It sure sounds to most posters here that you don't understand how a clamp on ammeter works, which suggests that you do not have the experience to safely accomplish the test that you probably need to do.
A clamp on ammeter works by detecting the magnetic field around a conductor produced by the current flowing through that conductor. There are many different types of clamp on ammeter (current transformer, open loop hall effect, closed loop hall effect, etc.) but they all work on this basic concept.
The current reported by a clamp on ammeter is the _net_ total current of _all_ the conductors in the clamp. If you run the clamp around a supply cord to an appliance, the current that you read _should_ be zero; if you get anything other than zero than you have a fault somewhere and current is flowing where it shouldn't. In a properly functioning circuit, the current flowing in one conductor in the supply cord should be _exactly_ balanced by the current returning in the other lead, so that the _net_ current is zero.
If you want to measure the current being used, you need to somehow separate the two conductors of the circuit and run the clamp around just _one_ of those conductors.
If you can power down the device and unplug it, then the 'line splitters' described above work perfectly. You plug the line splitter into your receptacle. You plug your appliance into your line splitter, and then you clamp around one side of the line splitter. The line splitter carries the various conductors on its two sides, with a hole in the middle, so that you can easily clamp around a single conductor.
If you cannot power down the system, then you need to gain access to a single conductor _somewhere_ in the circuit. An electrician with proper protective equipment could probably gain access to a single conductor at the circuit breaker panel that feeds the receptacle in question...if you are certain from your records which circuit breaker supplies the load in question.
If this equipment really _must_ be on all the time, then it may have a UPS built into the equipment rack, in which case you could probably unplug the rack momentarily in order to install the line splitter. Using the plug to disconnect a running load is not ideal, but the plug _should_ be rated to interrupt the full load current.
Finally, if this is a 20A receptacle on a 20A circuit breaker, remember that you are only supposed to supply 16A on a continuous basis, continuous being defined as any period greater than 3 hours. If you make your measurement and determine that your load is 17A, don't take this to mean that you could add 3A of equipment...instead take this to mean that it is time to get a new circuit added.
Regards,
Jonathan Edelson
