If a switch has a 6 amp capacity @ 120V AC, and 3 amps at 240v AC, how can I figure out the amp capacity at 22v AC?
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Short answer about 36 amps. Not to be rude, but the formula is right in front of you. As the voltage goes down, the amps go up. In your example the volts are about one sixth of 120, so the amps are six times (6).
What are you welding?
That's not actually true Elliot, the ratings are not linear on most switches. Usually, the ratings at 125VAC will be very similar to the ratings at lower voltages if not identical. Also, the ratings for AC and DC are frequently different, note that this switch is rated at 6A @ 125VAC, but only 4A @ 30VDC for a center-off switch and 3A @ 30VDC for other switch types.
If it's not specifically specified, I'd stick with the 125VAC rating, I doubt it's much different for 22VAC, and likely no different.
NKK Switches M2022SS1W01
Electrical Capacity (Resistive Load)
Power Level (silver): 6A @ 125V AC & 3A @ 250V AC
4A @ 30V DC for On-None-On & On-None-Off; 3A @ 30V DC for all other circuits
It's not true that you can apply arithmetic to calculate switch capacity versus voltage.
Read this pdf. I think I would limit my low voltage amps still to around maybe 10 or 12 amps. It's not DC, but there are other considerations for switches.
http://www.aeroelectric.com/ar...s/Switch_Ratings.pdf
Or what John says, even better, safer.
Learn something new every day. Today was my day to show off what I don't know.
Welcome to the club Elliot, I get to do that a lot, and I usually get called on it! 
Big_Boy_4005 posted:Learn something new every day. Today was my day to show off what I don't know.
If you never make a mistake, you never learn anything new.
Thanks guys. On its face the question seemed so intuitive. Clearly there are other factors to consider. I'm glad you guys are around to keep the rest of us out of trouble.
The voltage drop, and thus the current carrying capacity of the switch, is directly related to the current through the switch. When you think about it, the voltage drop, and thus the power dissipation of the switch, is the same no matter what voltage being handled is. The voltage ratings have more to do with the internal gaps inside the switch. A higher current switch at high voltages will draw a fairly large arc when it opens, the gap has to be sufficient to minimize that effect when the voltages falls to zero on the half-cycle. The reason the 250v rating was less in the first example was simply the size of the gap and the possible arc separating the contacts. You also have to think about the current carrying capacity of the contacts, in the previous example, a 36 amp switch is a very hefty switch in any voltage rating! The reason the DC ratings are so much lower is the voltage never falls to zero, so any arc that develops between the contacts when open will not be automatically quenched by the voltage crossing zero as it is in an AC circuit. A high voltage DC switch needs much wider spacing of the contacts when open to mitigate this effect.
All you ever wanted to know about switches.
Much good information here.
JGL
gunrunnerjohn posted:That's not actually true Elliot, the ratings are not linear on most switches. Usually, the ratings at 125VAC will be very similar to the ratings at lower voltages if not identical. Also, the ratings for AC and DC are frequently different, note that this switch is rated at 6A @ 125VAC, but only 4A @ 30VDC for a center-off switch and 3A @ 30VDC for other switch types.
If it's not specifically specified, I'd stick with the 125VAC rating, I doubt it's much different for 22VAC, and likely no different.
NKK Switches M2022SS1W01
Electrical Capacity (Resistive Load)
Power Level (silver): 6A @ 125V AC & 3A @ 250V AC
4A @ 30V DC for On-None-On & On-None-Off; 3A @ 30V DC for all other circuits
The Rating has a lot has to do with the contact make-up, what material (Note silver indicated), and the distance of the gap. Can the switch successfully open under the load specified by the rating??? Lower a voltage, in general there is less arc over problem, but as noted DC is different.
Also note the (Resistive Load). Inductive loads are different.
You're right Mike, the contact material does have a big effect on the ratings of the switch. The major takeaway I was shooting for is that the current rating of the switch at lower voltages doesn't go up substantially over higher voltages.
gunrunnerjohn posted:The voltage drop, and thus the current carrying capacity of the switch, is directly related to the current through the switch. When you think about it, the voltage drop, and thus the power dissipation of the switch, is the same no matter what voltage being handled is. The voltage ratings have more to do with the internal gaps inside the switch. A higher current switch at high voltages will draw a fairly large arc when it opens, the gap has to be sufficient to minimize that effect when the voltages falls to zero on the half-cycle. The reason the 250v rating was less in the first example was simply the size of the gap and the possible arc separating the contacts. You also have to think about the current carrying capacity of the contacts, in the previous example, a 36 amp switch is a very hefty switch in any voltage rating! The reason the DC ratings are so much lower is the voltage never falls to zero, so any arc that develops between the contacts when open will not be automatically quenched by the voltage crossing zero as it is in an AC circuit. A high voltage DC switch needs much wider spacing of the contacts when open to mitigate this effect.
All you ever wanted to know about switches.
Nice, but can you say, "which switch is which?" fast 10 times? There'll be a test at NJHR on Saturday!
cjack posted:Big_Boy_4005 posted:Learn something new every day. Today was my day to show off what I don't know.
If you never make a mistake, you never learn anything new.
I must have learned a LOT of new things.
If I could only remember some of them...
gunrunnerjohn posted:The voltage drop, and thus the current carrying capacity of the switch, is directly related to the current through the switch. When you think about it, the voltage drop, and thus the power dissipation of the switch, is the same no matter what voltage being handled is. The voltage ratings have more to do with the internal gaps inside the switch. A higher current switch at high voltages will draw a fairly large arc when it opens, the gap has to be sufficient to minimize that effect when the voltages falls to zero on the half-cycle. The reason the 250v rating was less in the first example was simply the size of the gap and the possible arc separating the contacts. You also have to think about the current carrying capacity of the contacts, in the previous example, a 36 amp switch is a very hefty switch in any voltage rating! The reason the DC ratings are so much lower is the voltage never falls to zero, so any arc that develops between the contacts when open will not be automatically quenched by the voltage crossing zero as it is in an AC circuit. A high voltage DC switch needs much wider spacing of the contacts when open to mitigate this effect.
All you ever wanted to know about switches.
John, do you think this switch described would be okay for a short spur with one engine at a time on it if the load never exceeds 10 amps at 22V AC? Will the contact faces be large enough to handle the bandwidth required for the DCS signal?
Big_Boy_4005 posted:Short answer about 36 amps. Not to be rude, but the formula is right in front of you. As the voltage goes down, the amps go up. In your example the volts are about one sixth of 120, so the amps are six times (6).
What are you welding?
Just want to make sure a switch this small will handle the DCS signal and current for one locomotive at a time on a spur at the 22V AC.
I don't think you'll have any issue with a 6A switch for one locomotive on a siding. It's hard to believe you'll exceed that value.
