It's not important what code you use, so don't get stressed over it. But the IRS wants to know what your business is, not the business that contracts with you. You are not the co-op, and you are not a utility. You are in the business of being a manager or an administrator. I would use something like 541600 Management, scientific, & technical consulting services, or 541990 All other professional, scientific, & technical services.
