If a pot of soup was placed on an electric hotplate before Shabbat to keep it warm for the Friday night meal, but the power shorted or the hotplate malfunctioned, may one bring the pot to the neighbors’ home (assuming there is a proper Erub) and place it on their hotplate?
The Rambam (Rabbi Moshe Maimonides, Spain-Egypt, 1135-1204), in Hilchot Shabbat (3:11; listen to audio recording for precise citation), writes that it is permissible to move food from one stove to another stove on Shabbat. The Rambam emphasizes that this is permissible even if the second stove is hotter than the first. He speaks of a "Kira" ("stove"), referring to a stove that is "Garuf Ve’katum" – covered – as opposed to an open flame, as it is forbidden to place food on an open fire on Shabbat. A hotplate certainly falls under the category of a covered stove, and thus it is permissible to move food from one hotplate to another on Shabbat.
There is, however, one important condition that must be met, namely, that when dealing with liquid, such as soup, it must still be very hot – at the level of "Yad Soledet Bo" – when it is placed on the second hotplate. Halacha follows the view that "Yesh Bishul Ahar Bishul Be’lah" – one violates the prohibition of cooking on Shabbat by reheating liquid, even though it had already been cooked. And thus once soup has cooled, one may not place it on a hotplate. However, if the pot of soup is still very hot when one reaches the neighbors’ hotplate, he may place it on their hotplate. If the pot contains solid food, such as rice, then it may be moved to a different hotplate and placed there even after it has completely cooled, following the rule of "En Bishul Ahar Bishul Be’yabesh," which means that once a solid food has been completely cooked, it may be reheated on Shabbat even after it has cooled.
This is the ruling of Hacham Ovadia Yosef in Hazon Ovadia (Shabbat, vol. 4, p. 394; listen to audio recording for precise citation). Hacham Ovadia notes that some authorities disputed this ruling, noting that the Shulhan Aruch does not codify the Rambam’s ruling, suggesting that he did not accept it. However, Hacham Ovadia writes, Maran indeed cites this ruling in his Bet Yosef, and its omission from the Shulhan Aruch does not indicate that it was not accepted as Halacha. Indeed, there are many Halachot which are mentioned in Bet Yosef and not in the Shulhan Aruch, and they are regarded as authoritative rulings. Hence, we may certainly follow the Rambam’s ruling and allow moving food from one hotplate to another Shabbat, on the condition that if it is liquid it is still hot.
Summary: It is permissible to transfer food from one hotplate to another on Shabbat, though if the food is liquid (such as soup), then this may be done only if the food is still very hot when it is placed on the second hotplate.
