אין מערבין ערובי תחומין אלא לדבר מצוה כגון שהיה רוצה לילך לבית האבל או לבית המשתה של נישואין או להקביל פני רבו או חבירו שבא מן הדרך וכיוצא באלו: הגה או שרוצה לילך לטייל ביום טוב או בשבת בפרדס שיש בו שמחה בזה מיקרי דבר מצוה (תה"ד סימן ע"ז) או מפני היראה כגון שהיה רוצה לברוח מן העכו"ם או מן הלסטים וכיוצא בזה ואז מותר לו לילך אפי' לדבר הרשות (טור והג"א פרק בכל מערבין) ואם עירב שלא לא' מכל אלו אלא לדברי הרשות הרי זה עירוב:

That one may only make an eruv of boundaries for a matter of mitzva. Contains 4 sections One may only make an eruv of boundaries for a matter of mitzva — such as wishing to go to a house of mourning, or to a wedding feast, or to greet his teacher or friend who came from a journey, and similar cases. Rema: Or if he wishes to go for a stroll on Yom Tov or Shabbat in an orchard, which brings joy — this is considered a matter of mitzva. Or out of fear, such as wishing to flee from non-Jews or bandits and the like, in which case he is permitted to go even for an optional matter. And if he made an eruv for none of these but for an optional matter, it is nonetheless a valid eruv.

One may not make an eruv of boundaries during twilight, but if he did make one it is valid.

If two people said to someone "go and make an eruv for us" — one made an eruv for him while it was still day and one made an eruv for him during twilight: the eruv made for him while it was still day was eaten during twilight, and the eruv made for him during twilight was eaten after nightfall — both have acquired a valid eruv. Some disagree.

When placing an eruv of boundaries one recites the blessing on the mitzva of eruv and says "through this eruv I shall be permitted to walk from such-and-such place two thousand cubits in each direction." If he is making an eruv for many he says "so-and-so and so-and-so shall be permitted" or "the residents of such-and-such place shall be permitted."