One may only light an item from a pre-exiting flame for the purpose of using the flame for [cooking] something that could not have been done [i.e. cooked] before Yom Tov without loss or diminishing of quality.^[1] [If however, the food could have been cooked before Yom Tov without any diminishing of quality or loss, then it may only be lit with an irregularity.^[2] In general, we assume that foods would diminish in quality if cooked before Yom Tov.^[3]] One may likewise extend a flame for the purpose of giving light to one’s home^[4], or to heat water to bathe minority of one’s body^[5], or for the sake of a Mitzvah, such as to increase lights in a Shul.^[6] It is however forbidden to extend a flame on Yom Tov if one does not need to use the flame at all for any purpose, such as he does not need it either for cooking, or for lighting one’s home, or for a Mitzvah.^[7]

Lighting a candle:^[8] It is permitted to light a candle on Yom Tov for the sake of using its light^[9], or for the sake of a Mitzvah^[10], such as to light candles in a Shul.^[11] It is permitted to light candles in a Shul even after the last prayer of Mincha on the afternoon of Yom Tov, and even if there is no one [else] in Shul.^[12] It is likewise permitted to light candles at home in honor of Yom Tov. Thus, even if one already said the blessing on the Yom Tov candles, he can light more candles if his intent is to do so in honor of Yom Tov.  It is however forbidden to light a candle on Yom Tov if one does not need to use the flame at all for any purpose and he has no intent at all to do so for the honor of Yom Tov.^[13] Such a candle is called a Ner Shel Batala. Thus, one may not light candles for the sake of showing off one’s wealth and generosity.^[14] [See Q&A regarding a Yartzite or Yizkor candle!]

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[1] Admur 495:5

[2] Vetzaruch Iyun as to what the definition of an irregularity is when extending a flame. Admur ibid does not give this oPiskeiy Teshuvosion of irregularity in his example of flame extension even though it was mentioned regarding the regular rule of Melacha that could have been done before Yom Tov.

[3] See Admur 495:5 “One cannot compare the taste of food that was cooked today to the taste of food that was cooked the day before.”

[4] Admur 495:5

[5] Admur 511:1

[6] Admur 514:14

[7] Admur 514:13-14; Michaber 514:5

[8] Admur 514:13-14; Michaber 514:5

[9] Admur 495:4-5

[10] Admur 514:14; Rosh 2:22

[11] Admur 514:14; M”A 514:14; Michaber 514:5; Rosh ibid

[12] Admur ibid; Michaber ibid; Rosh 5:8

The reason: The candle of a Shul is not considered Ner Shel Batala, and it is not considered that one is preparing on Yom Tov for the weekday when lighting it towards the end of the day, as there is a Mitzvah involved in having it lit even for that remaining hour, as it gives honor to Hashem when one lights a candle before Him, as the verse [Yeshaya 24:15] states “Al Kein Beurim Kavdu Hashem.” [Admur ibid] 

[13] Admur 514:13; Michaber 514:5; Rosh 2:22; Yerushalmi 5:2

The reason: As the Torah only permitted making a fire for non-food related purposes, if it has some necessity for Yom Tov. [Admur ibid]

[14] Admur ibid; M”A 514:12; Rashal 2:35