Primary fusing on power supply

We are using this power supply at 120VAC. It is a 24VDC 5A power supply. So 24x5 = 120W.

120W/120VAC = 1A draw.

So am I supposed to fuse it for 1A? or 2.6A?

What's the correct calculation?

Thanks!

UL, it all about the wire not catching fire. or as i like to say, the wur.

find out at what amps your inputs catch on fire, then subtract an amp.

our pe's in the car wash caught on fire at 5 amps w/ the bad made in mexico turk cordset run.

"We are using this power supply at 120VAC. It is a 24VDC 5A power supply. So 24x5 = 120W.

120W/120VAC = 1A draw"

If efficiency would be 100% and no loss when rectifying, which is never a reality.

The larger number goes for 100-120VAC. 2.6 is the draw.

As previously stated you have to consider wiring gauge and selectivity (upstream fuses)

If your power supply was 100% efficient, your input current would be 1A when your output current is 5A, as per your calculations. But no P/S is 100% efficient, they all lose quite a bit of energy as heat/etc.

The input current @ 120V will be anywhere up to 2.6A as stated on the label. If it were me, I'd be putting it on a 4A C/B. The main thing you're protecting is the wiring, not the appliance.

[edit] too slow this morning!

Select a wire size that can comfortably handle your worst case load. Then size the fuse/CB for that wire. In this case, worst case load is 2.6A, so you could get by with an impractically thin wire. Typically most folks don't like anything thinner than 16 or 18AWG for general panel wiring.

Then you don't want too big of a fuse, because then if the power supply feed shorts out, you could pop a fuse upstream, so you usually get back to something reasonable, like the 4A suggested above.

It gives you the current draw at 120V right on the label, there's nothing to calculate here. 2.6A at 120V, I'd probably put in a 3 to 5A slow-blow fuse, or better yet a 4A circuit breaker as previously cited.

But since you are curious, the specs on the power supply tell you that under perfect circumstances, it can put out 6A at 24VDC, which means 144W, not 120W, so 144W at 120V at a .7 power factor = 1.71A, not 1A (you also forgot Power Factor when converting DC Watts to AC Watts). On top of that, inexpensive Switch Mode Power Supplies like this are typically around 60-65% efficient line to load (as this one appears to be). The very best are maybe 85% efficient.

too much math. funny, we use a 4 amp breaker as well. seems legit. i'm old school, and love fuses. customer loves circuit breakers. he probably right. fuses tell the truth.

love the new stuff. they put the circuit breakers on terminal blocks now. too cool.

ASF said:

If it were me, I'd be putting it on a 4A C/B. The main thing you're protecting is the wiring, not the appliance.

Click to expand...

This is the point. You are not protecting the downstream device, but the supply wiring to it.

Fuses are to prevent fires. Even ultra fast fusing/breakers do virtually nothing to prevent damage to the connected device, even if on replacing it the device begins to 'work' again... more times than not, it is at least slightly damaged, and is only going to get worse with time.

rdrast said:

This is the point. You are not protecting the downstream device, but the supply wiring to it.

Fuses are to prevent fires. Even ultra fast fusing/breakers do virtually nothing to prevent damage to the connected device, even if on replacing it the device begins to 'work' again... more times than not, it is at least slightly damaged, and is only going to get worse with time.

Click to expand...

i like the boat btw.

Thanks everyone for the answers! Sizing the wiring is the easy part. It's calculating the fuse size that's confusing.

jraef said:

But since you are curious, the specs on the power supply tell you that under perfect circumstances, it can put out 6A at 24VDC, which means 144W, not 120W, so 144W at 120V at a .7 power factor = 1.71A, not 1A (you also forgot Power Factor when converting DC Watts to AC Watts). On top of that, inexpensive Switch Mode Power Supplies like this are typically around 60-65% efficient line to load (as this one appears to be). The very best are maybe 85% efficient.

Click to expand...

Ok, Explain the calculation like I'm 5...How did you get "144W at 120V at a .7 power factor = 1.71A, not 1A (you also forgot Power Factor when converting DC Watts to AC Watts)"?

So what did we just calculate with 1.71A? How does that relate to the 2.6A on the label?

The 25 page data sheet makes for interesting reading:

http://www.pulspower.com/fileadmin/Dateien/pdf/cs5e241.pdf

especially things like Power Factor, efficiency, inrush current, all dependent on actual measured input voltage and temperature.

But basically :

Current = Power divided by (voltage x power factor)

Nearbyatom said:

Thanks everyone for the answers! Sizing the wiring is the easy part. It's calculating the fuse size that's confusing.



Ok, Explain the calculation like I'm 5...How did you get "144W at 120V at a .7 power factor = 1.71A, not 1A (you also forgot Power Factor when converting DC Watts to AC Watts)"?

So what did we just calculate with 1.71A? How does that relate to the 2.6A on the label?

Click to expand...

In a DC circuit: A x V = VA, but also W
In an AC circuit: A x V = VA only, W (power) = A x V x PF

So I was assuming the PF of that power supply is .70, that's about the best you can get. Then from the nameplate, it tells you that the maximum output (DC side) is 6A at 25VDC, as long as the ambient is 30C or less. That is the absolute max power output of the unit, 24 x 6 = 144W output. Then 144W / 120V input = 1.2A, but because it is AC now, you have to divide that by the Power Factor as well, so 1.2/.7 = 1.71A Then because they tell you that the peak current is actually 2.6A, you can determine the efficiency by 1.71/2.6 = roughly .65, so it is 65% efficient, which is not uncommon for low-end power supplies.

I didn't go so far as to research that power supply, but it appears the actual rated PF is even lower, so the efficiency is likely higher in this case. But regardless, the 2.6A is what you work with.