http://www.straightdope.com/classics/a2_389.html
For sure the UL guy is way off base. In order to use holes to improve cooling, the holes must be configured in a way that increases surface area. The formula for this is that the diameter of the hole must be less than twice the thickness of the material. If it is not, a hole will actually make cooling worse, not better. for plug prongs, this would result in quite small holes.
I always thought the holes were there to aid people who are trying to test the cable, or the appliance the plug is attached to. If you are using a meter with probes, you can put one probe through the hole and let go; the probe will hang on the hole and stay in contact with the prong leaving both hands free to position the other probe.
I’m surprise Cecil didn’t call her on this.
Higher voltage on an appliance = lower amperage = less heat.
Um, what? For an Ohmic device, V = IR. If V goes up and R is fixed, I must also go up.
He’s talking about a case where the overall power is held constant. Then I (generally) decreases as V increases.
Thank you Una.
This is typically why much transmission is high voltage is you can send the same wattage at lower energy loss due to high amperage heat generation.
This is also why Dryers & Ovens are typically 220V and not 110V.
Overall 220V is more effiecient than 110V.
Ah, that makes sense. 
I always thought the holes in the prongs were there to keep the plug secure in the recepticle. Inside the recepticle slots, there is a tensioned tab of metal with a little dimple in it that each prong slides past when it is inserted. The dimple rests inside the hole in the prong, and since there’s a little tension being applied to it, it keeps the plug from falling out of the wall.
Can’t believe it took 20 years to get the right answer.
Oh, and some outlets have locking devices within the outlet that can ‘grab’ the holes and prevent the cord from being pulled out.
From here.
The holes also reduce the amount of material needed to make the prongs by roughly 10%
If you’re stamping out a million plugs a week, 10% less brass is going to add up to a tidy amount of money saved.
As for higher-voltage items not having holes in their plugs, I’ve got two that do have holes. My (recently replaced with a gas version) electric dryer runs on 240 volts at 30 amps, and has a hole in one of the blades. The twist-lock plug for my generator (also 30 amps at 240) has a hole as well. The twist-lock means that there’s no need for tension-increasing dimples - the hole is purely there for lockout purposes. In both cases (I’m going by memory) the neutral blade is the one with the hole. In a 240-volt device, the neutral wire/blade/prong is usually carrying less current overall, so less material there shouldn’t have an adverse effect.
Just to be clear, this is some sort of three phase system, with two hots and a neutral?
In Australia and New Zealand, where all standard appliances are 240V and most are three pronged (Live, neutral and ground), holes are a definate exception to the rule. I think I’ve seen one or two. But in Pakistan, where I lived a few (cough) years ago, they had the most messed up collection of voltages, plug shapes, and so on I’ve ever seen. Some of the plugs had a locking device inside, and if it couldn’t lock into the holes, no current would flow.
Nope. Plain old North American household power. In the case of my electric dryer, if the 240-volt heating element draws 20 amps and the 120-volt motor draws one amp, there will be a one-amp current flow in the neutral as one of the 120-volt legs that comprises 240 will be carrying 20 amps and the other will be carrying 21 amps.
The heating element can actually be ignored in this particular dryer as it has no connection to neutral at all. The only thing connected to neutral is the motor that spins the drum and the timer. Fancier dryers may have a 120-120 split element to allow lower-heat drying.
Surely this isn’t correct - unless I am missing something more complex. I would expect that, at any instant (and neglecting minor charge build up effects) the current flowing at the live pin is exactly equal and opposite to the current in the neutral pin. There should be no current flowing in the earth pin.
This is correct, isn’t it? Or do you have a (current) 'eater rather than a heater?
Very few brass plugs being made today, so the savings would be small and almost certainly eaten up in reprocessing the scrap for reuse. The dots punched out of low grade steel have no reclaimable value, IMHO; probably for brass as well.
I work for a Molding & Stamping Factory. We still make all of our plugs with one blade Brass. The holes are hardly 10% however. I will ask the stamping Engineers their opinions on the holes, but I am afraid the blades in question have not been changed in at least 25 years and no engineer has been here that long.
I was incorrect to use the term “three phase”, but this is apparently a system with two hots and a neutral. It is not the typical household plug with two parallel flat prongs and a round ground pin. There are two hot pins at 120 volts, out of phase with each other, and a neutral pin. Some parts of the appliance (the heating element, apparently) run on 240 volts, and get their current up one hot leg and down the other. Other parts (the motor) run on 120, and take current up one leg and down the neutral. Total current in is the same as total current out.
On a single phase circuit, which is what the 120 volt motor is essentially on, the load on the nuetral will be whatever the current draw of the device is. So, in this case the 1 Amp motor will have 1 Amp on the hot and one Amp on the nuetral. If two single phase motors are sharing the same nuetral and the motors are on different legs of the service then you subtract the two loads from each other. A 5 Amp motor and a 3 Amp motor sharing the same nuetral will produce 2 Amps on the nuetral (assuming your service legs are 180 degrees out of phase with each other).
In the case above you have the two heating elements canceling each other out (plus they aren’t referencing ground in this case) and the only load left is the motor load of 1 Amp. This 1 Amp will be on the nuetral conductor as well.
Interesting. What do you do with the scrap brass that is punched out of the holes?