As a firefighter and part time electrician... i would not use anything that will draw a good deal of current on those old plugs.... old plugs= old wires... ask the owner to upgrade the electrical.
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The house is a duplex, our floor has 100 amps: 2 X 30 and 2 X 20, people below us have 140 for some reason. Maybe its their new kitchen. Looks like the dryer is on its own 30amp wring which is good.
I screwed the cheater into the center screw of the faceplate. Not going to mess with anything for now, the wiring is a nightmare in the basement.
Ejs811, Look at your electrical panel. Sometimes it will state the capacity of your electrical service. Or, you can set up your gear and not worry about it. The only thing possibly drawing a lot of current is your amplifier and it depends on the load presented by your speakers and the volume you listen at. I would not count on your landlord rewiring any time soon.
There will be two mains fuses, at the top of your electrical panel. Read the sizes on those, to determine how much amperage is available, per phase. Outlet boxes in older homes ARE NOT USUALLY grounded. Especially if the wiring is knob and tube. Unless the wires were run through metal conduit(EMT), metallic liquid-tite, Bx cabling or have a third ground conductor to the box; it is NOT grounded. Grounding(running a pigtail) to the new outlet to an ungrounded box would be a waste of time/effort. Besides; the ground terminal on grounded outlets, is already grounded to the box through the mounting tabs/screws.
Call an electrician (maybe the owner will split the cost). I live in a 90 year old house and had a dedicated line installed for the HiFi and most of the recepticals replaced.
When the electician found some outlets with pre-war wiring, I said dont bother running new lines as they are not used for high current items. (it would have been expensive). Most of my oulets are now 3-prong and up to code.
Pre-war 2-prong wireing imposes a danger of setting building on fire and needs to be rewired new. Any tenant ringing a bell should not be neglected weather landlord loves or hates such. No matter how many amps you've got you may only use it OK for small appliances and microwaves, iron, corded drills may easily set these old wires on fire.
EJS, you can easily determine whether or not the screw that holds the wall plate over the outlet is grounded, by using a multimeter or a cheap neon bulb AC tester to see if 120 volts or so exists between that screw and one of the two slots in the receptacle (the smaller one if they are unequal in size).
Obviously, if you do that don't touch the metal tips at the end of the test leads while they are in contact with the outlet.
According to this Wikipedia article, knob and tube wiring was in common use in North America "from about 1880 to the 1930s."
"If your house is not up to code, it doesn't matter...houses are "Grandfathered" without needing updates. I owned older houses for rentals, and that is what I was told by the township."
Stringreen is 100% correct. There is no rule or law that says an owner has to conform to code changes as they happen. They change frequently.