We have learned (21:8-9) that according to most poskim, today’s streets are considered a karmelit. Thus, to permit carrying in the streets, it is sufficient to surround them with structures resembling doorways (tzurot ha-petaĥ), which form a kind of wall around them.
The basic form of a doorway is comprised of two doorposts with a lintel atop them. A lintel can be formed by laying a board across the top of two posts or running a string above the posts.
The main requirement for a tzurat ha-petaĥ is for the posts on the sides and the string or wire above them to be configured like a doorway. Since the lowest doorway is ten tefaĥim high, the poles of a tzurat ha-petaĥ must be at least that high to be considered valid for an eruv. The wire extended above them must also be at least ten tefaĥim above the ground. Accordingly, if any part of the wire droops lower than that, the entire area between the poles is considered breached, since there is no actual doorway in which any part of the lintel is lower than ten tefaĥim. And if the poles are more than ten amot (4.56 m) away from one another, the entire eruv is disqualified, as a breach of ten amot disqualifies an eruv.
Since the poles form the sides of the tzurat ha-petaĥ, they must be strong enough that a normal wind would not cause them to sway, and to support some type of door, even if only an extremely light one made of straw (SA 362:11).
According to most poskim, there is no limit to how wide a tzurat petaĥ may be, as it can retain the basic form of a doorway even if it extends for a thousand amot. However, Rambam maintains that when most of the eruv’s perimeter is comprised of tzurot ha-petaĥ, no tzurat ha-petaĥ may extend longer than ten amot. Le-khatĥila, when possible, his opinion should be taken into consideration. However, in practice, since it is very difficult to enclose towns and cities using tzurot he-petaĥ that are limited to ten amot, we are lenient and do not limit the width of a tzurat petaĥ (SA 362:10).
The wire that is stretched between the tops of the poles must be secured well enough that it will not become detached in a normal wind. Le-khatĥila, when possible, it should be stretched taut so that it will not even sway in the wind or droop down below the tops of the posts, since lintels do not normally sway or droop. However, be-di’avad, even if the wire sways or droops, the eruv is kosher (MB 362:65; AHS 362:37).