I recently installed, for the first time, a STEP Warmfloor system in a newly constructed house that I wired for a general contractor. The 1000W transformer for one of the zones is randomly blowing fuses on the output side and I am unable to determine why. I have gone over the design specs a number of times including checking the temperature vs resistance curves and correlated them with the resistances I recorded for each element when it was installed. I have also checked the amperage on both the out and the in sides on the transformer when it was presumably under highest load; that is when the floor was cold. In all cases the numbers seem to check out. The fuse-blowing seems to be independent of the temperature anyway. The circuit breaker for this circuit is not tripping. On the 1000W transformer there are two output legs with a 25A slow-blow fuse on each one. One of these seems to be blowing, but I believe the owner said the other one blew at least once as well. The design specs do cut pretty close to the maximum allowable for the 1000W. Even so, my understanding is that the maximum allowable usually contains about a 20% of a fudge factor. Since your site points out that transient draws can be pretty high on this system, does that mean that you need to design in even more of a fudge factor? Does the owner need to replace the 1000W with a 1500W and reallocated the elements among three legs? Or is there another possible issue?