# Switch for DC application

I have two solar panels in series totaling a maximum of 80 VDC and 500 Watts. I believe that would be a maximum of 6.25 Amps. Could a 20A 120/277V AC switch handle that since 6.25 Amps is a relatively low current? If not what about a 30A 120/277V AC be able to handle it?

Update: The reason I want a DPDT switch is so I can easily switch my two panels setup between being series and parallel. Each panel is 250 watts with a 40 volt output. Series would give me 80 VDC at 6.25 Amps and parallel would give me 40 VDC at 12.5 Amps. Because of shade issues on the RV, there would be times that one setup would be advantages over the other. The switch would not be switched often. I am wondering if a knife switch would serve my purpose better.

• Welcome to EE.SE! AC =! DC. – winny Feb 12 at 7:50
• Instead of switching the current directly, is there some other circuitry involved that could be switched off? – Andrew Morton Feb 12 at 20:13
• Is this a fixed wiring setup, or something that is intended to be stood up for temporary use then taken down again? – ThreePhaseEel Feb 12 at 23:56

NO. The AC ratings of a switch don't translate well to DC.

The critical rating of a switch is the current it can interrupt, not the current it can carry.

When you open a switch, any arc that forms across the contacts erodes them. With AC, the current goes through zero many times per second, helping the arc to extinguish. With DC, the current is persistent.

DC switches need to have oversized contacts compared to their equivalently rated AC counterparts. They often have contacts that open faster, or further.

In your particular case, 6A 80V DC into a 30A 277v switch. There may be enough de-rating there. The switch will certainly work a few times. Whether it will still be working after 1000 operations is another matter.

• The very same applies to AC fuses and DC fuses. Don't use AC fuses in DC applications. – Huisman Feb 12 at 8:01

Alternating current reverses it's direction of current flow many times each second, providing a built in point of zero-crossing. This means that AC switch contacts don't have to be built up as heavily as DC contacts, because any arc drawn when they open will tend to put itself out when the polarity reverses. Minimizing this arc is one major factor in designing the switch to last many, many operations, so switches designed for DC will allow for that. At lower current ratings it can be as simple as designing the switch to have more metal available so that any given amount of damage is negligible. 1970s era DC motor controllers used multiple silver plated contacts with a surface area the size of a large person's thumb print - these machines still run on older American submarines with those controllers.

If you are designing a system you intend to last, see if the manufacturer provides DC ratings for the switch you intend to use - lots of switches have both AC and DC ratings. If it's a hobby project and you've put fuses in all the right places, either one of the AC switches you mentioned will work - until maybe they don't anymore. And remember to account for the worst case - switches occasionally fail closed, so leave yourself the ability to unload the system some other way so you can pull the fuses out without drawing an arc across them.

• Hi Sean, Your explanation ix excellent. Now I understand why DC requirements are so different. Thank you so very much. Not sure what I will do now since I cant seem to find a DPDT DC switch that will carry my current. – Geometry Feb 13 at 19:19
• @Geometry, sourcing can be tough. Your best bet is to find a few part numbers from a robust electronic parts catalog such as Mouser. Even if you're in a location Mouser can't deliver to, you can check those part numbers against other services that can reach you. Also you might have to think outside of 'switch' and into 'switching' - you might be able to find a relay that can do the job and be controlled by a much smaller, more common switch. – Sean Boddy Feb 13 at 19:49

80VDC is a fairly low voltage, so arcing may not be a major issue - I don't know.

I was curious about the rotary-action isolation switch used on my 12-panel array (about 360 VDC, 3 kWp), and discovered that a small number of solar installation fires had been caused by the use of AC-rated switches in a solar DC installation. This surprised me, because the isolation switch is very rarely operated (typically, only to replace a failed inverter). So there may be a safety issue even with the switch just sitting in the closed position?

You can get specific DC Disconnectors which are designed for use with solar installations, and rated for DC operation.

My invertor has a hand operated u-link form isolator, to provide a large arc path (several cm), I believe this is also linked to as series electronic isolator (the part is designated Electronic Solar Switch). Even so, this isolator is speced for a limited number of disconnects and is designed as a consumable component.

An 800v/35A component is rated at 50 disconnects under short-circuit operation. spec