# AC to DC amperage safety rating conversion.

#### kid_kicker77

Joined Mar 23, 2009
3
I'm working on a project that I need to use switches with ~10 amps DC. Most of the components I run across are rated in AC.

I've seen the conversion of power, meaning power is the same:
120v (AC) * 10A = 1200W therefore,
1200W = 12v (DC) * 100A.

Meaning, what is meant for 120v @ 10A AC will handle 12v @ 100 Amps DC. Which sound ridiculous to me.

I've also seen posted many places that ratings are and should always be LOWER in DC than AC b/c of the current arcing abilities.

So if I'm looking for switches rated in AC ratings. What is the equivalent AC rating to 12v @ 10A DC?

#### KMoffett

Joined Dec 19, 2007
2,766
I've seen the conversion of power, meaning power is the same:
120v (AC) * 10A = 1200W therefore,
1200W = 12v (DC) * 100A.

Meaning, what is meant for 120v @ 10A AC will handle 12v @ 100 Amps DC.
This is entirely wrong.

Switch contacts are rated by the current that they can "break". AC is easier to break because the voltage goes to zero every half-cycle, so it automatically extinguishes the arcing current as the switch opens. DC continues to arc until the contacts are far enough apart, that at the spec'ed voltage, the arc can no longer be maintained.

Rather than derating AC switches, go to an automotive parts store (or online) and look at switches that are designed for DC.

Ken

#### kid_kicker77

Joined Mar 23, 2009
3
I can live with looking at DC switches and I think I'll be happy with 1 follow up question.

Is a switch rated for:
4A @ 75v DC, equivelant to: 25A @ 12v DC?

Thanks.

#### KMoffett

Joined Dec 19, 2007
2,766
Forget the I2*V2=P=I1*V1 formula. It does not apply to switch ratings. There is also the current carrying factor in switches, not related the AC or DC breaking current rating. A contact meant to handle up to 4A is not likely to handle 25A. If you need a 25A/12VDC switch, look for that rating.

Ken