sitelights
06-06-2004, 12:17 PM
At what point does a transformer begin to fail? When it no longer functions in a manner that meets the manufacturer's specifications.
I am not going to touch upon the 15v+ units. My interest is in the standard UL1838-compliant 300w, 25A, 120VAC multitap (11, 12, 13, 14, 15v or any combination of the usually three taps) transformers (AKA power supplies or power units) that are available from dozens of manufacturers. As much as possible I will enfeavor not to repeat myself so some words will be in ALL CAPS to indicate that details can be accessed by using the capitalized terms as keywords for the lowvolt search engine.
Transformers of the type I am addressing differ in a number of aspects. My primary focus will be on TRANSFORMER REGULATION, CAPACITY, AMPACITY, HEADROOM and LOAD.
Some transformers are better than others and some are much better in the meaning of their ability to hold voltage under LOAD and exhibit superior REGULATION. Manufacturers' specs on CAPACITY are plainly indicated on the label and one is instructed not to exceed the rating which is 300w or 25 amps. Raising the number of watts available is usually accomplished by using dual or triple or quadruple configurations in the same case; thus 300w, 600w, 900w and 1200w can be handled out of a single case. There is no limit on the number of 300w COILS are enclosed in the same case except CORE weight. Hence the name of the attachment "boatanchor.jpeg" (it is a 2X300) is somewhat descriptive. Imagine the weight of a 1200w unit. Note that multiple transformers each need their own primary THERMAL OVERLOAD, low voltage CIRCUIT BREAKERS and TERMINAL BLOCKS so a 1200w interior faceplate begins to look like the control panel of an antique airplane.
A 300w transformer is labeled as such because it can manage 300w of lamp LOAD. If it has to be derated (by, for example 25%) to less than 300w so as not to exceed the CAPACITY of the unit so if you can only attach a maximum lamp LOAD of 225w to the labeled 300w unit you are being ripped off; in that case it should be labeled a 225w transformer. A pint is a pint and, for example, it is illegal in the UK to serve less than a pint when you order your Guinness or whatever. Only pay for what you get.
Holding low voltage output to 12v under full LOAD can be done by even a substandard transformer if the voltage is measured at a higher than 12v tap connection point or very close to it. This is the rationale behind the 15v+ (up to 22v) used by the high volt tribe. To identify the transformer with the best REGULATION without elaborate test equipment is easy: just switch from one to another on the same system. The one that provides 12v under identical circumstances has the best REGULATION. The test must be done under exactly the same conditions since source primary voltage can be in the range of 110v to 130v depending on the season and proximity to the power company transformer. They have the same problem you do.
There are limits to a conductor's CAPACITY to handle electrical current; these limits define the AMPACITY of the wire which is related to its GAUGE and the heat resistance of the SHEATH. The NEC and UL have tables giving the AMPACITY of different types of electrical wire. In low voltage outdoor lighting applications the limits on two conductor STP3 stranded cable are given in UL1838 as wire gauge = AMPERES thus 18 = 10, 16 = 13, 14 = 15, 12 = 25, 10 = 30. AMPERES are calculated as SECONDARY volts multiplied by watts then divided by the PRIMARY volts yielding 12v X 300w /120 or 30A which is the 10 guage cable's AMPACITY. Using this formula a high volt transformer at, say, 20v output with the same load equals 50A and well beyond the AMPACITY of the 10 gauge cable. In order to stay within the AMPACITY of 12 gauge cable which is almost invariably used by the high volt tribe the maximum lamp load on that cable would be 150w; the 12v, 10 gauge system is twice as efficient as the 20v, 12 gauge system in terms of lamp load limits.
The 12v vs.20v comparison given above is an over-simplification of a fomula that is made complex by variables such as cable length, location of LOAD on the cable, source voltage and equipment performance.
HEADROOM is the remaining CAPACITY of the transformer after the initial designed lamp LOAD to carry additional fixtures. A true 300w transformer with, for example 10 20w lamps will have sufficient HEADROOM to handle up to 5 additional 20w lamps. It is good planning to have about 30% CAPACITY in reserve to accomodate your client's desire to have a bit more lighting. It is bad news when you have to say that it will take another transformer just to add 20w. The CAPACITY in this case has nothing to do with derating a 300w transformer or exceeding the AMPACITY of the cable.
All of the above can lead to some anomalous results due to all the variables but widely differing outputs on an identical system is not one of them. As in my post "meltdown" there is always the possibility of equipment either failing or wandering out of spec.
Note that the transformer in the attachment has been cannibalized for parts: Grasslin DIGI 42; photocontrol, hi-lo switch, both magnetic breakers, the straight and angled male spade lug leads, the cover and even the power cord. The terminal block was salvaged later. It is our practice to never invoke the warrranty on anything (RGAs are a waste of time) so the parts are now spares and the case has been consigned to the Dumpster.
