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Thread: How big a transformer do I need?

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    How big a transformer do I need?

    I have a low voltage control application where I need a 120 to 24 VAC transformer and I'm trying to figure out how big it needs to be. Specifically, the load side will have two relay coils and two LED pilot lights for a total of 330 ma @ 24 v = 7.92 va. There will be two runs of about 50 feet of 18AWG wire to the control switches so voltage drop may be an issue.

    How many VA should the transformer be able to handle on the load side? Running it at 100% didn't seem like a good idea.

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    Check the specs on the relay coils. Very often relays and contactors will draw more current when first energized then once they are 'sealed'. If this is the case then your transformer and wiring needs to support sufficient current to energize the coils in the first place.

    -Jon

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    I think I'm going to be using these:

    https://www.functionaldevices.com/do...ets/RIB24P.pdf

    Just gives one figure for current.

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    Also I forgot to ask: Do I need to put a fuse on the secondary? Can it be one of those inline glass fuses?

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    Quote Originally Posted by Coppersmith View Post
    Also I forgot to ask: Do I need to put a fuse on the secondary? Can it be one of those inline glass fuses?
    Properly selected primary overcurrent device can protect the secondary - but if your load is only 7.92 VA that is a primary current of .066 amps, though you probably have at least a 10VA transformer which would have a rated primary current of .083 amps as a base for required protection. In your case primary only protection could be 300% that would be about right at 1/4 amp.
    I live for today, I'm just a day behind.

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    Quote Originally Posted by Coppersmith View Post
    I have a low voltage control application where I need a 120 to 24 VAC transformer and I'm trying to figure out how big it needs to be. Specifically, the load side will have two relay coils and two LED pilot lights for a total of 330 ma @ 24 v = 7.92 va. There will be two runs of about 50 feet of 18AWG wire to the control switches so voltage drop may be an issue.

    How many VA should the transformer be able to handle on the load side? Running it at 100% didn't seem like a good idea.
    Quote Originally Posted by winnie View Post
    Check the specs on the relay coils. Very often relays and contactors will draw more current when first energized then once they are 'sealed'. If this is the case then your transformer and wiring needs to support sufficient current to energize the coils in the first place.

    -Jon
    Jon is correct. AC relays have a "surge" or "inrush" until the armature pulls in and seals. One example (from stackexchange):
    Nominal Inrush @ 60 Hz 20 VA
    Nominal Sealed @ 60 Hz 5.25 VA
    Inrush is 3 ~ 4 times sealed.

    My experience has been that marginal sized control transformers will cause relay chatter. The issue is to keep the voltage from sagging during the inrush period to where the relays chatter before they seal.

    Unless real estate (footprint) is an issue, my inclination would be to up size and least 2X. I suspect there is little cost difference. Most all applications I have seen, 50VA is usually about the smallest - regardless of relay coil loading.

    As for wire size, I don't have any calculated data. Although, most installations where the relays are remote to the buttons, tend to be at least #16 and often #14


    Quote Originally Posted by Coppersmith View Post
    Also I forgot to ask: Do I need to put a fuse on the secondary? Can it be one of those inline glass fuses?
    As kwired noted, does not have to be. However I recommend primary and secondary fuses. I've had a couple of cases where the control wiring failed, drawing enough current to burn up the xfm, but not enough to open the primary fuse. Eventually the transformer shorts out, catches fire, and then the primary fuse opens. Makes a real mess in the control cabinet. Sure, in-line glass is fine. I tend to go with 125% slo-blo.

    the worm
    Without data you’re just another person with an opinion – Edwards Deming

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