VA rating for switches? Why not Watts?

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wmtunate

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Joined
Jun 7, 2006
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From what I understand, VA (volt-amps) ratings are the same as Watts for DC circuits (no inductance).

I'm seeing a lot of toggle switches with a contact rating of ".4VA @ 20VAC or DC". By my math that's 20mA at 20VDC, right? (.4/20 = .020). That seems awfully low, especially when the same switch with tin contacts is rated for "5A @ 120VAC or 28VDC". What am I missing when it comes to these VA ratings for switches? I'm sure there's some sort of conversion I'm not doing here.
 
Ignore the AC thing for a sec... I'm interested in DC applications for now.

VA (volts * amps) is a way to spec power rating. For DC loads, it's the same as wattage (from what I understand). However, the ratings I'm seeing are extremely low (.4VA @ 20VDC, i.e. .4W @ 20VDC), which is a 20mA rating, where it should be in the neighborhood of 5A or so. Why the discrepancy?
 
The voltage and current ratings for switches are not at the same time... Max voltage is when open and zero current. Max current is when closed and very small voltage... So power from V x I is inappropriate in this context.

JR
 
So, if I only know the VA rating at a certain DC voltage, how do I know how much max current this thing can handle?
 
You have both max voltage and max current. You can't recalculate them for the same power but higher voltage or current, as John said already.
 
Thanks Roger, I guess I was just overlooking the two different applications.

[quote author="Wavebourn"]You have both max voltage and max current. You can't recalculate them for the same power but higher voltage or current, as John said already.[/quote]

So my 20mA @ 20VDC calculation is correct then? I guess that makes sense for a signal level application, but it still seems low.

I was actually more interested in a lower voltage application (12VDC).

EDIT:
OK, I think I've got it. So either way, I shouldn't (for this particular switch) exceed 20VDC or 20mA.

Similar switches give current ratings at 2 different voltages (5A @ 120VAC / 2A @ 250VAC) which would lead me to believe that I could recalculate max current for a particular voltage using the power formula. I guess that's not always the case...
 
[quote author="wmtunate"]

So my 20mA @ 20VDC calculation is correct then? I guess that makes sense for a signal level application, but it still seems low.

I was actually more interested in a lower voltage application (12VDC).[/quote]

So, 20 mA / 12 V.
 
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