Sizing an inline fuse

[gobbler]

Ok guys, how does one size a fuse for a particular application. I've got a sprayer w/ a pump that pulls 7amps. I'm wiring up a switch and want to put in an inline fuse. How do you decide what size fuse you should use ???


Jim

 

[tc35dforme]

gobbler,

i'm no expert....but in general, the fuse is sized to protect the wiring not the load device. for example a # 14 wire can handle about 15 amps...so you'd want a 15 amp fuse on that wire even if the load is only 7.0 amps. this way, if there is a short, the fuse will blow before the wire heats up enough to burn up !!

for your application, i'd run a # 12 wire ( bigger is better !! ) and start with a 10 amp fuse. if the fuse blows, i'd step up to a 15 amp. you're still safe with that wire size up to 20 amps. slow blow fuses are good for motor loads to help tolerate the high inrush amperage when the motor starts.

good luck !!

 

[Phred]

gobbler,

In the Engineering world:
Keep in mind that an electric motor draws more current on startup. In some cases a lot more current for a very short period of time. Also if you stall the motor it will draw more current. So the ideal way to go about this would be to look at the motor and see if it specs a max or peak current in adition to the contious operating current which is the 7 amps.
You could then use a fuse that is rated just slightly over that max or peak current. If the number is not on the motor then the manufacturer could tell you the details.

In the real world:
Start with a 7 amp. Since the start current is so short in duration you won't need a 15 amp fuse. So start with the 7 amp and if that blows slowly increase it by 1-2 amp increments until it does not blow.

Also if you use a slow blow fuse, it would be less sensative to the current spikes of startup and still provide protection, just not as sensative protection.

Remeber it is better to have a fuse that is just bigger than the normally operating current so if a problem occurs you can minimize the damage to the wiring, motor, etc...

Phred

 

[gobbler]

Thanks guys,

I picked up an automotive (blade) inline fuse holder last nite. I've already got various fuses, so I went with the blade type instead of the glass tube type. I'll be finishing the wiring after work today.

 

[ddl]

the idea of starting out with a certain size fuse and increasing until the fuse does not blown was actually written in the NEC code book in one of its versions.

Dan L

 

[Wood_Butcher_Dav]

The amount of current that a wire will carry also depends on the insulation around the wire and how tight and many are in a bundle. The Alpha Wire Company specs the following:
18 AWG PVC @18 amps
16 AWG PVC @24 amps
14 AWG PVC @33 amps
12 AWG PVC @45 amps
10 AWG PVC @58 amps
This is single conductor in free air 30C ambient Temp

Dave

 

[dgood1]

Ok! now I am confused. I always thought that according to the NEC #14 is fused at 15A #12 is fused at 20A. Now I know that is for AC wiring. Is DC different on the fusing?

 

[Phred]

Dave is right. It depends on the wire (romex, THHN, etc..), and the operating enviroment.

I think in this case the wire size is not improtant as long as it can handle a heafty safety margin over the 7 amps. Obvoiulsy even if you used #8 wire (Which is good to more than 30 amps) you would not want a 30+amp fuse to "protect" a 7 amp motor.


Dan; Those NEC guys really had to give that a lot of thought!

Standard engineering practice; Calculate the correct value to three decimals, double that value for a safety factor, and when that does not work keep trying values till something does work!



Phred (AKA Fred)

 

[ddl]

I think some confusion concerning wire size is abound here. The normal practise is to select the wire size accorrding to the load the wire is to carry. The other confusion is that not only is the wire fused to protect the wiring, it is also assumed that the load is also fused to protect it. Normally since the load and wire size is matched, one fuse size is used to protect both wire and load. If you choose to use a wire able to handle a larger load then you are using, maybe because it is handy, then you better fuse to protect the load for the wire size it self will protect the wire since the load will not damage the wire.

Dan L

 

[Wood_Butcher_Dav]

I'll add a little more confusion. Line loss or line resistance will reduce the voltage that the motor sees as to the voltage at source (battery/alternator.) The number of connector contacts will also add to the total resistance. Which means a larger wire will have less DC resistance than a smaller diameter wire. This should not make much of a difference on a small tractor. If I were going to wire up my tractor for a 7-amp motor I would use a 16 or 14AWG wire. I’d make sure that all the contacts were sized for the wire that I was using.

Dave