The Halacha, in Eruv follows the more lenient opinion — Eruvin 46a
תלמוד בבלי מסכת עירובין דף מו/א
מאי כללא דאמר רבי יהושע בן לוי הלכה כדברי המיקל בעירוב
The Gemara here lays down a principle: “Halacha k'divrei hameykeil b'eruv” — the Halacha, in an argument concerning matters of eruv, follows the more lenient opinion. Rashi notes that although this specific sugya concerns eruvei techumin, the principle applies to all types of eruvin.
Unfortunately, this principle is often misapplied. Indeed, some modern authorities even extend this principle to the point of using it to support their own lenient rulings!
Although there are some authorities who interpret the principle in its narrow, literal sense, and opine that it only applies to issues that concern the eruvei chatzeiros — the collection of bread that unites the Jews in the eruv (see, for example, Ritva to Eruvin 85b), many authorities maintain that Halacha k’divrei hameykeil even in issues that concern the enclosure itself (see Beur HaGra, Orach Chaim 358:5; see Noam vol. 1 pp. 214-215 and Nesivos Shabbos 15:2 and notes 4-5 for a list of sources that deal with this principle and summaries of their opinions).
Dagul Me'Revavah (Orach Chaim §394; see also Beur HaGra loc. cit.) derive from our Gemara that the principle applies even when it leads us to rule in accordance with an individual authority against the majority of authorities.
The Chazon Ish, Orach Chaim, 112:10, proves that this principle does not apply to later authorities. Whenever we deal with an issue discussed by the Acharonim (any authorities subsequent to the redaction of the Shulchan Aruch) we follow a weighted majority opinion. We must weigh the stature and reasoning of each rabbinic authority when considering his views and counting his ruling in the halachic equation.