Wire size for use with 30A fuse?

Thread Starter

danton133

Joined Mar 12, 2010
7
I don't know weather to be using the term wire or cable here. Can they be used interchangeably?

I have a roll of 1.5mm multicore wire/cable that says its rated for 21amps.

I assume the voltage is not relevant. It will take a maximum of 21A regardless of voltage is this correct?

For sizing wire/cable for a 30A fuse how much discrimination is needed should the wire be able to take 33A, 35A, more?

Usage is within an electrical control panel, obviously lengths are relevant somehow and I'm guessing the best way to do this is use the regs?
 

Reloadron

Joined Jan 15, 2015
6,324
Normally for 30 amp service fused or breaker for 30 Amps I use AWG 10 (2.59mm). That applies to Cu Copper wire. Voltage does make a difference but anything below 600 volts my choice would be THHN for in cabinet use. You want to look at, as mentioned an Ampacity chart and consider wire type for your application. In a cabinet for example the ambient temperature which can be expected and overall environment. You also want to meet any existing codes for your location be it industrial or residential.

Ron
 

MaxHeadRoom

Joined Jul 18, 2013
24,248
The Can/US code states it is not permissible to use a fuse larger than the capacity of the cable it is feeding.
For enclosure wiring regulations, see NFPA79, there is a PDF version out there, if needed.
 

Tonyr1084

Joined Sep 24, 2015
6,624
The Can/US code states it is not permissible to use a fuse larger than the capacity of the cable it is feeding.
For enclosure wiring regulations, see NFPA79, there is a PDF version out there, if needed.
Not only is this code - it's good sense. A wire that can handle 21 amps with a fuse rated for 30 amps is just looking for a fire. Basically a fuse is just a wire with a calibrated breaking point. The fuse is ALWAYS the weak link in any wiring system. The reason why is because you want to control where the wire breaks and how. So the fuse will burn out but not start a fire because the fusible element is entrapped inside a capsule. Any sparks are contained. If you use too large a fuse then instead of burning out the fuse the wire will burn out somewhere along the pathways it takes. Inside walls, passing through studs - all a serious and dangerous fire hazard.

I lived in a house that had substandard wiring. I had a TV with a shorted power supply. Down stairs, the TV would blow the fuse (not breaker, a fuse). But up stairs in the attic, rather than blow the fuse the lights would dim significantly. Basically the wiring was just a very large heater circuit. I had the good sense to unplug the TV right away; and before I did any more work on the TV I ran a 12 gauge wire from the basement fuse panel all the way to the attic - uninterrupted. With a 20 amp fuse in the panel, when I plugged the TV in and turned it on - the fuse blew. That's EXACTLY what it's supposed to do. The old wiring would have just gotten red hot and started a fire in a 1800's home, thoroughly dried out timbers just WAITING for an excuse to burn. Fuses are ALWAYS weaker than the wire they are connected to. Let me restate that - the fuse must have a lower amperage capability than the wires or it's the wires that will burn out. On a wire rated for 21 amps I would not use anything higher than a 15 amp fuse. Why? Because 20 amps is too close to the wires rated capacity. Likely the wire will handle more current, but under the right conditions it will start a fire if pushed beyond its rating.

Bottom line - use the right wire and fuse for your wiring system. 12 gauge wire will handle and is rated for 20 amps. 10 gauge wire will handle and is rated for 30 amps. 8 gauge wire (short runs only) will handle 50 amps whereas longer runs, you must use 6 gauge. And in every circumstance you must use the right fuse or breaker for each according to its rating. As for the 8 gauge short run at 50 amps - I could be wrong, but if I remember it correctly then maybe. Still, I'd opt for the 6 gauge wire even if it IS a short run.
 

MrChips

Joined Oct 2, 2009
25,046
The proper term is wire. Wire refers to the single conductor without the insulation. A cable may consist of many insulated conductors in a common sleeve.

Voltage matters as well as cable length. The voltage drop on a long cable would be more significant in a 12V system than a 120V or 240V system.

For 30A breaker on 240V system, use 10AWG.

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MisterBill2

Joined Jan 23, 2018
10,580
The current ratings for wire that are used in the NFPA electrical code are based on the wire heating, not on the conductor failing. So there is a great deal of difference. Resistance is also a consideration, and that does relate also to heating, but it does effect the performance of the load device.
 
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