The code does not ask for a stamp
NEC 220.87 defines a method for establishing an existing load from measured demand. It says nothing about who may sign the result. That is not an omission. The National Electrical Code sets out how electrical work is to be done, and who is allowed to submit engineering to a permit authority is a matter of state licensing law and the authority's own rules.
The jurisdiction does, or does not
Whether a load calculation has to carry a Professional Engineer's seal is decided by the authority having jurisdiction, within what the state's engineering licensing law allows. Some authorities require sealed calculations on commercial permits. Some accept a contractor's calculation for defined kinds of work. The rule is per state and per authority, and it cannot be worked out from an address, so the only reliable answer is to ask the authority that will review the permit.
Where the authority requires a seal, an unsealed calculation is returned, and the project waits while a sealed one is produced. That is why it is worth knowing before the submittal rather than after.
Who can seal it
A Professional Engineer licensed in the state where the facility is, sealing under their own licence and their own insurance. The seal says that a named engineer has reviewed the work and takes responsible charge of it. It is that engineer's credential, and it goes on the document with their name, their licence number and their jurisdiction.
Wattif holds no engineering licence and seals nothing. On a US Capacity Determination Report, a named, licensed Professional Engineer reviews the package and seals it under their own licence. The report carries their name and number, and it says separately that the measurement and the drafting are ours.
Questions people ask
Does NEC 220.87 require a PE stamp?
No. The article defines the method for establishing an existing load from measured demand and says nothing about who signs the result. Whether a seal is required is set by the authority having jurisdiction and by state licensing law.
Can an electrician sign a load calculation?
It depends on the authority and the kind of work. Some authorities accept a licensed contractor's calculation for defined work and require an engineer's seal for other work, and the line is drawn differently from one jurisdiction to the next. Ask the authority that will review the permit.
What does the engineer actually check before sealing?
That the existing load was established by a method the code allows and from evidence that supports it, that the new load is calculated per its own article, that the arithmetic closes against the service or feeder rating, and that the assumptions are written down. A seal is the engineer taking responsible charge of that.