Why are connectors rated in amps and not watts?

[rg12]

Hey Dudes,

For example, if my XT60 connectors are rated at 60A, then it's common sense that I wouldn't be able to run 100v at 60A because that would fry the suckers (tested it).
I'm also pretty sure that I can run 120A on them with 10v because that would be only 1200w.

So why?

 

[Drunkskunk]

The Amp rating is a measure of the actual number of electrons that are passing through the connector. For a 60 amp connector, you are passing 3,720,000,000,000,000,000,000 electrons per second ( (60 X 6.2) x 10^18 )
It doesn't matter what the wattage is, or the voltage. Amperage is a constant. Voltage is essentially just the pressure behind electrons, and wattage is the amount of work that that many electrons can do at that pressure.

So no, your 60 amp connector can't handle 120 amps just because you run it at an equivalent wattage. It can only handle those 3,720,000,000,000,000,000,000 electrons.

 

[rg12]

But wouldn't the heat be lower with 120A setup and the heat is what matters to the connector?

 

[teklektik]

What matters is the heat developed from the voltage drop across the connector not the voltage of the rest of the circuit. We know from our old friend I^2*R that heat will be developed based on the intrinsic resistance of the connector and the current running through it.

• Big connector with large cross section and large contact area = lower reistance = more amps allowed to develop a specified amount of heat.
• Small connector with small cross section and small contact area = higher resistance = less amps allowed to develop the same specified amount of heat.

So, with a more or less fixed 'safe' amount of heat generation, the connector amp rating primarily depends on the size and material of the connector as well as the contact area. The voltage drop in the rest of the circuit really doesn't enter into it.

You were correct to be looking at Watts - it's just that what counts are the Watts of waste heat in the connector not the Watts consumed by other elements of the circuit.

FWIW:

• 
  I^2*R comes from:
  voltage = current * resistance (the voltage developed across the connector because of resistance)
  power = current * voltage
  power = (current) * (current * resistance)
  power = I^2 * R

 

[wesnewell]

There's other reasons for frying connectors other than the connector itself. If you run 60A through wire not rated for 60A on either or both sides of the connectors. The heat has to go somewhere. Guess where it goes? But I think the main reason would be a bad connection of the wire to the connector via a bad solder joint. And for 60A you should be using 9awg wire on both sides of the connector with good solder joints. You could get by with smaller wire as long as you aren't pulling 60A constant, but if it's constant I would be using 8awg minimum just to be safe. Actually for 60A constant I'd use a connector with a higher rating, like XT90's or 8mm bullets.

 

[rg12]

Thanks guys for the info
It was more of a theory question...I'm using XT60's for my LiPo packs and for the high current end voltage I just migrated to the EC5's which are bad ass (120A) since they can be pulled apart so much easier than the XT60's...

 

[liveforphysics]

Thanks guys for the info
It was more of a theory question...I'm using XT60's for my LiPo packs and for the high current end voltage I just migrated to the EC5's which are bad ass (120A) since they can be pulled apart so much easier than the XT60's...

In practice the ratings on RC connectors is rated for RC types of duty cycles... I've had XT90's fail through melting the solder joint from the wire to the pins solder cup at sub 50A continuously.

 

[rg12]

Thanks guys for the info
It was more of a theory question...I'm using XT60's for my LiPo packs and for the high current end voltage I just migrated to the EC5's which are bad ass (120A) since they can be pulled apart so much easier than the XT60's...

In practice the ratings on RC connectors is rated for RC types of duty cycles... I've had XT90's fail through melting the solder joint from the wire to the pins solder cup at sub 50A continuously.

Not that I believe everything that is advertised but many times people get fakes if they buy from eBay or something and those little things are pretty simple looking so not very hard to exactly duplicate them.

 

[Punx0r]

RC connectors are good for what they are, but Luke is right: Look at the size of proper industrial DC connectors like Andersons (always see them on forklift chargers for example).