*T rating of a switch*

Rob2019

Member
Location
Toronto
Occupation
Technician
What does it mean by *T rating of a switch*?
Where can I find more info on different types of general purpose switches?
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
A "T" rating is the maximum current a switch can handle for a tungsten load, like an incandescent lamp. Tungsten has a very high cold to hot Positive Temperature Coefficient (PTC) of resistance, meaning the resistance increases as temperature increases. Why this is important is that when a tungsten filament is 'cold" (off), and is first turned on, there is little to no resistance and the load looks almost like a short circuit for a fraction of a second, allowing current to "rush in", or we say it has a "high inrush current". So high in fact that it can cause switching elements in the circuit to weld even though the normal operating current, i.e. once the filament is glowing white hot, is relatively low. So the T rating of a set of contacts in a switch is lower than the "thermal" or normal load carrying current rating of the contacts.
 

Rob2019

Member
Location
Toronto
Occupation
Technician
A "T" rating is the maximum current a switch can handle for a tungsten load, like an incandescent lamp. Tungsten has a very high cold to hot Positive Temperature Coefficient (PTC) of resistance, meaning the resistance increases as temperature increases. Why this is important is that when a tungsten filament is 'cold" (off), and is first turned on, there is little to no resistance and the load looks almost like a short circuit for a fraction of a second, allowing current to "rush in", or we say it has a "high inrush current". So high in fact that it can cause switching elements in the circuit to weld even though the normal operating current, i.e. once the filament is glowing white hot, is relatively low. So the T rating of a set of contacts in a switch is lower than the "thermal" or normal load carrying current rating of the contacts.
Many Thanks Jraef, that's great info(y)
 

suemarkp

Senior Member
Location
Kent, WA
Occupation
Engineer
The modern switches are "AC Only" and marked as such. These are silent and don't make the click of the older AC/DC switches (probably why they originally called them snap switches because of the snap sound). The AC Only switches don't need to be derated for tungsten lighting loads and 80% for motors (see NEC 404.14(A)). NEC 404.14(B) says 50% for motors on AC/DC switches and its rating if T rated for tungsten lighting loads.

Look in the switch catalog for an AC/DC switch. Not sure if you can still find them. Otherwise, go to a place where they have pulled out stuff from older houses (second use stores) and you will probably find some of the old ones that make the loud click/snap when you move the toggle handle.
 
Last edited:

GoldDigger

Moderator
Staff member
The modern switches are "AC Only" and marked as such. These are silent and don't make the click of the older AC/DC switches (probably why they originally called them snap switches because of the snap sound). The AC Only switches don't need to be derated for tungsten lighting loads and 80% for motors (see NEC 404.14(A)). NEC 404.14(B) says 50% for motors on AC/DC switches and its rating if T rated for tungsten lighting loads.

Look in the switch catalog for an AC/DC switch. Not sure if you can still find them. Otherwise, go to a place where they have pulled out stuff from older houses (second use stores) and you will probably find some of the old ones that make the loud click/snap when you move the toggle handle.
👍
The snap comes from the high contact separation speed needed to extinguish a DC arc without suffering damage.
 

moonshineJ

Member
Location
USA
:DCertainly good to learn new thing from an experienced person. Are these T rated switches with 'SNAP' still available & used in new installs?
You may find plenty of those in other parts of the planet. They have a spring-loaded snap (quick-acting) mechanical contact(s). You have to replace em after a certain mileage due to the wear/tear/broken mechanical components. Those filled with mercury will virtually last until the universe dies (if rated properly).
 
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