A project dealing only with either IEEE/ANSI/UL or IEC standards is not an issue. If there will be mixed standards within a single project, then compliance becomes an issue.
In the Philippines where implementing agencies are not too strict as to which standard to implement, interfacing between the IEC and IEEE/ANSI/UL will not be too much of a problem.
What if strict compliance requirements are essential like in the oil & gas industry? How is the implementation of mixed standards going to be?
Leading electrical equipment manufacturers are now designing their equipment to comply with both standards out of the box. In most cases, however, this is not possible. One good example is cable. Cable is also where most interfaces happen.
In green field installations, the strict demarcation between standards helps ease the problem of the interface. In brownfield installation, however, it may be easier said than done.
I once got a problem with joining IEC cables to AWG cables in Zone 2 hazardous area. As on-site modification will void any certification on the junction box, I need to be careful with the selection of glands and terminal blocks. Terminal blocks should be able to accommodate the small and the larger cable size.
This is just a simple example. A more complicated example will be joining two switchgear from different standards. How could you still maintain compliance without voiding the warranty of the equipment?
In actual practice, if two(2) switchgear compliant with different international standards are required to be joined together as installing them separately is not an option due to space limitations (i.e. installations in offshore platforms where space is always an issue), a transition tier is installed, one side to match the IEC designed switchgear and the other one is for the ANSI/IEEE switchgear.