Siman 368
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א עֵרְבוּ וְאַחַר כָּךְ נִתְקַלְקֵל הָעֵרוּב וּבָא אֶחָד מִבְּנֵי הֶחָצֵר לַחֲזֹר וּלְתַקְּנוֹ, אִם בָּא לְעָרֵב מִמִּין הָרִאשׁוֹן, אֲפִלּוּ כָּלָה, אֵינוֹ צָרִיךְ לְהוֹדִיעַ וַאֲפִלּוּ מְעָרֵב עֲלֵיהֶם מִשֶׁלָּהֶם. וְאִם בָּא לְעָרֵב מִמִּין אַחֵר, אִם כָּלָה הָרִאשׁוֹן צָרִיךְ לְהוֹדִיעַ לָהֶם, אִם מְעָרֵב מִשֶׁלָּהֶם; וְאִם מִשֶּׁלּוֹ, אֵינוֹ צָרִיךְ לְהוֹדִיעָם כְּלָל. וְאִם לֹא כָּלָה הָרִאשׁוֹן אֶלָּא נִתְמַעֵט, אַף עַל פִּי שֶׁמְּעָרֵב מִשֶׁלָּהֶם אֵינוֹ צָרִיךְ לְהוֹדִיעָם.
1 Laws of When the Eruv Becomes Invalidated After It Was Made — 5 Sections If an eruv was made and afterward became invalidated, and one of the courtyard residents comes to fix it again — if he comes to make the eruv from the same type of food as the first, even if the original is completely gone, he need not inform the others, and he may even make the eruv on their behalf using their food. If he comes to make it from a different type of food: if the original is completely gone, he must inform them if he is using their food, but if he is using his own he need not inform them at all. If the original has not completely gone but merely diminished, even if he is using their food he need not inform them.
ב נִתְוַסְפוּ דִּיּוּרִים בֶּחָצֵר, אִם הַבַּיִת פָּתוּחַ לִשְׁתֵּי חֲצֵרוֹת צָרִיךְ לְהוֹדִיעָם, מִפְּנֵי שֶׁאוּלַי אֵינָם רוֹצִים לְעָרֵב עִם אֵלּוּ.
2 If new residents were added to the courtyard — if a house opens to two courtyards, he must inform them, since perhaps they do not wish to make an eruv with these others.
ג כַּמָּה הוּא שִׁעוּר הָעֵרוּב בִּתְחִלָּתוֹ, בִּזְמַן שֶׁהֵם שְׁמוֹנֶה עֶשְׂרֵה אוֹ פָּחוֹת שִׁעוּרוֹ כִּגְרוֹגָרוֹת לְכָל אֶחָד; וְאִם הֵם יוֹתֵר מִשְּׁמוֹנֶה עֶשְׂרֵה, אֲפִלּוּ הֵם אֶלֶף אוֹ יוֹתֵר, שִׁעוּרוֹ מְזוֹן שְׁתֵּי סְעֻדּוֹת, שֶׁהֵם שְׁמוֹנֶה עֶשְׂרֵה גְּרוֹגָרוֹת, שֶׁהֵם כְּשִׁשָּׁה בֵּיצִים, וְיֵשׁ אוֹמְרִים שֶׁהֵם כִּשְׁמֹנֶה בֵּיצִים.
3 What is the minimum amount required for an eruv at the outset? When there are eighteen or fewer participants, the amount is a dried fig's worth per person. If there are more than eighteen — even a thousand or more — the amount is two meals' worth, which is eighteen dried figs, equivalent to six eggs. Some say it is equivalent to eight eggs.
ד נִתְמַעֵט מִשִּׁעוּרוֹ אַחַר שֶׁנִּכְנַס שַׁבָּת רִאשׁוֹנָה וְנִשְׁתַּיֵּר בּוֹ אֲפִלּוּ כָּל שֶׁהוּא, כָּשֵׁר אַף לִשְׁאָר שַׁבָּתוֹת.
4 If the eruv was reduced below the minimum amount after the first Shabbat began, and any amount at all remains, it is valid for all subsequent Shabbatot as well.
ה אִם נִתְעַפֵּשׁ פַּת הָעֵרוּב וְנִפְסַל מִלֶּאֱכֹל, הֲרֵי הוּא כְּמוֹ שֶׁכָּלָה לְגַמְרֵי וְצָרִיךְ לְעָרֵב מֵחָדָשׁ. הַגָּה: וְלָכֵן נָהֲגוּ לַעֲשׂוֹת הָעֵרוּב חַלַּת מַצָּה שֶׁאֵינָהּ מְמַהֶרֶת לְהִתְעַפֵּשׁ (כָּל בּוֹ וּבֵית יוֹסֵף סי' שנ"ד), וְעוֹד דִּיכוֹלִים לְשָׁמְרָהּ בִּימֵי הַפֶּסַח וִיכוֹלִים לִשְׁמֹר הָעֵרוּב כָּל הַשָּׁנָה (דַּעַת עַצְמוֹ) וְזֶה טוֹב יוֹתֵר מִלְּעָרֵב כָּל שַׁבָּת וְשַׁבָּת, שֶׁלֹּא יִשְׁכְּחוּ מִלְּעָרֵב. מִיהוּ אִם רוֹצִים לְעָרֵב כָּל עֶרֶב שַׁבָּת וְלֶאֱכֹל הָעֵרוּב כָּל שַׁבָּת, הָרְשׁוּת בְּיָדָם. וְע' לְקַמָּן סִימָן שצ"ד.
5 If the eruv bread became moldy and unfit to eat, it is as if it were completely gone and the eruv must be remade. Rema: Therefore the custom developed to make the eruv from matzah, which does not mold quickly, and furthermore it can be kept through Passover and used as the eruv all year long. This is better than making a new eruv each Shabbat, lest people forget to make one. However, if one wishes to make a new eruv each Shabbat eve and eat the eruv each Shabbat, one may do so. See below Siman 394.