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Old 11-10-2010, 01:26 AM   #1
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Default help with transformer wiring

I'm building a larger electric system now, and will be using a seperate low-voltage switch panel. I have not yet dealt with transformers, so I need some advice/help on protecting mine.
I have sized my components and it seems this one is an acceptable amount of overkill. 100VA, 240/120v primary, 24/12v secondary

I am confused by fuses/circuit protection for this. Here's my thinking: 120v 100VA = .83amp So... if I put a 1 pole 1 amp C curve breaker on the primary line side, is this enough protection for the transformer and its components downstream (they are just relays and led switches)? Should I worry about protecting the secondary side as well?

Thanks so much in advance if anyone can help!

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Old 11-10-2010, 11:01 PM   #2
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Yes, you should also protect the secondary side conductors. 100VA for 12V would be 8.3A, so you should use a 10A fuse on the secondary.

FWIW, Fuses protect the conductors feeding the components. If the components short out for some reason, the fuses will blow, hopefully keeping too much current from flowing through the conductors, melting the insulation, and then shorting to ground or starting a fire.

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Old 11-10-2010, 11:21 PM   #3
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The normal method is to protect the primary side. So you are correct.

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Old 11-10-2010, 11:27 PM   #4
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Normal for whom?

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Old 11-11-2010, 12:08 AM   #5
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Quote:
Originally Posted by evanmars View Post
Normal for whom?
I can understand the thinking behind only needing to fuse on the HV side (not saying that is what you "should" do though) - if you draw more than 8.3 amps on the LV then the HV side would draw more than .83 A and blow the HV fuse. Correct ?
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Old 11-11-2010, 01:19 AM   #6
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Thanks guys

I still need to clarify a couple things though...
Is there a larger inrush current that I should account for on a transformer like this? Meaning should I oversize the primary protection? I've read that the protection should sometimes be 2-3x the rating of the transformer... but I think that's only for motors (?)

If I fuse the secondary side, should it only be on the positive pole or both that and negative? I'll be using the 24v rather than 12v.

If the primary is .83 amps, and the secondary is 8.3 amp (if it was 12v), then should the wiring be sized according to those currents? Doesn't make much sense to me if so...

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Old 11-11-2010, 01:22 AM   #7
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OK, if your secondary circuit is expected to draw 8.3 amps then I guess only fusing the primary side would be ok. However, if you are only running a secondary circuit of, say, 5 amps, then the circuit could draw up to 3 or so amps more than it should before the primary side fuse would blow.

You only fuse the positive side. 100VA @ 24V = 4.17A, so a 5A fuse. Yes the wiring should be sized for the max current, so a minimum wire size of 22ga. which is good up to 7 amps. If you wire your panel with 18ga., you should be more than good.

Now that I think about it some more, I guess if you use 18ga. wire, then your secondary circuit can handle up to 16 amps, so I guess your primary side fuse should blow way before then...

I guess fuses are so expensive, it is better to leave them out to save a few $0.01.

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Old 11-11-2010, 01:43 AM   #8
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here is some basics
transformer 120v to 12v
Power (watts)= volt x amps
output 12 volt with a 10 amp draw so (12x10=120va)
would require
120 volt @ 1 amp on primary (120x1 = 120 va)
If you are fused on primary side and you overload the secondary the fuse will blow.
Fuse the secondary (without primary fuse), if you over load with burn the transformer.

So fuse the primary with 1amp slow.
secondary wiring should be in the 16 gauge range.

You can put fuses in the secondary neg and pos circuits if you like. (just a waste of time and money)
You only need fusing on the secondary if you have multiple branch circuits that can not handle the secondary current.

Small transformers have minor inrush current, not like a motor at all.

Hope that helps.

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Old 11-11-2010, 02:08 AM   #9
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That helps alot, thanks!

I'd prefer to use this rather than a fuse:



And 16 awg all around. That will be consistent with all the other 120v wiring in my panel. It will also be run in flexible conduit to the control arm.

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Old 11-11-2010, 02:14 AM   #10
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Quote:
Originally Posted by Budzu View Post
I'm building a larger electric system now, and will be using a seperate low-voltage switch panel. I have not yet dealt with transformers, so I need some advice/help on protecting mine.
Budzu, you are already getting good advice on fuses. I will add my vote on only fusing the primary side. Having a lot of fuses is just confusing, it clutters up the control panel, and a short on the secondary will blow the primary fuse any way.

However, I want to suggest a little different approach on the transformer. I am building a 10 gallon E-brewery and I was considering using 24VAC for the switches and relay coils. As I searched for transformers I realized that DC supplies are a lot cheaper and readily available. I ordered this one but I have not receved it yet. It is a 24VDC 2A switching power supply. All 24VAC relays you may have will also run from 24VDC. So will the pilot lights. I'm just saying.

By the way, good call on the camlocks. I just got mine.
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