Controller Fuse Question - Need Help

[NineInchNails]

I built a controller a while back, but then had a house fire. No one was hurt and it wasn't due to DIY electric. I was able to salvage the controller, but I had to strip it 100% down, clean it up and reassemble it again. It runs a PID for temp control or it can switch over and have variable control using a Potentiometer. I want to install proper fuses before plugging it in.

ALUMINUM BOX BEFORE FIRE:


AFTER FIRE:


I planned to use this to control a Camco 2963 5500W 240V Ripple Foldback Ultra Low Watt Density element.

The PID runs one 40A SSR and the Potentiometer runs a 40A SSR (shown here)

I believe I need a .5A Fast Blow Fuse and a Panel Mount Fuse Holder to protect the Auber PID.

5500W 220V would draw 22.9A (25% more would be 28.75A FUSE). Could I get away with a 25A fuse for each SSR? Should I bump up to 30A for each SSR? I'm having a hard time finding a 30A Panel Mount Fuse Holder.

I could use some input on this.

 

[lschiavo]

As long as you have 10 gauge wiring, you could fuse the element at up to 30A. This is exactly how a water heater is fused. A smaller fuse is fine if it will hold.

I have always been confused why people even fuse pids. I contacted auber once a while back and they said fusing was not needed for the device as it was internally protected. I just looked at a manual and they now show a fuse at the power input. I could not find in the instructions where they recommended a size.. The specs show power consumption at less than 5W though. So at 120V, the max current should be 5/120=0.042A. That's a small fuse.

 

[NineInchNails]

Yeah Auber doesn't say what fuse to use, but everywhere online recommends a 0.5A fast blow if you want to use one. Auber sells fuses and housings, but not a 0.5A fast blow fuse. The smallest fuse they appear to sell is 20A. Perhaps Auber has had enough people screw up the basic wiring to the point where they began recommending a fuse? I don't know. Maybe it is unnecessary. I put together a small PID controller before, I didn't use any fuses and never had a single issue.

Fuses are cheap so the idea just stuck in my head to use them. Maybe I'm overthinking it, but with such an elaborate build (in my opinion), it kinda scares me to plug it in. Maybe smoke will come out

 

[stevehaun]

Just buy a fuse holder and a 1 amp fuse. May be overkill but won't hurt anything. You can buy fuses like this at radio shack or Home Depot.

 

[NineInchNails]

Just buy a fuse holder and a 1 amp fuse. May be overkill but won't hurt anything. You can buy fuses like this at radio shack or Home Depot.

Wouldn't too large of a fuse be a bad idea?

I was under the impression that 0.5A was the proper size to protect it.

 

[lschiavo]

Wouldn't too large of a fuse be a bad idea?

I was under the impression that 0.5A was the proper size to protect it.

Yes. It will not stop the device from overloading which is the purpose of the fuse in the first place. 50mA is the proper size if powered with 120V IMO.

.5A is 10x oversized based on 5W load.

 

[doug293cz]

Remember that fuses are only to protect the wiring, to keep it from overheating and starting a fire. Fuses are sized for the wire, not the actual load, although smaller fuses will limit the over current to a lower maximum in the case of a short. Fuses do not protect the devices that are downstream from them. A fuse won't stop a device from failing. If the device is the cause of an over current condition that would blow the fuse, the device has already failed. Thus the fuse in front of the PID just protects the (probably smaller) wire that is use to provide power to the PID.

30A fuses are fine for the main power input, and will protect all 10AWG (or larger) wire throughout the box. You need to fuse both hot lines. You do not need more than two 30A fuses in your box.

Either a 0.5A or 1A fuse is probably ok for the power feed to the PID, as long as you are using 20AWG (or larger) wire to power the PID.

Brew on

 

[lschiavo]

Remember that fuses are only to protect the wiring, to keep it from overheating and starting a fire. Fuses are sized for the wire, not the actual load, although smaller fuses will limit the over current to a lower maximum in the case of a short. Fuses do not protect the devices that are downstream from them. A fuse won't stop a device from failing. If the device is the cause of an over current condition that would blow the fuse, the device has already failed. Thus the fuse in front of the PID just protects the (probably smaller) wire that is use to provide power to the PID.

30A fuses are fine for the main power input, and will protect all 10AWG (or larger) wire throughout the box. You need to fuse both hot lines. You do not need more than two 30A fuses in your box.

Either a 0.5A or 1A fuse is probably ok for the power feed to the PID, as long as you are using 20AWG (or larger) wire to power the PID.

Brew on

Agree 100%. Fuse the wire. I've said it myself many times around here. I've seen plenty of diagrams that the fusing is in place to protect the pid. I assumed that here. That may or may not have been the op's intent. Good point. You're always on the spot Doug.