The only numbers I will ever spell are one and zero, and only when using them as a pronoun, or for emphasis, respectively.
Is there ever a reason to not to use symbols when dealing with numbers? Why would "fourteen whatevers" ever be preferable to "14 whatevers". It's just so much easier to read numbers as symbols, not spelled out.
(Caveat, not including multipliers, like "273 billion").
Typically when I type out professional emails or documents that contain numerical values, I write out the number followed by the digits in brackets if it is ten [10] or below for cases of amount, unless I am listing out the counts of items, then I only use digits.
"The updated electrical design will require three [3] new, pad-mount 500kVA transformers to replace the three [3] existing 225kVA transformers,each located on floors four, five, and six."
Can I ask why, though? I'm also an engineer and I just never spell it out, if I can avoid it (so far, luckily, haven't had push back since I'm on delivery and not proposals or anything like that.)
To me, it's just more annoying to read it as words, and no matter what you do, mistakes can still happen, including when it's spelled out.
I work in MEP and our emails are always considered legal documents as they can be used as evidence if ever we are taken to court. So we always treat them very technical and try to over explain everything so clients/plan reviewers/contractors can't misinterpret. It's kind of an old school thing, but the head of our department is an old school guy.
I'll write out a count without a digit if it's immediately next to a value. Like without other qualifiers: "three 500 kVA transformers", versus "3 500 kVA transformers" (horrible), or even "three [3] 500 kVA transformers" (acceptable, but perhaps cluttered)
(Also note the space between value and unit—technically required but I'm not consistent about it)