Do you fuse for the Wire or Device?

JeremyAdair87

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I know I should go to an electrical forum for this but I am comfortable here.

I've seen two ways to fuse.

In one scenario you put a 15 amp fuse on the wire to make sure the 120v never gets hot enough to start a fire. I would use an electrical reference for the wire gauge. I am simplifying.

I've also seen say a 5 amp fuse instead because the device connected to the 120V should never pull more than 4 amps.

I got into a recent debate with another EE and I am curious which is correct. In most places that I've seen you fuse the main wires coming in for protecting the wire and then go much smaller for the devices down stream. It seems crazy to put a 15 amp fuse on a 2.3 amp controller with nothing else in parallel on the power of that wire.
 
Both are correct

Usually the feed to a machine or device is protected for the wire size feeding it. Once you are inside of a panel the fuse still needs to be sized to protect the wire BUT you can go down in size to protect the device. There are exceptions to the feeders for motor starting, where the fuse can be larger than the wire size as long as there is another means of checking current such as overload heaters.
 
I've seen others on the forum suggest (in a debate about whether or not to ground 24v) that most of the hardware is already protected internally against surges and that fuses in the panel are all for fire protection in the wires. I offer that without my own comment or opinion, as I've just started learning the ropes (wires?) of EE.

I believe the discussion was in this thread...
http://www.plctalk.net/qanda/showthread.php?t=110307&highlight=ground
 
I can't speak for 'merica, of course, but here in upside down land the general gist of it is that you need protection against two things: short circuit, and overcurrent. This applies to every component in the circuit, be that a simple piece of wire or a $10k PLC chassis.

To use the simple example of a DC power supply. I put a circuit breaker on the line side, because I need to protect the wiring to the PS and the PS itself from short circuit and overcurrent. As far as short circuit protection goes, the size of the breaker is largely irrelevant (within reason, of course). If I get a short circuit on the wiring or internally within the PS, any breaker will trip before the 240V causes any real problems, provided I haven't done something silly like put an 80A breaker on it. As far as overcurrent goes, well, I know that the PS will only ever draw, say 2A. Any more than that and it's already been damaged and no circuit protective device will save it; at that point it's just a matter of making sure it doesn't catch fire. The wiring is good up to around 10A, so any breaker from 2A to 10A would be fine. Typically, I'd use a 4A - give me a little headroom and prevent nuisance tripping, but low enough to trip on a genuine fault.

On the load side of the PS, I could argue that the PS has internal protection for both short circuit and overcurrent, and no external protection is required. I tend to put a breaker in anyway, but that's more for ease of distribution and power cycling than anything else. Quite often I'll have a 5A PS with a 6A breaker on it - the breaker is providing short circuit protection, and the PS is providing overcurrent protection. Of course, in practice, the PS will provide the short circuit protection as well - it's internal cutout works a lot quicker than a breaker can trip.
 
I can't speak for 'merica, of course, but here in upside down land the general gist of it is that you need protection against two things: short circuit, and overcurrent. This applies to every component in the circuit, be that a simple piece of wire or a $10k PLC chassis.

To use the simple example of a DC power supply. I put a circuit breaker on the line side, because I need to protect the wiring to the PS and the PS itself from short circuit and overcurrent. As far as short circuit protection goes, the size of the breaker is largely irrelevant (within reason, of course). If I get a short circuit on the wiring or internally within the PS, any breaker will trip before the 240V causes any real problems, provided I haven't done something silly like put an 80A breaker on it. As far as overcurrent goes, well, I know that the PS will only ever draw, say 2A. Any more than that and it's already been damaged and no circuit protective device will save it; at that point it's just a matter of making sure it doesn't catch fire. The wiring is good up to around 10A, so any breaker from 2A to 10A would be fine. Typically, I'd use a 4A - give me a little headroom and prevent nuisance tripping, but low enough to trip on a genuine fault.

On the load side of the PS, I could argue that the PS has internal protection for both short circuit and overcurrent, and no external protection is required. I tend to put a breaker in anyway, but that's more for ease of distribution and power cycling than anything else. Quite often I'll have a 5A PS with a 6A breaker on it - the breaker is providing short circuit protection, and the PS is providing overcurrent protection. Of course, in practice, the PS will provide the short circuit protection as well - it's internal cutout works a lot quicker than a breaker can trip.

Perfectly put.

One of the best pieces of literature I've read from an aussie!
 
"Do you fuse for the Wire or Device"

I've not read the whole thread, but IMHO you should fuse for the lowest current capability of either.

If the wire can't carry the current for the device then it is under-rated, and will inevitably be a fuse for the device.

But wires aren't designed to be fuses, they will get hot first, and the insulation may burn causing a fire.

Fuses are designed to get hot, and burn out, without the possibility of a fire.
 
Fuse for the device, insure that the wire feeding the device is adequate for the current required for the device. Branch circuit should be fused for the expected load X 1.25, the wire should be adequate for that same current level.
I like to use with, few exceptions #12 and larger for power feeds #14 and smaller for control it helps me to quickly define the use later, of course wire color helps as well.
Also you really don't use a fuse to protect the device, you use a fuse to isolate the device from the system if there is a failure in the device.
 
Last edited:
Step 1: You choose what fuse you need for your device
Step 2: You choose what wire size you need for that fuse
 
Fuse for the device, insure that the wire feeding the device is adequate for the current required for the device. Branch circuit should be fused for the expected load X 1.25, the wire should be adequate for that same current level.
I like to use with, few exceptions #12 and larger for power feeds #14 and smaller for control it helps me to quickly define the use later, of course wire color helps as well.
Also you really don't use a fuse to protect the device, you use a fuse to isolate the device from the system if there is a failure in the device.

Best explanation so far. I just couldn’t put it in words properly.
 

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