Having a cup of wine [i.e. Kos Shel Bracha] while Bentching:
A. The dispute:
See Yesterdays daily halacha.
B. The final ruling:
There is no final Halachic arbitration regarding whether we rule like the opinion which holds that it is an obligation to Bentch over a cup of wine or like the opinion who rule that it is not an obligation, and therefore each individual may choose to be stringent or to be lenient. Nonetheless, the following is the widespread custom accepted on this matter:
When a Zimun is present: Practically, the custom today is like the third opinion to not require a cup of wine during Bentching even if one has a Zimun of three or ten, and it is only done during a Zimun as a Mitzvah Min Hamuvchar when there is wine available at home. Accordingly, there are times today that we Bentch without a cup of wine [even when one is with a Zimun]. [Indeed, the custom of the world today is to not to be particular to Bentch over a Kos Shel Bracha even when wine is available at home and there is a Zimun present, whether of three or ten. However, by a Seudas Mitzvah, the custom is to do so.] Nonetheless, there are some Jews who are meticulous to always recite Birchas Hamazon over a cup of wine, as rules the stringent opinion.
Without a Zimun: There are some Jews who are meticulous to always recite Birchas Hamazon over a cup of wine [even when Bentching without a Zimun], as rules the stringent opinion. Some are meticulous when they recite Birchas Hamazon without a Zimun to have a cup of wine on the table but not to hold it in their hands, in order to fulfill their obligations according to all. However, the custom of the world is never to Bentch over a cup of wine when one is without a Zimun, as is the right practice according to Kabbalah, and hence a Kos Shel Bracha is only done when Bentching with a Zimun. [This applies even on Shabbos.]
The custom of the world is to be particular not to Bentch over a cup of wine unless there is a Zimun present, and even when a Zimun is present, it is not treated as obligatory, and is only done on special occasions, such as by a Seudas Mitzvah, if wine is readily available. This applies even by a Zimun of ten, that a cup of wine is not obligatory, and is only done on special occasions. Nonetheless, there are some who are particular to always Bentch over a cup of wine, and those who desire to be stringent may do so.
Shabbos and Yom Tov:
Some Poskim rule that even those who are accustomed to not Bentch over a Kos Shel Bracha during the weekdays, should do so on Shabbos and Yom Tov if there is a Zimun present and have available wine. Practically, however, many are not particular in this and Bentch with a Zimun without a Kos Shel Bracha even on Shabbos and Yom Tov.
The Chabad custom:
It states in Hayom Yom, and is recorded in Sefer Haminhagim: “One recites Birchas Hamazon over a Kos Shel Bracha even if there aren’t 10 people present.” It is unclear as to the intent of the statement. Some understand this to mean that according to Chabad custom one is to be particular to always Bentch over a cup of wine whenever one Bentches with a Zimun, even a Zimun of three. Others, however, understand this to simply mean that there is no need to avoid Bentching over a cup of wine unless there is a Zimun of ten, and that even if there is a Zimun of three men, one may choose to Bentch over a cup of wine. However, there is never an intent to say that it is obligatory according to our custom. Practically, the widespread custom amongst all Chabad Yeshivos, and congregations, is like the second approach and not be particular to always Bentch over a Kos Shel Bracha even on Shabbos, even when there is a Zimun of ten, unless it is a special occasion. This was also the acclaimed custom of the Rebbe Rashab, and the witnessed custom of the Rebbe, to not to be particular to always have a Kos Shel Bracha by a Zimun.
The reason behind a Kos Shel Bracha:
The reason behind the concept of bentching over a cup of wine, is because it is considered honorable and noble to say blessings of praise to God over a cup of wine. According to Kabbalah, the reason for having a Kos Shel Bracha is because it corresponds to the attribute of Malchus, which is also referred to as the attribute of salvation from which all blessing spills from [i.e. Kos Yeshuos Esa].
If there is wine on the table, is one required to recite a Kos Shel Bracha upon Bentching, and should it be removed from the table if one chooses not to do so?
Some Poskim rule that if there is wine on the table, then it is an absolute obligation to bentch over a cup of wine. [Practically, while it is clear from the Poskim that we do not rule this way, it is proper to remove the wine from the table if one does not plan on Bentching over a cup of wine.]
Does the concept of Kos Shel Bracha apply also when a woman Bentches?
Seemingly, there is no difference between a man and a woman in the idea of having a Kos Shel Bracha upon Bentching, and hence according to the first opinion above who holds it to be obligatory even upon the individual, it would seemingly be obligatory also on women. Nonetheless, according to Kabbalah, women are not to Bentch over a cup of wine even if they are making their own Zimun.
