Most commercial buildings take power from ComEd at secondary voltage. Somebody else's transformer steps it down before it reaches you, and you never think about it. Once an account gets large enough, or when a new service is being designed, a second option appears: take the power at primary voltage, before the step-down, and own the transformer yourself.
The pitch for it is simple and true. The primary delivery rate is lower per kilowatt than the secondary rate. The reason it is lower is the part that decides whether the trade is any good.
What you are actually buying
ComEd's delivery rates differ by service voltage because the two arrangements involve different amounts of utility equipment. A secondary customer is served through transformation ComEd owns, maintains and replaces. A primary customer is served upstream of that, and the transformation becomes the customer's problem.
So the discount is not a reward for being a large account. It is the utility declining to charge you for equipment it no longer has to provide. Where ComEd does supply transformation to a primary-served customer, a separate primary voltage transformer charge applies, which is the same idea running the other direction.
That reframes the question. It is not "should I take the cheaper rate." It is "am I better off owning this transformer than renting it."
The three costs the rate comparison leaves out
Capital. A service transformer, the pad, the protection, the switchgear and the engineering are a project, and it is a project in six figures for many sites. It is paid once and it is paid before any saving arrives.
Maintenance and replacement. The equipment is now yours for its life. Testing, oil, inspection, and eventual replacement are on your books, and they arrive on their own schedule rather than yours.
Outage responsibility. When utility-owned transformation fails, ComEd restores it. When yours fails, you are the one procuring a replacement, and lead times on distribution transformers have not been kind in recent years. For some operations that exposure is a rounding error. For a facility that cannot be down, it is the whole analysis.
The arithmetic that tells you whether it is even close
You can do the first pass yourself, from the bill in front of you.
Find your billed kilowatts. It is on the delivery portion of the bill. This is the quantity every rate on your account is multiplied by, and it is what scales the saving.
Multiply your billed kilowatts by the rate difference, then by twelve. That is the annual saving, before any of the costs above. The rate difference between service voltages is published in ComEd's filed informational sheets for your delivery class; ask ComEd or read it off the sheet rather than accepting a number from whoever is proposing the work.
Compare that annual figure to the installed cost. If the simple payback runs past the useful life of the equipment, the conversation is over and you have saved yourself a study. If it comes back in a few years, the decision is real and the ownership and outage questions above are what settle it.
The reason this simple pass is worth doing first is that the saving is linear in your kilowatts while the cost of the equipment is close to fixed. Below a certain size the answer is no for arithmetic reasons alone, and no amount of design work changes it.
Where our own coverage stops, stated plainly
Our tariff engine reconstructs ComEd demand charges at secondary voltage. Primary service is a documented gap: the primary voltage transformer charge is a component we do not yet model, and reconstructing a primary account's delivery charge from the primary distribution rate alone would understate it. We record that as an open gap rather than shipping a number we cannot reconcile.
So if you are weighing this, we can tell you what the mechanism is and what to check, and we would want a real primary-served bill before quoting you a reconstructed figure. That is the honest position, and it is more useful than a confident one.
What is the difference between primary and secondary voltage service?
Secondary service is delivered after the utility's transformation, at utilization voltage. Primary service is delivered upstream of it, and the customer provides and owns the transformation.
Is ComEd primary voltage service cheaper?
The delivery rate per kilowatt is lower, because ComEd is providing less equipment. Whether the total cost of service is lower depends on the capital, maintenance and outage exposure you take on in exchange.
How large does a business need to be for primary service to make sense?
There is no threshold in the tariff. The saving scales with your billed kilowatts while the transformer cost is roughly fixed, so the answer is arithmetic: multiply your billed kilowatts by the rate difference and compare twelve months of it against the installed cost.
Who fixes the transformer on a primary-served account?
You do. That is the substance of the trade, and it is the part of the decision the rate comparison does not show.