I am not going to touch upon the 15v+ units. My interest is in the standard UL1838-compliant 300w, 25A, 120VAC multitap (11, 12, 13, 14, 15v or any combination of the usually three taps) transformers (AKA power supplies or power units) that are available from dozens of manufacturers. As much as possible I will enfeavor not to repeat myself so some words will be in ALL CAPS to indicate that details can be accessed by using the capitalized terms as keywords for the lowvolt search engine.
Transformers of the type I am addressing differ in a number of aspects. My primary focus will be on TRANSFORMER REGULATION, CAPACITY, AMPACITY, HEADROOM and LOAD.
Some transformers are better than others and some are much better in the meaning of their ability to hold voltage under LOAD and exhibit superior REGULATION. Manufacturers' specs on CAPACITY are plainly indicated on the label and one is instructed not to exceed the rating which is 300w or 25 amps. Raising the number of watts available is usually accomplished by using dual or triple or quadruple configurations in the same case; thus 300w, 600w, 900w and 1200w can be handled out of a single case. There is no limit on the number of 300w COILS are enclosed in the same case except CORE weight. Hence the name of the attachment "boatanchor.jpeg" (it is a 2X300) is somewhat descriptive. Imagine the weight of a 1200w unit. Note that multiple transformers each need their own primary THERMAL OVERLOAD, low voltage CIRCUIT BREAKERS and TERMINAL BLOCKS so a 1200w interior faceplate begins to look like the control panel of an antique airplane.
A 300w transformer is labeled as such because it can manage 300w of lamp LOAD. If it has to be derated (by, for example 25%) to less than 300w so as not to exceed the CAPACITY of the unit so if you can only attach a maximum lamp LOAD of 225w to the labeled 300w unit you are being ripped off; in that case it should be labeled a 225w transformer. A pint is a pint and, for example, it is illegal in the UK to serve less than a pint when you order your Guinness or whatever. Only pay for what you get.
Holding low voltage output to 12v under full LOAD can be done by even a substandard transformer if the voltage is measured at a higher than 12v tap connection point or very close to it. This is the rationale behind the 15v+ (up to 22v) used by the high volt tribe. To identify the transformer with the best REGULATION without elaborate test equipment is easy: just switch from one to another on the same system. The one that provides 12v under identical circumstances has the best REGULATION. The test must be done under exactly the same conditions since source primary voltage can be in the range of 110v to 130v depending on the season and proximity to the power company transformer. They have the same problem you do.
There are limits to a conductor's CAPACITY to handle electrical current; these limits define the AMPACITY of the wire which is related to its GAUGE and the heat resistance of the SHEATH. The NEC and UL have tables giving the AMPACITY of different types of electrical wire. In low voltage outdoor lighting applications the limits on two conductor STP3 stranded cable are given in UL1838 as wire gauge = AMPERES thus 18 = 10, 16 = 13, 14 = 15, 12 = 25, 10 = 30. AMPERES are calculated as SECONDARY volts multiplied by watts then divided by the PRIMARY volts yielding 12v X 300w /120 or 30A which is the 10 guage cable's AMPACITY. Using this formula a high volt transformer at, say, 20v output with the same load equals 50A and well beyond the AMPACITY of the 10 gauge cable. In order to stay within the AMPACITY of 12 gauge cable which is almost invariably used by the high volt tribe the maximum lamp load on that cable would be 150w; the 12v, 10 gauge system is twice as efficient as the 20v, 12 gauge system in terms of lamp load limits.
The 12v vs.20v comparison given above is an over-simplification of a fomula that is made complex by variables such as cable length, location of LOAD on the cable, source voltage and equipment performance.
HEADROOM is the remaining CAPACITY of the transformer after the initial designed lamp LOAD to carry additional fixtures. A true 300w transformer with, for example 10 20w lamps will have sufficient HEADROOM to handle up to 5 additional 20w lamps. It is good planning to have about 30% CAPACITY in reserve to accomodate your client's desire to have a bit more lighting. It is bad news when you have to say that it will take another transformer just to add 20w. The CAPACITY in this case has nothing to do with derating a 300w transformer or exceeding the AMPACITY of the cable.
All of the above can lead to some anomalous results due to all the variables but widely differing outputs on an identical system is not one of them. As in my post "meltdown" there is always the possibility of equipment either failing or wandering out of spec.
Note that the transformer in the attachment has been cannibalized for parts: Grasslin DIGI 42; photocontrol, hi-lo switch, both magnetic breakers, the straight and angled male spade lug leads, the cover and even the power cord. The terminal block was salvaged later. It is our practice to never invoke the warrranty on anything (RGAs are a waste of time) so the parts are now spares and the case has been consigned to the Dumpster.