C. The practical ramifications:
Drinking the Kos Shel Bracha after Birchas Hamazon of Shalosh Seudos: According to the widespread custom today to not be particular to Bentch over a Kos Shel Bracha, one may not drink from the cup of wine used for Kos Shel Bracha during Birchas Hamazon until after Havdalah. Thus after Birchas Hamazon one is to leave the Kos Shel Bracha on the table, and not say a blessing over it. The custom is to then Daven Maariv and after Maariv to say Havdalah. Unlike the ruling in the Shulchan Aruch, the custom today is to say Havdalah over this same cup of wine, as opposed to using another cup of wine for Havdalah. [It is best for the person who led the Zimun, or at least participated in Birchas Hamazon, to say Havdalah over this cup of wine. In the event that no one will be saying Havdalah over the cup of wine, it is nevertheless proper to say Birchas Hamazon over a Kos Shel Bracha.]
If one has enough wine for either Havdalah or Kos Shel Bracha after Bentching of Melaveh Malka: If one is not always particular to say Birchas Hamazon over a cup of wine, it is forbidden for him to eat before Havdalah, and he is thus to rather say Havdalah over the wine and not have wine for Kos Shel Bracha. If, however, one is always particular to say Birchas Hamazon over wine, then if he does not plan to receive more wine until Sunday night, he may eat prior to Havdalah and then after Bentching use the wine to say Havdalah.
What to do if forgot to eat Afikoman and remembered in middle of Hallel: If one forgot to eat the Afikoman, and he only remembered after beginning the recital of Hallel, then if he is not particular throughout the year to recite Birchas Hamazon on a cup of wine, then he is to complete the Hallel and Haggadah and drink the [4th] cup, and then rewash his hands [and recite the blessing of Al Netilas Yadayim] and recite the blessing of Hamotzi on the Afikomen [and eat a Kebeitza of Matzah]. He is to then recite Birchas Hamazon without a cup of wine. If he is particular throughout the year to recite Birchas Hamazon on a cup of wine, then see the footnote for the exact order that he should follow in this case.
 See Admur 182:1; 296:13; 299:6; 477:8-9; Michaber 182:1; Tur 182; Piskeiy Teshuvos 182:1
 Admur 296:13 in parentheses; 299:6; 477:8-9
Opinion of Michaber: While the Michaber 182:1 does not arbitrate between the three opinions, it is implied that he rules like the 3rd opinion, especially being that so rule the Rambam and Rif. [Kaf Hachaim 182:1]
 Admur 182:1 in parentheses and 477:8-9; M”A 174:2; M”B 182:4
 Admur 477:8
 Aruch Hashulchan 182:1; Ashel Avraham Tinyana 182; Orchos Chaim 182:2 in name of Meorei Or; Piskeiy Teshuvos 182:1
 Piskeiy Teshuvos 182:1; See Q&A regarding Shabbos and Yom Tov
 See Admur 296:13; 299:6; 477:9; Aruch Hashulchan 182:3 that every God-fearing Jew should be stringent in this when a Zimun is present; Piskeiy Teshuvos 182:1
 See Admur 296:13; 299:6; 477:9; Ashel Avraham of Butchach that so was his custom; Piskeiy Teshuvos 182:1 footnote 9 that so was the custom of his grandfather
 Admur ibid; Rama 182:2; Taz 182:2; Beis Yosef 182; See Bach 182
As according to the opinion who requires a cup of wine even when Bentching alone, indeed the cup is in front of him, as according to all the holding of the cup is only required as a Mitzvah Min Hamuvchar. [Admur ibid]
 See Poskim above! Conclusion of Kaf Hachaim 182:1 and 15 that one is to follow this ruling
 Admur 182:1 in parentheses; Kuntrus Achron 271:4 “According to our custom that we do not Bentch over a cup Beyachid”; Piskeiy Teshuvos 182:1
Contradiction in Admur: Vetzaruch Iyun from Admur 46:1 who lists the Kos Shel Bracha of Bentching as part of the weekday list of 100 blessings, accounting for two blessing, one before blessing one after blessing. However, according to the worldly custom this would only be done if there is a Zimun present, and even then only count for the person resetting the blessings. Vetzaruch Iyun as to why in 46:1 Admur would list blessings that are not commonly follow today.
 Zechor Leavraham 20; Kaf Hachaim 182:1
 Seder Hayom Seder Seudas Leil Shabbos that even one who does not Bentch over a cup of wine during the week they should do so on Shabbos; Aruch Hashulchan 182:1 that some are particular to do so on Shabbos and Yom Tov; Piskeiy Teshuvos 182:1 footnote 7
 See Rivivos Efraim 1:150; Orchos Rabbeinu 1:83;
 See Hiskashrus 214:19; 497:16; 523:17; 553:19; Shulchan Menachem 1:120; See Sefer Hasichos 5751 2 p. 596 in the footnote where the Rebbe mentions all the opinions on this matter, of the does not give any final arbitration
 14th Kisleiv
 Sefer Haminhagim Chabad p. 22
 Response of Rav Leibel Groner z”l to me through correspondence: Regarding the Hayom Yom, I was told by eltera chassidim that it means when you bench even with only a mezuman to bench on a kos, which i try especially on shabbos when there is wine on the table. However, the rebbe did not bench on a kos.
