I recently supplied This Answer, which quoted almost an entire article from NEC. Is there a problem (legal or otherwise) with doing this?
An occasional use of a sentence or paragraph from the NEC(r) in a non-commercial setting would probably be considered a Fair Use under the copyright act. There is no precise limit concerning what constitutes Fair Use and what constitutes an infringement, but the small occasional use that you suggest appears to fall in the Fair Use category.
If this becomes a regular use of material for a commercial purpose, that may change the response.
I hope this is helpful.
In the first instance having a link back to the original source is a must - assuming that the original content is available on the web.
In the second instance you have quoted a rather lot of information in the answer. This could be an issue regarding use of potentially copyright content.
It would be far better to summarise the information in your own words quoting one or two relevant sentences and explaining why things are the way they are. However, if the content you are quoting is not on the web this might not give people enough information to solve their problem.
I don't know what the answer is here. You have to be aware that Stack Exchange may be issued with a copyright violation notice (of some kind) and have to take the content down. The easiest way of doing this would be to delete the answer.