COMMUNITY FORUM

From the American Electricians Handbook
"The T rating of an ac, dc switch is its ability to control satisfactorily a tungsten-filament lamp load of that rating. The peculiar characteristic...is that the resistance of the tungsten-filament to the passage of current is extremely low when the lamp filament is cold.As the temperature of the lamp filament increases, the current decreases...Therefore, switches used with these loads must be capable of ...handling the inrush current, which will be considerably greater than the normal hot operating current of the circuit."

This isn't as much of a problem as it used to be. At one time, offices & classrooms were lit by 300 watt 0r 500 watt mogul based lamps with a silvered bulb end. Tore out many of these early in my career.

Tom

[This message has been edited by Tom O (edited December 01, 2002).]

Speaking for all the persons who read the forum, I thank all the persons who kindly reply. This is a great way to share information.
The original post was about “matching” the circuit breaker to the devices on that circuit. In my personal case, the bathroom has a 20-amp GFCI that started my question.
(I have not checked the light switch in this box. I don’t know if the lighting is “Tungsten” or if there is an “inductive load” – don’t think so. Shouldn’t the lighting device instructions say what rating the switch needs to be? I just bought a replacement luminaire for a closet, it says “ask a licensed electrician” to confirm the conductors are ok. No info about about the switch rating.)
Anyway, I’m upgrading the kitchen wiring and wondered if the GFCI receptacles should be rated 15 or 20 amp. The “diy” store clerk said 15-amp devices are OK, but I don’t believe everything they say.

I made these basic observations: 20-amp circuits use 12-gauge wire. 15-amp circuits use 14 gauge wire. Is that not correct? Is it not logical to think that a 15-amp device is similar to 14 gauge wire – that it does not have sufficient “material” to carry 20 amps? But no need to answer, I'll install the 15-amp devices and bury the dead horse.

Other observations: Many homeowners change their house wiring, purchase a lot of materials for house wiring projects, and/or purchase many more appliances, lighting, and other tools that connect to the house wiring. Our current system of house wiring (I’m not concerned with commercial and industrial) needs new/improved products and procedures, to make it safer and easier. The homeowner needs to safely make simple electrical connections with zero defects. This may require mistake-proofing, similar to the 3.5 inch computer diskettes being insertable only one way. But the starting place is to provide authoratative, consistent, clear, and understandable explanations. And who really can “ask a licensed electrician” every time they buy an electrical product?

Sincerest thanks for your help.

quote:

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Member gs99 posted December 01, 2002 11:10 AM
Speaking for all the persons who read the forum, I thank all the persons who kindly reply. This is a great way to share information.
The original post was about “matching” the circuit breaker to the devices on that circuit. In my personal case, the bathroom has a 20-amp GFCI that started my question.
(I have not checked the light switch in this box. I don't know if the lighting is “Tungsten” or if there is an “inductive load” – don't think so. Shouldn't the lighting device instructions say what rating the switch needs to be? I just bought a replacement luminaire for a closet, it says “ask a licensed electrician” to confirm the conductors are ok. No info about about the switch rating.)
Anyway, I'm upgrading the kitchen wiring and wondered if the GFCI receptacles should be rated 15 or 20 amp. The “diy” store clerk said 15-amp devices are OK, but I don't believe everything they say.

I made these basic observations: 20-amp circuits use 12-gauge wire. 15-amp circuits use 14 gauge wire. Is that not correct? Is it not logical to think that a 15-amp device is similar to 14 gauge wire – that it does not have sufficient “material” to carry 20 amps? But no need to answer, I'll install the 15-amp devices and bury the dead horse.

Other observations: Many homeowners change their house wiring, purchase a lot of materials for house wiring projects, and/or purchase many more appliances, lighting, and other tools that connect to the house wiring. Our current system of house wiring (I'm not concerned with commercial and industrial) needs new/improved products and procedures, to make it safer and easier. The homeowner needs to safely make simple electrical connections with zero defects. This may require mistake-proofing, similar to the 3.5 inch computer diskettes being insertable only one way. But the starting place is to provide authoritative, consistent, clear, and understandable explanations. And who really can “ask a licensed electrician” every time they buy an electrical product?

Sincerest thanks for your help.

---

Look, the information about T rated and motor horsepower rated switches was only to illustrate that not all of the amperage ratings we see on electrical devices are meant to mean the same thing. T rated switches are only those that have been tested to handle a certain amount of tungsten filament load on DC circuits. Arcing at switch contacts is not a problem on most AC loads when the switch used is rated for the current to be switched. Motor rated switches are built to withstand the high starting current of a motor which can be a problem even on an AC circuit. That basically applies to motors rated one full horse power or greater. How much current a receptacles terminals can handle is usually not an issue for 15 & 20 ampere pattern duplex receptacles. The amperage rating in the case of receptacles is the pattern of the contacts and therefore the pattern of the cord cap blades they are designed to accept rather than how many amperes the terminal screw strips can carry between terminals. A long winded but more technically accurate way to describe a fifteen ampere receptacle is a receptacle built to NEMA 5-15R were 5 = 120 volt three wire grounding, 15 = fifteen ampere connection pattern, and R = Receptacle. If you pick up a twenty ampere, 240 volt receptacle and examine it you will find that the metal used is about the same as that used on a fifteen ampere, 120 volt receptacle. Those parts do get more robust as the amperage increases but since the US NEC permits two or more fifteen ampere receptacles on twenty ampere circuits you generally won't find much difference in those two ratings.
On the subject of switches the rating is on the ability of the mechanism and the contacts to safely connect and disconnect the load. That rating is based on the amperage and nature of the load as well as the type of current to be switched. That does not mean your instincts are entirely wrong. If I install a regular "snap" switch as the building disconnecting means in a residential outbuilding it behooves me to make sure that the switches rated for any load it is likely to have to open or close on. Once again a switch can usually carry much more current once closed than it can safely connect or disconnect.
If you were to obtain the UL listing information for the fifteen ampere receptacles you are about to install you would probably find that they can feed through more than twenty amperes. The reason that GFCIs have there feed through ampacity marked on them is that they contain a switch that opens the circuit under load.
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Tom

[This message has been edited by tdhorne (edited December 01, 2002).]

tdhorne,
Thanks Tom, for that explanation; it provides more light on the subject.
On a prior reply I asked “What’s the purpose of the device rating?”
When you say “not all of the amperage ratings we see on electrical devices are meant to mean the same thing.”, it appears that the expression “amperage rating” may be misleading and erroneous, at least to the average consumer.
Should there be several ratings: its ampacity (the one I was focusing on); for receptacles, the kinds of plugs that can safely be inserted, for switches their usage rating, or possibly others?

To borrow some lingo from computer programming, the expression “amperage rating” is a property describing the device object.
Consider another object – the box. One property of boxes is “material type”. Listed boxes are either metallic or non-metallic (none are semiconductor). Decisions are made on that property being accurate and meaningful. What if the property “material type” also meant to include another property - the internal space of the box? Wouldn’t that be confusing?

To the trained and licensed electrician these matters may be simple. But from many postings on forums like this and personal experiences, I see many questions that could be resolved by an improved design and/or presentation of info. In the NEC, in electrical terms, in UL listing information, in products and their installation instructions. The average “diy” homeowner does not need to know the complications of commercial and industrial code.

Perhaps there should be a “basic” level of products and expertise that would include mistake-proof designs, reducing the inspection time. Of course these products could be utilized by contractors also.

Another (future) possibility is to have electronic agents in each device and appliance, permitting the proper amount of current to flow when and where allowed, similar to the protocols on computer networks?

I need to get back to work on this kitchen.

Gs99

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