 Meaning, that it is our custom to always fulfill the Mitzva Min Hamuvchar to bentch over a cup of wine whenever a Zimun is present, as is likewise the ruling of Kabbalah.
 Rav Ginzberg in Hiskashrus ibid; Response of Harav Eli Landa Shlita that so was the custom of his father to not be particular in this, and that so he himself has witnessed “have we ever seen or heard by Anash that we are particular to Bentch over a cup of wine by a Zimun of three? I have never witnessed or heard of such a thing.”
 So I received from Harav Eli Landa Shlita, based on what he witnessed by his father Harav Yaakov Landa who was a constant guest by the table of the Rebbe Rashab, and was part of list of people who were given the opportunity to lead the Zimun, and he, Rav Yaakov Landa, was not particular to always Bentch with a Kos Shel Bracha when a Zimun was present [even on Shabbos], hence strongly pointing to the conclusion that also by the Rebbe Rashab this was not done.
 So testified to me Harav Leibel Groner z”l through personal correspondence
 Levush 182; Kaf Hachaim 182:2; See also Siddur Im Dach 104b and 105c
 Ruach Chaim [Falagi], brought in Piskeiy Teshuvos 182:1 footnote 7
 So I witnessed done by Harav Eli Landa Shlita on Shabbos
 Kaf Hachaim 182:2 based on Levush 182 and Zohar and Shaar Hamitzvos
 Admur 299:6-7
 Admur 299:6 records a distinction in this matter:
If one is always careful to have a Kos Shel Bracha when he says the grace after meals, then he is to drink from the wine after he finishes Birchas Hamazon, prior to saying Havdalah. The reason for this is because according to the opinion which requires Kos Shel Bracha by Bentching, the cup of wine is part of the meal, and the same way one may continue eating a meal past night if he began prior to sunset, so too he may drink this wine.
However, one which at times says the grace after meals without a [cup of wine], being that he relies on the opinions which say Birchas Hamazon does not require a cup of wine, it is forbidden for him to drink prior to Havdalah from the cup of wine used to say Birchas Hamazon. Rather he should immediately after Birchas Hamazon say Havdalah on another cup of wine, if available, and have the first cup included in its blessing of Hagafen, and then immediately drink it after drinking the cup of Havdalah. [ibid] [Practically today we are no longer accustomed to always have a Kos Shel Bracha when saying the grace after meals. Therefore we must say Havdalah prior to drinking the cup.]
Other Opinions: The above distinction in Admur is taken from the Magen Avraham 299:7. However the Elya Raba 299:6 rules one may always drink the Kos Shel Bracha before Havdalah even if he is not always particular to say Birchas Hamazon over a cup. The Kaf Hachaim [299:20] rules when one is saying a Zimun with three people he may drink the cup prior to Havdalah, immediately after Birchas Hamazon.
 Ketzos Hashulchan 94 footnote 9; Tehila Ledavid 299:1; Kitzur Halachos p. 118; Biur Halacha 299 “Mivareich”
Ruling of Admur in Shulchan Aruch 299:7: From the letter of the law, immediately after Birchas Hamazon one is to say Havdalah in order not to delay between Birchas Hamazon and the Kos Shel Bracha. The reason for this is because one is required to drink the Kos Shel Bracha immediately after Birchas Hamazon, according to those which rule the cup is an obligation. (Furthermore, even according to those which rule Birchas Hamazon does not require a Kos, they nevertheless agree that it is a Mitzvahs Min Hamuvchar to do so, and hence he is to drink it as close as possible to Birchas Hamazon.) [299:7]
Explanation of custom: Tehila Ledavid ibid explains the above ruling of Admur was referring to a person who has already Davened Maariv, in which case we tell him to say Havdalah immediately after Birchas Hamazon. If one however has not Davened Maariv he is to first Daven. The reason for this perhaps is because it is better to say Havdalah within Shemoneh Esrei before one says Havdalah over wine. [Kitzur Halachos ibid]
 Ketzos Hashulchan 94 footnote 9; Tehila Ledavid 299:1; Kitzur Halachos p. 118
Ruling of Shulchan Aruch 299:7: From the letter of the law, immediately after Birchas Hamazon one is to say Havdalah on another cup of wine, if available, and have the first cup included in its blessing of Hagafen, and then immediately drink it after drinking the cup of Havdalah. The reason for this is because Havdalah and Kos of Birchas Hamazon are two separate Kedushos, and hence require two separate cups of wine due to the rule “we do not fulfill Mitzvos in bundles”. (This applies even according to the opinion which rules Birchas Hamazon does not require a Kos, as nevertheless it is a Mitzvah Min Hamuvchar to do so, and hence one should not use the same Kos for both Havdalah and Birchas Hamazon.)
