Hi there, this is my first post and I am new to the forum, though I've been reading it for a long time. This seems to be the best place to get informed and educated answers to the how's and why's of the more esoteric situations one encounters from time to time. I apologize in advance for the length of this post, but I want to give as much background info as possible.
I am in the process of setting up machinery and distributing 1-ph and 3-ph power to it, in a small machine / fabrication shop. The shop has a few machines that require 3 phase power, and a few machines that require 240v single phase power and 120v power is needed only for lighting and receptacles. The shop is located in its own building in an old industrial complex and service is distributed to the building via a sub-station within the complex. The building that the shop is in, is masonry/brick construction circa about 1914 and has no common building steel that goes to the earth. The service coming into the building is 480/277Y three phase, however, it is provided via a 3 conductor SER cable and therefore has no neutral. Voltage readings for the service are approx. 470v between each of the three phases and approx. 272v from each phase to ground. I do not yet know how the service is returned to ground at the sub station, but I am trying to get the park's maintenance dept. to let me into it to find out. There is a #6 solid bare copper conductor connected to the ground bus of the 480v panel, which runs out of the panel, through the exterior wall of the building and then goes into the asphalt on the ground. I do not know if this is tied into anything below ground / how deep it goes/ etc.
The SER cable enters the building and goes directly to a service disconnect, which then feeds a 480v/277 3-ph panel board. The 480v 3ph panel board feeds a 37.5kva 480v delta - 120/240v single phase xfmr, via a 480v 2 pole breaker, with 2 - #3 THHN conductors and 1 - #8 THHN bonding jumper. Finally, the xfmr feeds a 120/240v single phase panel via 3 - #3/0 THHN conductors and 1- #4 THHN bonding jumper. The xfmr primary connections are H1 & H2 and the secondary connections are X1, X2, X3, X4. There are no factory provisions for bonding in the xfmr case and there are no grounding electrodes / grounding electrode conductors (GEC's) running to the xfmr. The #8 and #4 bonding jumpers from the 3-ph and 1-ph panels are made common through a 2 conductor lug that is thru-bolted to the xfmr case, inside the case.
X1 & X4 feed L1 and L2, respectively, in the 1-ph panel to provide 240v between phases and X2 & X3 are connected via a jumper to provide 120v, and feed the neutral bus of the 1-ph panel via 1 - #3/0 conductor connected to the X2 terminal (sorry if this is redundant, just trying to paint a clear picture). Here's the rub - at the 1-ph panel, voltage readings are as follows : L1-L2 ~240v, L1-N ~120v, L2-N ~120v, L1-Ground ~110v, L2-Ground ~130v.
I believe that this imbalance is being caused by either the lack of bonding the X2/X3 terminal to the xfmr case or lack of bonding the neutral bus to the ground bus in the 1-ph panel. My main concern is that this system is not grounded properly and therefore, potentially unsafe. I want to ensure that it is safe and that if a ground fault current should occur, there will be a proper ground fault current pathway established, the ocpd's will function properly, machines won't get damaged and no one will unassumingly get whacked. All of the distribution conduit will be either EMT or greenfield/MC, with metal boxes. My questions are as follows:
1) Is this system ungrounded / unsafe?
2) Is the imbalance being caused by the lack of bond between the derived neutral and earth/ground?
3) Should X2/X3 be bonded to the xfmr case or should the neutral bus be bonded to the ground bus in the 1-ph panel? Is one of those situations better than the other?
4) If X2/X3 gets bonded to the xfmr case, should the bonding jumper connect to the same lug (in this case X2) as the neutral conductor or can/should it go to the free lug, X3?
5) Should there be a bonding jumper from xfmr to 480v 3-ph panel?
6) Should the xfmr be attached to its own grounding electrode via a GEC since it's a separately derived system?
7) If yes to Q.5, how many grounding electrodes are necessary and is it ok to use sprinkler pipes (they are dry until the heads are tripped)?
