What is the difference between a load calculation and a load study?

A load calculation

Article 220 of the NEC sets out how to calculate the load on a service or feeder: add up the connected equipment, apply the demand factors the code allows for each kind of load, and compare the total with the rating. It is the method for a facility that does not exist yet, because there is nothing to measure, and it is designed to be conservative, since the service it sizes has to carry whatever the facility turns out to do.

Applied to a facility that has been operating for years, the same method answers a different question from the one being asked. It says what the facility could draw if far more of its equipment ran at once than ever does. That figure lands high, and a project adding load on top of it is told the service is full.

A load study

NEC 220.87 allows the existing load on an operating service to be established from evidence: the maximum demand over 12 months of utility records, or over a 30-day recording where those are not available, taken at 125%. The new load is then calculated the normal way, under its own article, and added. A load study is that work, done from the facility's own evidence, with the one-line rebuilt and every figure traced to its source.

The study does not replace the calculation. It replaces one input to it, the existing load, with a measured figure the code recognises, and the new load is still calculated under Article 220 because it is not installed yet.

When each one applies

A space that has never operated has no existing demand, so there is nothing for NEC 220.87 to use, and the Article 220 calculation is the only method. That is what an authority wants on new construction and it is the right answer there.

An operating facility can use either. The calculation is always available. The study is available where the facility can produce the evidence, which is 12 months of demand records or a valid 30-day recording, and where the service is not carrying solar or peak shaving that would make the recording read net rather than load.

Why the numbers differ, and which one the authority wants

The calculation counts equipment. The study counts what happened. In an operating facility those are different figures, and the gap between them is the spare capacity that a project can use without an upgrade. It is not a trick or an exemption. It is a method the code sets out precisely because the calculation is known to run high on existing facilities.

Authorities accept both, and the study has to document its evidence: which path was used, the record itself, its duration and interval, and where the meter was connected. A study that cannot produce that is sent back. Whether it also needs a Professional Engineer's seal is the authority's rule, set by state licensing law.

Questions people ask

Is a load study a load calculation?

It contains one. The new load is calculated under Article 220 in the normal way. What the study changes is the existing load, which is measured rather than calculated, as NEC 220.87 allows.

Can I get a load study for a facility that has not opened yet?

No. A space that has never operated has no existing demand, so there is nothing to measure, and the Article 220 calculation is the method for it.

Does a load study replace Article 220?

No. It replaces one input, the existing load, with a measured figure. The new load is still calculated under Article 220, and the comparison with the rating is the same.

Will the authority accept a measured existing load?

The method is in the code, and authorities accept it where the evidence is documented as the section requires. Whether the submittal also needs an engineer's seal is set by the authority and by state licensing law.