Non-Kohanim reciting Birchas Kohanim and performing Nesias Kapayim:
It is forbidden for a non-Kohen to perform Nesias Kapayim. A non-Kohen who performs Neisas Kapayim transgresses a Biblical positive command. This applies even if the non-Kohen performs Nesias Kapayim together with the other Kohanim. [This applies even if the non-Kohen does not say the before blessing of Asher Kidishanu prior to saying Birchas Kohanim..]
What aspect of Nesias Kapayim is it forbidden for a non-Kohen to perform or say?
May a non-Kohen say the verses of the blessing without raising his hands, or intending to perform Nesias Kapayim?
Placing the hands on the person’s head: Due to the above prohibition against lifting the hands while saying Birchas Kohanim, some are accustomed not to even rest both hands on the head of the person being blessed when blessing him with Birchas Kohanim, and rather rest only one hand on his head. Other Poskim rule it is permitted and encouraged for one to do so. Practically, the widespread custom is to be lenient and allow the placing of two hands on the head of the person being blessed, and so was the custom of the Chacham Tzevi and Rebbe Rashab.
The Nussach of Birchas Habanim:
Blessing children on Erev Yom Kippur:
Blessing children on Friday night:
Blessing someone prior to their travel:
Blessing children under the Chuppah:
This is a personal that occurred to me [the author]: In the year 5762, I had a dream in the early morning in which I saw the Rebbe. In the dream I witnessed a Chassid raising his hands and blessing the Rebbe with Birchas Kohanim. After he concluded, I also raised my hands and began blessing the Rebbe with Birchas Kohanim. [I am not a Kohen.] I began saying Yivarechicha when the Rebbe suddenly turned to me and began waving his hand downward repeatedly. I understood this meant for me to stop saying the blessing and I did. I then woke up and went to Mikveh, which was downstairs, dazed and trying to understand the dream’s meaning. As I exited the Mikveh at 6:20 in the morning, I was greeted by a very old random Jew who asked me for charity which I gave him. He then began blessing me with Birchas Kohanim saying that he is a Kohen and he is allowed to bless me however I am not a Kohen and hence I cannot bless. Needless to say, I almost fainted. I later found the sources in Poskim who differentiates between saying the blessing itself, which is allowed by a non-Kohen, and raising the hands, which is not, hence explaining the Rebbe’s downwards hand motion to place down my hands.
 Admur 128/2; Rama 128/1; Kesubos 24b
 Admur ibid; Kesubos ibid
The reason: The Torah states [Bamidbar 6/23] “Speak to Ahron and his sons and tell them that so they should bless Bnei Yisrael.” From here we learn that the command excludes all non-Kohanim, implying that they cannot bless, and a negative command that derives from a positive command is considered a positive command. [Admur ibid; Rashi on Kesubos ibid]
Alternative reasons: Some Poskim rule the reason a non-kohen is invalid is because of the blessing, as it is Biblically forbidden to say a blessing in vain. If however he does not say a blessing, then there is no prohibition involved. [Riy in Tosafus Shabbos 118, brought in M”A 128/1] The practical ramification is regarding whether he may join other Kohanim under the Duchan. [See next footnote]
 Admur ibid; Rama ibid [although concludes with Tzaruch Iyun in parentheses]; M”A 128/1; Shaareiy Teshuvah 128/1; M”B 128/3
