Fuse for 24V Transformer

I'm wiring a remote ON/OFF switch for a large room (window mount) air conditioner which is otherwise accessible only via a 8 foot step ladder. Historically, everyone has been going to the fuse panel to do this. I need to use a low-voltage 24VAC control circuit to keep everyone happy (don't bother to ask me why). The A/C is on a 240 VAC,

20 amp circuit. . The 240 VAC to 24 VAC transformer I have is a White-Rogers (Steveco) 90-T40M2 and it will be controlling a Supco DP30242 (W-R 90-244) contactor. The same fellow who insists on having the 24 VAC control circuit, also wants the transformer's 240 VAC input to be fused. . As the transformer's output is 40 volt-amps (I know, almost but not the same as Watts), my simple math tells me that if the transformer was 100% efficient, it should only draw a max of 167 mA. In looking at fuses, I see that Bussman makes all sorts of 5mm x 20mm fuses under 1 amp. I'm considering using 300 mA fast acting fuses as that's what's readily available from my local Grainger store. . Am I on the right track here ? I just hope my 'friend' doesn't insist that the 24 VAC wiring also be fused. Or should it, just to be absolutely safe ? The transformer will be installed in a tight location where there will not really be any air flow to draw off heat. And it will likely be energized (even with the A/C OFF) all the time, all year long.
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Serious Machining
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Try 1/4A slo-blow for the input side of your transformer. Grainger has those, too. A fast blow fuse will pop on you. That's because the 11 ohm DC resistance of your contactor coil might cause momentary inrush current..

Don't worry -- you don't need to fuse the secondary of the transformer.

And the 90-T40M2 is made to be mounted in an electrical box, I believe. Since your contactor will only require 6VA when on, you shouldn't have any problem with heating.

Buy your friend a cold six of beer if he's giving you free advice -- he seems to be leading you in the right direction.

Good luck Chris

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I think you want a slow fuse on that transformer fast fuses before transformers are succeptible to surges when the power comes on. (something I only learned about recently)

you could use an in-line fuseholder for that... no big deal.

if something shorts out your wiring it could save the transformer.

how much current will it be suppllying most of the time?

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The fuse on the input side of the transformer is NOT to protect the transformer. It is to protect the wiring on the input (240V) side, in case the transformer draws too much current. So you don't need a fuse close to some computed value based on

40 va. A slow-blow 1 amp fuse would be fine in an assembled piece of equipment.

In fact, if your transformer is UL listed for installation in a j-box, the mains branch circuit breaker provides the correct protection. Bastardizing things by adding a fuse not called for by the manufacturer's instructions will make things *less* safe, not safer.


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I do quite a bit of work with 24 volt control circuits and I've never felt the need to fuse the input side of the transformer..... I always fuse the low voltage side...and usually use transformers with built in circuit breakers. Transformers for the most part dont just fail....Ive seen them 50 years old still kicking it....problems usually arise on the low voltage side in my experience.

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a fuse on the primary will stop a fire if the transformer fails. (many transformers have it built in)

a thermal switch on the transfromer and or a small enough fuse on the secondary will save the transformer if a fault develops in ther low voltage stuff.

Bye. Jasen

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