The common is the leg that hurts
Take the defaults: 26.5 V at the controller, 500 ft of 18 AWG, a 5 VA holding solenoid. On its own the zone valve sees 25.11 V, having lost 1.39 V in the wire. Switch the master valve on — a second 5 VA solenoid sitting 20 ft from the controller — and the zone drops to 24.45 V.
That extra 0.66 V did not come from anything happening on the zone hot, which is still carrying one solenoid. It came from the common, which is now carrying two. Every hot on an irrigation system serves one valve; the common serves all of them at once, and its drop is shared by everything energised. This is the part that a normal voltage drop calculation does not model, because a normal branch circuit does not have several loads returning down one shared conductor.
Pull-in is the worst instant
A solenoid draws more while its plunger is moving than it does once it has pulled in. The defaults are 11 VA at inrush and 5 VA holding, which is more than double the current at the one moment the valve most needs voltage. On this run the zone sees 23.02 V while pulling in with the master already holding, and 22.33 V in the worst case where both are pulling in together.
Which means a run can pass a holding-voltage check comfortably and still fail intermittently, opening on cool nights and refusing on hot afternoons when the copper resistance is higher. Anybody chasing a valve that works sometimes should be looking at the pull-in figure and not the holding one.
How far the sizes reach
With a 20 V low limit typed in — the sort of figure a solenoid data sheet gives, entered by you rather than assumed here — the table at the bottom bisects the run length at which the pull-in voltage lands exactly on that number. On the defaults, with the master sharing the common, 18 AWG reaches 1,061 ft, 16 AWG 1,687, 14 AWG 2,682, 12 AWG 4,264 and 10 AWG 6,779.
Those are not ratings and they are not advice. They are arithmetic on the number you typed, and if you type a different low limit every figure in the table moves. What they are useful for is the shape: each step up in size buys 59 percent more distance, every time, because resistance falls with the circular mil area and the AWG steps are geometric. The doubling comes every two sizes, not every one.
A splice is a resistor
The extra-resistance field on the form is worth playing with before trusting any of the rest. Put two ohms into it — a plausible figure for a corroded twist connector sitting in a valve box that floods — and the holding voltage on the defaults falls from 24.45 V to 23.30, while the worst pull-in case falls from 22.33 V to 20.27. Put it on the common instead of one hot and every valve on the system moves together, which is the signature of the fault.
That is generally a more productive place to look than wire size on a system that used to work and then stopped, because wire gauge does not change over time and connections do.
Questions people ask
What size wire do I need for sprinkler valves?
It depends on the run length, the transformer voltage, the solenoid VA and how many solenoids share the common, so there is no single answer. Put your own figures in and read the distance table, which bisects the run length at which pull-in voltage reaches the low limit you enter from your solenoid data sheet. On the defaults here — 26.5 V, a 5 VA solenoid with 11 VA inrush, a master valve sharing the common, and a 20 V limit — 18 AWG reaches 1,061 ft.
Why does adding a master valve make a distant zone stop working?
Because the master shares the common conductor. Each zone hot carries one solenoid, but the common carries every solenoid that is energised, so bringing a master valve on roughly doubles the current in the common and doubles the drop on that leg. On the defaults that costs the zone 0.66 V, which is enough to push a marginal run over the edge at pull-in.
Should I size for inrush or for holding current?
Inrush is the moment that decides whether the valve opens at all, and it is the higher draw. A run sized on holding current can hold a valve open perfectly once it is open and still fail to pull it in, which shows up as a valve that works intermittently rather than one that is dead. The page reports both, and the worst instant is when a zone and a master are pulling in together.
Where do I find the VA figures for my solenoid?
The valve or solenoid data sheet, which normally quotes both an inrush and a holding figure at a nominal voltage. They differ between manufacturers and between models from the same manufacturer. Nothing on this page supplies them, and the placeholders in the fields are there to be replaced.
Is this the same as a voltage drop calculation for a lighting circuit?
No, and that is why the page exists. A branch circuit has one load on two conductors. An irrigation system has several loads on several hots and one shared common, so the common leg carries the sum and the arithmetic has to solve the loops together. For an ordinary two-conductor circuit the voltage drop calculator on this site does the job properly, ampacity included.