The reason: In addition to the regular transgression explained above, some Poskim explain that a non-Kohen is commanded by a positive command to be blessed [Chareidim, brought in Haflah Kesubos ibid] and if he goes up to the Duchan rather than staying in the crowd then he is not getting blessed and transgresses this command. [Haflah ibid, brought in Shaareiy Teshuvah 128/2]
Other opinions: Some Poskim rule it is permitted for a non-Kohen to join the other Kohanim in Nesias Kapayim, so long as he does not recite the blessing. [Option brought in Rama ibid in parentheses and explained in Darkei Moshe 128/1 based on Riy in Tosafus Shabbos 118b, based on Rebbe Yossi in Shabbos 118b who says he would join the Kohanim on the Duchan if requested; Chasam Sofer 128 rules that it is permitted to perform Nesias Kapayim with other Kohanim; See M”A ibid and Machatzis Hashekel ibid] The reason why it is permitted is because this opinion holds there is no prohibition for a non-Kohen to say Nesias Kapayim, other than the fact he is saying a blessing in vain, and hence when said with others [and he omits the blessing] it is valid. [M”A ibid] Alternatively, it is because there is never an Issur of Baal Tosif in a case of Hiddur Mitzvah, when one performs a Mitzvah Berov Am. [See Chasam Sofer ibid]
 M”A ibid unlike Riy ibid; Bach 128; Olas Tamid 128/4; M”B 128/3; Kaf Hachaim 128/40; See previous footnotes
 Bach 128, brought in M”B 128/3
 M”B ibid that so is implied from P”M 128 A”A 1 [seemingly from the fact he did not suggest that Rebbe Yossi did not raise his hands]
 M”B 128/3 in name of Magen Giborim; Kaf Hachaim 128/39
 Biur Halacha 128/1 “Viein Lezar”
 Biur Halacha 128/1 “Viein Lezar”
 See Mahariy Assad O.C. 41; Keren Ledavid; Kapei Ahron 64
Some Poskim rule it is forbidden to say Hashem’s name in the blessing due to it being in vain. [Noda Beyehuda Kama 6]
 See Poskim in previous Q&A!
 Custom of Gr”a, brought in Torah Temima Bamidbar 6/23; Siddur Reb Shabsi; Maavor Yabok Sisei Renanos 43; See Piskeiy Teshuvos 128/6
 Siddur Yaavetz Seder Leil Shabbos; Sheilas Yaavetz 2/15 that so was the custom of his father the Chacham Tzevi
 See Daas Sofer 1/14; Beir Moshe 4/25; Tzitz Eliezer 11/8; Yechaveh Daas 5/14
 Brought in Sheilas Yaavetz ibid
 Otzer Minhagei Chabad p. 194
 See Piskeiy Teshuvos 271/1 footnote 5
 Sefer Haminhagim p. 124 [English Edition]; Mateh Efraim 619/2; Kitzur Shulchan Aruch 131/16; Otzer Minhagei Chabad p. 194
 Sefer Haminhagim ibid
 Mateh Efraim ibid
 Hisvadyos 1987 Vol. 1 p. 131; So is the Rebbe’s custom in Birchas Hatemimim [Sefer Haminhagim ibid], and so is implied from Kitzur Shulchan Aruch ibid. to say the blessing of Yivarechicha. In Sefer Haminhagim, it does not mention the dialect of the blessing. Regarding the issue of a non-Kohen reciting Birchas Kohanim see Otzer Minhagei Chabad p. 195. The M”B 128 in Biur Halacha “Zar Over Beasei” rules the prohibition only applies when one raises his hand or has intent.
 Kitzur ibid
 Kitzur Shulchan Aruch ibid; See Otzer ibid that the Rebbe Rashab would go to his mother to receive a blessing.
 Oatzer ibid
 Sefer Hachaim [brother of Maharl] 3/6; Maavor Yabok Sifsei Renanos 43; Siddur Yaavetz; Pela Yoeitz Brachos; Kaf Hachaim 262/17; See Piskeiy Teshuvos 271/1
 Siddur Yaavetz ibid; Kaf hachaim ibid
 Makor Chaim of CHavos Yair 270; Menachem Tziyon in name of Rimnavor in Derushim Lipesach that he would only do so on special occasions and not every Shabbos; This custom is not recorded in the writings of the Arizal
 Rabbi Leibal Groner replied to my question as to what is the Chabad custom: “In majority of instances I did not hear of or witness this custom in Chabad homes, however there were some Chabad families I knew of who did follow this custom.”
 Biur Halacha 128/1 “Zar Over Beasei”