curt swartz
Electrical Contractor - San Jose, CA
- Location
- San Jose, CA
- Occupation
- Electrical Contractor
Hello everyone. It?s been awhile since I have posted anything here.
I?m working on a project installing new 480/277 wye service equipment. This is a 5 section switchboard (1 UG pull section and 4 ct/meter sections) with a 1200 amp cross buss. There is no main OCP device but each meter section has a service disconnect. The service lateral conductors are sized and installed by the utility.
The switchboard manufacture has installed a 3? piece of 1/0cu THHN between the neutral buss and grounding buss to serve as the main bonding jumper. I was surprised they used this small of conductor and so is the engineer I?m working with on this project. 250.28(D) requires the MBJ to be sized according to Table 250.66. If the supply conductors are larger than 1100cu or 1750al then it should be at least 12.5% of the supply conductors. Even if 8 sets of 1/0cu (105600 X 8 = 844800) were used to supply this panel table 250.66 would require 2/0 cu. In reality the utility will install 1 to 4 sets of 1000AL which would require a much larger MBJ. Interestingly 12.5% of 84480 is 105600 (1/0).
Is the switchboard manufacture required to comply with 250.28(D) since this is a UL listed piece of equipment? Does UL have another standard for them to follow? Since the size of the MBJ is based on the size of the supply conductors (amount of metal) could they be sizing the MJB based on the size (amount of metal) of the cross buss?
I?m interested in what others think.
I?m working on a project installing new 480/277 wye service equipment. This is a 5 section switchboard (1 UG pull section and 4 ct/meter sections) with a 1200 amp cross buss. There is no main OCP device but each meter section has a service disconnect. The service lateral conductors are sized and installed by the utility.
The switchboard manufacture has installed a 3? piece of 1/0cu THHN between the neutral buss and grounding buss to serve as the main bonding jumper. I was surprised they used this small of conductor and so is the engineer I?m working with on this project. 250.28(D) requires the MBJ to be sized according to Table 250.66. If the supply conductors are larger than 1100cu or 1750al then it should be at least 12.5% of the supply conductors. Even if 8 sets of 1/0cu (105600 X 8 = 844800) were used to supply this panel table 250.66 would require 2/0 cu. In reality the utility will install 1 to 4 sets of 1000AL which would require a much larger MBJ. Interestingly 12.5% of 84480 is 105600 (1/0).
Is the switchboard manufacture required to comply with 250.28(D) since this is a UL listed piece of equipment? Does UL have another standard for them to follow? Since the size of the MBJ is based on the size of the supply conductors (amount of metal) could they be sizing the MJB based on the size (amount of metal) of the cross buss?
I?m interested in what others think.
