Job #3453

Project: Fanuc Machine Shop

STATEMENT OF PROBLEM:

This is a design-build project where CCC hired Tandem Engineering to perform design services for relocating approximately 22 milling and CNC type machines from a Madison heights facility to an auburn hills facility. The machines are fed from a transformer and 208V bus duct.

Near the end of design (90%), the electrical engineer notified us that the late addition of a machine exceeded the bus power capacity and that another transformer would be needed and length of 208V bus duct. This was problematic as the project was fast-tracked and the bus duct and bus plugs were long-lead items. The engineer also noted that the electrical design was based on an 80% diversity factor, assuming that no more than 80% of the machines would be running simultaneously. This was never discussed during schematic design.

After conferring with Fanuc, they wanted the system sized to accommodate 100% diversity factor, as they were not sure if all machines would need to be running simultaneously for their operations.

ROOT CAUSE:

The electrical engineer made an assumption on operations and designed to industry norms, rather than asking and designing to the Owner's specific needs. 

COURSE OF ACTION:

The design was able to be altered between 90% and 100% design, but this late change required additional procurement from the Owner since they were sourcing the transformer. This late development frustrated the team. On the plus side, the bus duct length could remain the same and just be in two sections. CCC was also able to capture this cost in our GMP.

A few other electrical issues occurred with the bus:
 - Design clarity on the bus plugs. To prevent confusion with subcontractor and when procuring refurbished equipment, The bus plugs must be numbered or a schedule developed. Example is the machine tag schedule could have the bus plug number next to it, or number them on the one-line diagram.

- Short-circuit rating. During the new 208V bus duct submittal review, the Engineer had to do a short circuit analysis to verify the rating of the proposed refurbished duct was sufficient. The design gave a boilerplate value instead of an actual calculated value. The engineer should have looked at the existing 480V feeder bus duct and calculated the necessary short circuit rating for the new 208V bus duct during schematic design instead of listing a boilerplate value. This delayed procurement of a critical item. 

LESSON FOR FUTURE PROJECTS:

When the design team is sizing power capacity for equipment, The specific equipment operations must be discussed with the Owner so the proper diversity factor can be applied in the design. 
The design team should number or create a schedule for bus plugs for clarity in quantity. 
The design team must calculate short circuit rating by analyzing existing feeder, instead of choosing boilerplate values.