When you finally have enough engineering-required work in front of you to do something about it, the first instinct is usually to hire an engineer. It feels like the natural next step. Bring the capability in-house, control the priorities, own the relationship. For a small number of contractors with steady, predictable work in a narrow set of areas, that instinct is right. For most, hiring solves less than it looks like it will.
Four places the two options diverge
Breadth of coverage
What it is. The range of specialties available to you on any given project: arc flash analysis, protection coordination, power quality, relay settings, and commissioning support.
The trade-off. One engineer, however good, is deep in some of these and shallow in others. When a project lands outside their strongest area you are outsourcing anyway, on top of the salary. A firm carries the full range, and the engineer who does your arc flash study is the one who specializes in arc flash.
How the cost behaves
What it is. Whether the expense is fixed or moves with the work you actually win.
The trade-off. A hire is fixed overhead whether the work shows up that month or not. A partnership scales with your workload and gets marked up or passed through in the bid the way any subcontract would. For most contractors the partnership costs more per hour and less per year.
Where the knowledge lives
What it is. Who accumulates the history on your clients, your project types, and how you like to work.
The trade-off. An in-house engineer learns your business and takes that knowledge with them if they leave. A firm builds the same history across multiple people, so it survives turnover. The catch is that you have to commit to one primary partner rather than shopping every job, or the history never accumulates.
Availability and control
What it is. Whether you can direct priorities when a client is waiting on you.
The trade-off. This is the honest advantage of hiring. Your engineer works on your schedule. A firm serves other clients, so responsiveness has to be established up front and confirmed against your real deadlines. It is a legitimate consideration, it is manageable, and it is the thing to press on while you are deciding.
Which one fits your firm
The useful way to frame this is not control against convenience. It is one person's knowledge against a whole firm's.
If your engineering work is steady, predictable, and concentrated in one or two specialties, a hire can carry it and the fixed cost is justified. If the work is varied, occasional, or growing in ways you cannot forecast, a partnership covers more ground for less exposure.
The contractors who grow into more complex work almost never get there by hiring their way in. They get there by partnering with a firm that already carries the depth they would otherwise be trying, and struggling, to fit into a single seat.