fuses DC vs AC

[Jmac00]

I have a friend (yes I have ONE FRIEND ) that is putting together a "small cottage" solar system.

He seems to be doing all the right things, but he asked me what the difference is between a DC fuse and a AC fuse? He said his "instructions" (I think he's reading a book on PV systems) call for a DC fused disconnect between the solar panels and Charge controller and a DC fused disconnect between the Charge controller and the batteries. Now I agree that there needs to be disconnects between devices, but do they need to be fused?

I answered "I don't have a clue, but it seems to me an Amp is a Amp, and it doesn't care which way the electrons are going"

He's pretty insistent that that there is some difference?

so I ask? Is there such a thing as a DC fuse? Why would it be different than a regular AC slow blow fuse?

[Naptown]

Yes there is a difference
Because of the nature of DC current it will arc when a fuse blows. The amount of separation between the ends on a DC fuse are longer and many times there are other things within the fuse to prevent arcing.
Use only fuses that are DC rated for the voltage intended. And yes the disconnects should be fused. The need and configuration of the fuses and disconnects depend on the system design. There should be a fuse on the battery to the inverter also.

[Jmac00]

No kidding? I did not know that, Thanx for the response.

so I take it these fuses directional? or polarity sensitive?

Is there a website I can go to, to see the construction of a DC fuse.

should you fuse both legs from the device(s) or just the positive leg?

[Naptown]

fuses are not polarized they work in either direction.
On a floating system both poles should be fused but that is a design question we don't have enough info on for your particular case.

[Sunking]

so I take it these fuses directional? or polarity sensitive?

In general no they are not directional. DC can sustain an arc over significant distances at low voltages, and almost impossible to extinguish at high voltages. AC on the other hand crosses has zero sequence voltage 60 times a second, so it does not sustain an arc as easily at least until the air or gas is ionized.

should you fuse both legs from the device(s) or just the positive leg?

Dpends if the system is grounded or not and the system voltages. Low voltage non grounded system must have both polarities fused. grounded systems do not.

[Jmac00]

Is there a chart for sizing DC fuses. I know that a 10ga wire requires a 30amp fuse, 12ga needs a 20 amp and 14ga needs a 15 amp, and so on.

[Sunking]

Is there a chart for sizing DC fuses. I know that a 10ga wire requires a 30amp fuse, 12ga needs a 20 amp and 14ga needs a 15 amp, and so on.

There is no chart for DC fuses as they are treated no different than AC.

You are also stating things incorrectly. 30 amps is the largest over current device that can used on a #10 AWG wire under normal circumstances. You can use a 1-amp fuse on a 10 AWG wire, you just cannot use larger than 30.

Here is Table 310.16 that you are looking for. But be warned when using low voltage you will in most cases have to use much larger wire than the minimum requirement to control voltage drop