Why is Romex illegal in Chicago?

Romex is illegal in Chicago because the Chicago Electrical Code requires nearly all wiring to run in metal conduit, usually EMT. That is a local amendment, not an NEC rule.

The tradition dates to the city's fire history and a strong union preference for raceway systems. Most surrounding suburbs follow the same practice; the rest of Illinois generally does not.

NYC has its own version: nonmetallic cable is restricted by building type and size, so conduit and armored cable dominate there too.

Everywhere else, Romex is fully code-legal in dwellings. If you work across jurisdictions, the wiring method is the first thing to check, because it changes the material list and the labor completely.

Conduit-city work lives and dies by fill limits, so keep the EMT conduit fill calculator open while you plan runs.