Does your building have enough power for a new tenant?

Almost every version of this conversation starts with a single number. The building is 400 amps, or 800, or 1200, and the question is whether that is enough. It is the right instinct and the wrong measurement.

Your service rating is a ceiling. What matters for a new tenant is the headroom: the ceiling minus what the building is already using. A 1200 amp service that is nearly fully committed has less to offer a new tenant than a 600 amp service in a half-empty building. Two owners can give the same answer to "what size is your service" and be in completely different positions.

There is a second thing hiding in that number. Amps alone do not describe capacity, because the voltage and the phase configuration change what an amp is worth. Two hundred amps at 208 volts three-phase works out to roughly 72 kVA. Four hundred amps at 480 volts three-phase is about 333 kVA, which is more than four times the capacity for twice the amperage. So the useful figure is never one number, it is three: amps, volts, and phase.

Those three numbers live on the label of your main switchboard or panel, and reading a label is not something you need anyone else for. Photograph it rather than copying a figure out, because the combination is what carries the meaning.

Establishing what is already used is the harder half, and it is where the two available methods diverge sharply. The traditional route adds up the connected load on paper: every circuit, every piece of equipment, every fixture, with the code factors applied. It is thorough, it is defensible to a plan checker, and it tends to produce a conservative answer, because it accounts for loads that in practice never all run at once.

The second method is the one most owners have never been offered, and it is often the one that changes the outcome.

Tenant type predicts electrical load far better than square footage does. Two suites of identical size can differ by a factor of several. The ranking below is relative rather than a set of figures, because the figures depend entirely on the specific equipment and it would be dishonest to publish numbers that read as an estimate.

If the numbers come back short, a new service is one answer and it is usually the most expensive and the slowest. It is worth knowing what sits between.

Separate the panel problem from the capacity problem. A panel with no spaces left is not the same as a service with no capacity left, and the two get confused constantly. If the service has room and one panel is simply full, a subpanel or a reconfiguration solves it at a fraction of the cost and time.

Retire load you are still carrying. Lighting was a large share of connected load in any building designed around fluorescent or HID fixtures, and a retrofit can cut that share substantially. In a marginal building this is sometimes enough on its own, and unlike a service upgrade it pays back part of itself through the energy saving. It deserves to be priced against the upgrade rather than treated as a separate project.

Manage the peak rather than the total. Capacity is about simultaneous demand, so staggering or shedding non-critical loads can bring a peak under a ceiling that a naive total would exceed. This is how EV charging often gets accommodated in buildings that could not support the same ports running flat out.

Rebalance what is already there. Loads accumulate unevenly across decades of tenant changes, and an existing service is not always distributed the way the current use would suggest. Sometimes capacity is present but sitting in the wrong place.

Five things turn this from an open question into arithmetic. None of them requires opening any equipment, and having them ready is the difference between a real answer and a site visit that produces another site visit.

1. 1

   Photograph the service equipment labels

   The main switchboard or panel label carries the main breaker rating, the voltage, and the phase configuration. Photograph the label itself rather than writing the numbers down, because the combination matters and a single transcribed number loses it. Do not remove any covers to find it.

2. 2

   Pull twelve months of utility bills, with the demand figures

   Not just the kilowatt-hours. Commercial accounts are billed on peak demand as well as consumption, and that demand history is the most useful document in this whole process. It is what makes the next section possible, and you already have it.

3. 3

   Find the single-line diagram if the building has one

   Older buildings often do not, and that is normal rather than a problem. If one exists it is usually in the original permit set, with the building department, or with whoever last did significant work. It saves real time when it turns up.

4. 4

   Get the tenant equipment list or their drawings

   The tenant knows what they are installing even when nobody has written it down as electrical load. An equipment schedule, a kitchen layout, or a set of MEP drawings turns guesswork into arithmetic. A restaurant tenant who cannot produce an equipment list is the single most common reason these projects stall.

5. 5

   Know your lease delivery date before anyone quotes you

   The date you have committed to shapes which options are actually available. Some paths take weeks and some depend on a utility timeline nobody in the building controls, and knowing the deadline up front prevents a technically correct recommendation that cannot be delivered on time.