step down transformer fuse ratings

Not solar related at all.
Having a little battle with one of my electrical people. What I have is a 480 to 120 volt step down transformer rated at 1KVA.
480 volt input fuses should be 1000W / 480 volt = 2 amps. round up to the nearest minibus fuse is a 3 amp.
120 volt output fuse should be 1000W / 120 volt = 8.3 amps. Round up again to 10 amps.
Is my math correct? They are telling me input should be a 6 amp as well as the output.
I am getting tired of running miles at 2AM replacing fuses at $15-20 each and transformers at $200 a throw.
Thanks a bunch guys and gals.


  • waynefromnscanadawaynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: step down transformer fuse ratings

    Well, a 1kva transformer would be rated for 1kva continuous, unless otherwise stated. That does not mean it's limited to 1kva surge, ESPECIALLY whenever power is first applied after an outage, or having been switched off. At these times, depending on the combined residual magnetism in the core at the instant of reconnection and the phase of the incoming power at that same instant, there can often be a sudden, though short lived, rather huge surge bordering on a short circuit, depending on the wire resistance in the primary coil/s. This is sometimes referred to as "the crack of the whip". Thus using a fuse so close to the xformers continuous rating will often result in repeated fuse blowing whenever power is restored to the xformer, or if there are spikes etc in the supply. For this reason, a slow-blow fuse, if available for your application is recommended, as well as a fuse of greater amperage than the max continuous expected. The secondary (120 volt output) fuse should be sized to both protect the cable it feeds from overheating, and to blow if the normal expected loads/surges are exceeded. You don't mention what this xformer feeds, so if it's a high starting surge motor for example, you may need a larger / slow blow fuse here as well, although it must still protect the cable. Hope this helps to understand what's going on.
    You mention replacing transformers as well, what's with that? Are they being pushed to the max load they can continuously handle? Undersized? Overheating? Fed with a voltage higher than they should be? Roof leaking rain on them?
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: step down transformer fuse ratings

    you are throwing me too saying transformers are being replaced. if they are it would be due to the primary 480v side being allowed up to 6a of current and something was wrong to begin with that caused the primary to draw more than its usual up to around 2a. it would not be a problem from the secondary as this is fused lower than need be at 6a unless the problem would be a short across the secondary before the point of the fuse allowing primary current to rise higher than speced and blow it out.

    there's no way the fuse current rating would be the same fuse on the input and output. your basic line of thinking is correct that the output current up to the 1000va point will be almost 4x higher than on the input side. i say almost due to losses in the transformer. your calcs are basically correct and the slow blow fuse would be an answer for those annoying quick high current spikes.
  • toppertopper Solar Expert Posts: 113 ✭✭
    Re: step down transformer fuse ratings

    Application. 1 KW transformer feeds a 500watt halogen light and some control circuitry/relays (2 amps surge @120 volt).
    Where the problem arises is in the winter, these freaking guys see an outlet and plug in their 1500watt circulating block heaters, resulting in transformer meltdown on the primary 480v side.
    Solution would be proper sized fuses. Any dummy can replace a fuse.
  • BB.BB. Super Moderators, Administrators Posts: 32,006 admin
    Re: step down transformer fuse ratings

    Lock the outlets so nobody can plug in or put in a small 3-4 amp fuse/breaker to limit current to the outlet (obviously I am missing something about why you cannot--and "silently" killing power to somebody's block heater will not win many friends either)... And what about changing out to non-halogen lighting. There are starting to install LED street lighting big time in our area (will be interesting to see how well they last after 1-2 years).

    If this is a work light--Halogens have very nice color temperature and even a bit of heat too. Probably worth the cost of power (if grid connected).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • westbranchwestbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: step down transformer fuse ratings

    Rather than fuses, what about a small circuit breaker right above the plug in box and they will hear it trip when they plug in that circ block heater...AND... a sign that it is MAXIMUM 500W load
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: step down transformer fuse ratings

    you are missing his point that this is NOT a regular utility outlet as it is transformed down from 480v to 120v with a limitation of 1000va due to the limitation presented by the transformer. who cares if they don't like being limited for if they don't like it they can buy him a bigger transformer so as to power their extra loads presented to the circuit.

    i recommend a smaller fuse on the input at say 3a and make it slow blow. the output could be on a breaker as westbranch has suggested or you can use a fuse here too, but with a fuse you will be buying lots of them as they continue to attempt to take more capacity from the outlet than it is capable of giving.

    i am still confused here as you indicated the output was fused at 6a too. this would stop the damage to the transformer as it is blowing out sooner than even if you had the 10 fuse there. this would've and should've prevented damage to the transformer. your presented loads are just over 6a by calculation and could blow often without them plugging in with extra loads.
  • toppertopper Solar Expert Posts: 113 ✭✭
    Re: step down transformer fuse ratings

    Finally got somewhere this AM. I was sure my math was right.
    As stated prior, this is not a normal application. We DO have a sign on the outlet stating the maximum capacity. Unfortunately some rig hands caint read good, or just don't care.
    Exterior breakers will not work as the enclosure has to be explosion proof.
    However, our builder has agreed that they were wrong in initial fuse ratings. They are now reducing the 480 volt input rating to 3 amps and 110 volt output to 6 amps. They are also replacing all fuses with breakers on both input and output. I thank you all for your input. One can always count on your wealth of knowledge.

    We are starting to look at hard wiring our lighting and eliminating the outlets al together. It's a pain but would save a lot of BS in the end.
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