AC/DC current ratings

[Daz69]

Hi - I need to find a plug/socket combination that can handle approx 20A 12v DC. I would like to use the easily available (and cheap!) Neutrik Speakon (loudspeaker) series, but they are rated at 30 A rms continuous & 40 A rms 1 min @250v AC.

Does 30A rms on AC equate to 30A with 12v DC? or am I being stupid? Obviously the 7500w on 250v ac is dropped down to just 360w when used on 12v DC.

Thanks -- Darren

 

[stevez]

Lots of ways to think of this. DC amps and AC rms amps are close enough to say they are the same for some applications. The size of a conductor is dependent on amps not volts. The insulation depends on volts not amps.

There are a couple of things about connections worth mentioning.

1. Not all parts of a connector have the same cross sectional area as the wires going in and out. Connectors aren't always made of copper so they aren't quite as conductive.

2. The contact points or surfaces aren't perfect and there is some "ohmic" loss. You can actually measure the resistance of the contact - sometimes expressed in milliohms.

The net result of #1 and #2 can be localized heating in the contactor which probably explains the 40 amp for one minute limitation example.

For some connectors there is some consideration for making or breaking under load. This can be particularly harsh in some situations and result in a substatial piece of hardware. If you set this aside it would be reasonable to say that if your connector equals or exceeds the load in amperes whether DC or AC rms then it's probably ok. If the voltage rating meets or exceeds your requirements it's probably sufficient. Again, expect some heating under load.

 

[Nigel Goodwin]

For some connectors there is some consideration for making or breaking under load. This can be particularly harsh in some situations and result in a substatial piece of hardware.

Yes, this is the main reason for DC ratings often been considerably less than AC ratings - as long as you don't unplug it while it's under load it's not likely to make any difference.

 

[Daz69]

Thanks for the advice guys, I cant see that I will ever have to disconnect under load.

Darren