The count comes from the pitch, and nothing else does
Modules go down the middle of the can at a fixed spacing. Take the centreline length of the stroke, divide by the pitch, add one for the module at the far end, and multiply by the number of rows the stroke width needs. That is the whole model. With the defaults — a 34 in centreline at 6 in pitch, one row — it is six modules a letter and 72 across a twelve-letter set.
Halve the pitch to 3 in and the same letters take 12 modules each, 144 in the set. The wattage doubles, the supply count moves, and the drawing looks identical. This is why a set that was quoted off a photograph and built off a datasheet comes out at a different number.
Why the supply count is not just watts divided by watts
At the defaults each module draws 0.72 W, so 72 modules is 51.84 W. A 60 W supply loaded to 80 percent gives 48 W, which holds 66 modules. Seventy-two modules divided by 66 says two supplies, and that is the easy answer.
The harder answer is that a letter cannot be split across two supplies without running secondary conductors from one can to the next, which nobody wants to do inside a raceway. So the useful figure is whole letters per supply: 66 modules divided by 6 per letter is 11 letters, and 12 letters needs two supplies. The page prints both divisions and takes the larger. On this set they agree. On a set where they do not, the letter-grouping number is the one to buy against, and the difference is usually one supply.
The load fraction is yours
The field asks what percentage of the supply rating you intend to use, and it defaults to 80 because that is a common shop habit, not because anything requires it. Set it to 100 and the same job holds 83 modules a supply and comes out at one supply instead of two. Set it to 60 and it goes the other way. There is no number this page can give you here — the supply maker publishes what they want, some of them derate by ambient temperature, and enclosed raceways in the sun are not the bench the rating was measured on.
Series runs are a separate limit
The modules-per-supply figure is about heat and current in the supply. The modules-per-run figure is about voltage drop down the chain, and it binds independently. At 20 per run and six per letter, each letter is one chain, so the run count equals the letter count. Put 26 modules in a tall letter and it becomes two chains inside one can, with two returns coming out the back, and the hole pattern in the letter back has to know that before the backs are riveted on.
What the arithmetic cannot see
Whether the face lights evenly is not a count. It depends on the depth of the can, how far the module sits off the return, how translucent the face material is, and what colour vinyl is over it. A shallow can with dark vinyl hot-spots at a spacing that would look perfect on a deep can with white acrylic. The only honest test is one letter, built the way the set will be built, lit after dark. Everything on this page is the material order that follows that test, not a substitute for it.
Where it hands off
The DC side of a supply — headroom, efficiency losses and what else is sharing the rail — is on the power supply rail calculator. Long low-voltage runs from a remote supply to a distant letter are a voltage drop problem, and at 12 V a surprisingly short run matters. What may be connected, by whom, and what has to be inspected is a question for the electrical inspector with jurisdiction and not for a calculator.
Questions people ask
How many LED modules does a channel letter need?
Divide the stroke centreline length by the module pitch and add one. A 34 in centreline at 6 in pitch gives six modules for one row, doubled if the stroke is wide enough to need two rows. The pitch is the number that decides it, and it comes from the module maker for your can depth and face material rather than from a general figure.
How many modules can one sign power supply run?
Take the supply rating, multiply by the fraction of it you are willing to load, and divide by the watts per module. A 60 W supply at 80 percent is 48 W, which holds 66 modules of 0.72 W. Then check it a second way: how many whole letters that is, because splitting a letter across two supplies means running secondary conductors between cans.
Why does the page give two different supply counts?
One is the raw wattage division and the other groups whole letters onto a supply. They disagree whenever the letters do not divide evenly into the per-supply module limit. The letter-grouping figure is the buildable one, so the page takes the larger of the two, and the gap between them is usually a single supply.
What happens if I exceed the modules per series run?
The far end of the chain sits at a lower voltage than the near end and reads dimmer, which shows up at night as one letter in a word looking tired. The limit is published by the module maker for that module at that voltage. A long letter that needs more than one run is normal — it just means two chains inside the can and two returns out the back, which the hole pattern has to allow for.
Does tighter module spacing fix a hot-spotting face?
Sometimes, and it is the expensive way to find out. Hot spots are usually about how far the module sits from the face and how translucent the face is, not only about spacing along the stroke. Building one test can and looking at it lit costs an afternoon and settles it before four hundred modules are stuck down at the wrong pitch.