However, if one does not have another cup of wine readily available then he should say Havdalah on the same cup he used for Birchas Hamazon. He should not delay waiting for more wine as one is required to drink the Kos Shel Bracha immediately after Birchas Hamazon, according to those which rule the cup is an obligation, (and even according to those which hold it is not an obligation it is nevertheless a Mitzvah Min Hamuvchar to do so). [Admur 299:7]
Other Opinions: Ashel Avraham Butchacher 299 rules one may always say Havdalah over the same cup of wine used for Birchas Hamazon, as since the wine for Birchas Hamazon is no longer an obligation the concept of not doing Mitzvos in bundles does not apply.
Explanation of our custom today: Tehila Ledavid ibid explains the above Halacha in Admur was referring to a person who has already Davened Maariv, in which case we tell him to say Havdalah immediately after Birchas Hamazon. If one however has not Davened Maariv he is to first Daven. In such a case there is no longer a problem in using the same cup for Havdalah, as there has been an interval between Birchas Hamazon and the blessing over the Kos Shel Bracha. This is based on Admur 299 Kuntrus Achron 4 which rules the rule of “Not to perform Mitzvos in bundle” only applies if there is not a large interval between them.
Others answer that our custom is based on the Rambam which allows and encourages using the same cup for Havdalah, and we hence did not accept this current ruling of the Shulchan Aruch. [Ketzos Hashulchan ibid] See Kitzur Halachos ibid
 Ketzos Hashulchan 94 footnote 9
The reason: As the person who said Birchas Hamazon is to say the blessing over the wine afterwards. Perhaps however today that everyone says Birchas Hamazon quietly to themselves anyone who participated in the meal can say this blessing. [ibid]
 Sheivet Haleivi 8:242
 Admur 296:13
 So is implied from Admur, although no clear ruling is given regarding if one may be lenient in a case that he will receive wine on Sunday.
 Admur 477:9; M”A 477:4 and 7; Maggid Mishneh 8:13; Rashba Hameyuchasos 202; Raavad in Tamim Deim 32
Other opinions: Some Poskim rule that if one did not remember until after reciting the blessing over the wine then he is not to eat the Afikoman and rather rely on the Matzah that was guarded from the time of kneading that he ate in middle of the meal. However, in those areas that it is a custom to make Matzas Mitzvah from the time of the harvest, then even if you only remember after the saying of Hallel, he is to wash his hands and say a blessing of Hamotzi and eat the Afikoman. [Michaber ibid and Sephardi ruling; See previous footnotes for the full details of this opinion]
 See previous case and Admur 164:2
 One is not to suffice with simply eating a Kezayis of Matzah, as a) one is to initially eat two Kizeisim, as explained in Halacha E, and b) One cannot say a blessing over the washing if he eats less than a Kebeitza, as explained in Admur 158:2 and Seder Netilas Yadayim 18.
 The reason: See above!
Other opinions: Some Poskim rule that in such a case he is to say another blessing over a cup of wine after Birchas Hamazon and not worry over the fact that it appears as if he’s adding to the cups. [Rama ibid; See Taz 477:3; M”A 477:7; M”B 477:15; Kaf Hachaim 477:26]
 One who is particular to always recite Birchas Hamazon over a cup of wine: One who is stringent upon himself throughout the year to always recite Birchas Hamazon over a cup of wine, then if he remembers prior to mentioning G-d’s name in the concluding blessing which follows the saying of Hallel (which is the blessing of Baruch Ata Hashem Melech Mehulal etc Baruch Ata Hashem Keil Melech Gadol etc, as explained in chapter 480:1) then he is to stop in the place that he remembers and wash his hands and eat the Afikoman and recite Birchas Hamazon on this [4th] cup. He is not to drink it immediately after Birchas Hamazon but rather should repeat from Lo Lanu and finish the Hallel and Haggadah, and then drink the [4th] cup. Now, although one is not to perform 2 Mitzvos on one cup of wine, nevertheless, since here the custom has spread like the opinions who hold that Birchas Hamazon does not require a cup of wine, therefore in truth from the letter of the law he is only performing one mitzvah on this cup which is the Mitzvah of completing the Hallel and Haggadah. [Admur ibid; M”A 477:7; Machatzis Hashekel ibid; Kaf Hachaim 477:28] See also Admur 299:7; 271:12