see the sense in running another 250MCM conductor when I already have avaliable in my cable the (2) 2/0 EGC's.

Can you please explain more on the bonding jumpers?
 
Status
Not open for further replies.
Top