8) If yes to Q.2/3 & Q.4 & Q.5, should all jumpers and GEC be made common?
Thanks and sorry if this is a bit long winded
I am in the process of setting up machinery and distributing 1-ph and 3-ph power to it, in a small machine / fabrication shop. The shop has a few machines that require 3 phase power, and a few machines that require 240v single phase power and 120v power is needed only for lighting and receptacles. The shop is located in its own building in an old industrial complex and service is distributed to the building via a sub-station within the complex. The building that the shop is in, is masonry/brick construction circa about 1914 and has no common building steel that goes to the earth. The service coming into the building is 480/277Y three phase, however, it is provided via a 3 conductor SER cable and therefore has no neutral. Voltage readings for the service are approx. 470v between each of the three phases and approx. 272v from each phase to ground. I do not yet know how the service is returned to ground at the sub station, but I am trying to get the park's maintenance dept. to let me into it to find out. There is a #6 solid bare copper conductor connected to the ground bus of the 480v panel, which runs out of the panel, through the exterior wall of the building and then goes into the asphalt on the ground. I do not know if this is tied into anything below ground / how deep it goes/ etc.
The SER cable enters the building and goes directly to a service disconnect, which then feeds a 480v/277 3-ph panel board. The 480v 3ph panel board feeds a 37.5kva 480v delta - 120/240v single phase xfmr, via a 480v 2 pole breaker, with 2 - #3 THHN conductors and 1 - #8 THHN bonding jumper. Finally, the xfmr feeds a 120/240v single phase panel via 3 - #3/0 THHN conductors and 1- #4 THHN bonding jumper. The xfmr primary connections are H1 & H2 and the secondary connections are X1, X2, X3, X4. There are no factory provisions for bonding in the xfmr case and there are no grounding electrodes / grounding electrode conductors (GEC's) running to the xfmr. The #8 and #4 bonding jumpers from the 3-ph and 1-ph panels are made common through a 2 conductor lug that is thru-bolted to the xfmr case, inside the case.
X1 & X4 feed L1 and L2, respectively, in the 1-ph panel to provide 240v between phases and X2 & X3 are connected via a jumper to provide 120v, and feed the neutral bus of the 1-ph panel via 1 - #3/0 conductor connected to the X2 terminal (sorry if this is redundant, just trying to paint a clear picture). Here's the rub - at the 1-ph panel, voltage readings are as follows : L1-L2 ~240v, L1-N ~120v, L2-N ~120v, L1-Ground ~110v, L2-Ground ~130v.
I believe that this imbalance is being caused by either the lack of bonding the X2/X3 terminal to the xfmr case or lack of bonding the neutral bus to the ground bus in the 1-ph panel. My main concern is that this system is not grounded properly and therefore, potentially unsafe. I want to ensure that it is safe and that if a ground fault current should occur, there will be a proper ground fault current pathway established, the ocpd's will function properly, machines won't get damaged and no one will unassumingly get whacked. All of the distribution conduit will be either EMT or greenfield/MC, with metal boxes. My questions are as follows:
1) Is this system ungrounded / unsafe?
2) Is the imbalance being caused by the lack of bond between the derived neutral and earth/ground?
3) Should X2/X3 be bonded to the xfmr case or should the neutral bus be bonded to the ground bus in the 1-ph panel? Is one of those situations better than the other?
4) If X2/X3 gets bonded to the xfmr case, should the bonding jumper connect to the same lug (in this case X2) as the neutral conductor or can/should it go to the free lug, X3?
5) Should there be a bonding jumper from xfmr to 480v 3-ph panel?
6) Should the xfmr be attached to its own grounding electrode via a GEC since it's a separately derived system?
7) If yes to Q.5, how many grounding electrodes are necessary and is it ok to use sprinkler pipes (they are dry until the heads are tripped)?
8) If yes to Q.2/3 & Q.4 & Q.5, should all jumpers and GEC be made common?
Thanks and sorry if this is a bit long winded
