| All this discussion of 240-volt appliances has me wondering about the
| fairly recent US and Canadian electrical code changes that encourage us
| to install the t-slot 20 Amp receptacles in places like kitchens. But
| now I wonder why. Aside from providing some indication that the branch
| circuit was wired with slightly heavier wire, are there any consumer
| products out there that use the 20-amp style plug?
| I'll have to look to see if I can find the NEC discussions on the rule
| amendment that allowed the T-slot plugs. I'm curious as to the
| motivation for this.
If you are referring to "allowing" the NEMA 5-20R outlets, they have been
allowed for a long time, at least as far back as the 1960's when I first
saw one. They are required for a single dedicated outlet that has 20A
circuit protection (although arguably, this is pointless). Where a NEMA
5-20R is used, the circuit must be protected at 20A and thus must have a
rating of 20A. But you can use NEMA 5-15R on a 20A circuit which has more
than one outlet.
The NEMA 5-20R has a T-slot only on the grounded conductor blade. This allows
it to fit either a NEMA 1-15P, NEMA 1-20P, NEMA 5-15P or NEMA 5-20P. The NEMA
5-20P has the grounded conductor blade turned 90 degrees and offset outward to
maintain the 1/2 inch spacing between the blades. The NEMA 2-20P may also fit
but this would have problems if plugged in that way since it would be for an
older 240V ungrounded appliance now getting only 120V.
The NEMA 6-20R also has one T-slot, but that T-slot is on the side of the
outlet corresponding to the side the hot conductor is on in a 5-15/20.
Both conductor slots are hot at 120V relative to ground, but the one on
the right when the ground pin is down (left when ground is up) is the one
that has the T-slow for these 240V outlets.
The legacy T-slot outlet was an ungrounded outlet that had the T-slot hole
for both conductors. I still occaisionally see them around on very old
wiring. The intent was to be a single product usable for either 120V or
240V circuits. You can see the risks it exposed. It would not be allowed
today, and may not have ever been technically allowed.
Based on the 80% rule and assuming exactly 120V and 240V, appliances could
use these plugs for these wattage or volt-amp ratings:
Although I would rather use 240V in these (and other [*]) cases, I have seen
some appliances with a NEMA 5-20P though I don't recall if any were things
you might commonly see in a kitchen.
Is your reference to "encourage us to install the t-slot 20 Amp receptacles"
in reference to the NEMA 5-20R?
There is a difference between "encourage" and "allow".
I do not see anything in the NEC 2008 code that encourages or requires NEMA
5-20R outlets on the kitchen circuits (that have to be 20A). They are allowed
and in all places I've seen that might be recent installs, are used. As long
as the circuit has at least 2 outlets (a duplex receptacle does this), then
NEMA 5-15R outlets are sufficient. See NEC 210.21(B)(1) and (3).
The only case where you would be required to have NEMA 5-20R outlets in a
kitchen is if you meet NEC 210.11(C)(1) through the use of a dedicated branch
circuit for each single outlet, per NEC 210.21(B)(3).
I would actually prefer to use NEMA 5-15R only to meet the minimum kitchen
requirements in NEC 210.52, and supplement that with 2 or more NEMA 5-20R
on separate dedicated branch circuits. Heavier appliances would thus be
kept apart from the multi-outlet circuits. I'd also have NEMA 6-20R for
even heavier kitchen appliances like a 2400 watt microwave.
My layout might be like this:
Let me know if you find the rule in NEC you are looking for that allows the
T-slot plugs (I think you mean outlets) if it is not NEC 210.21(B)(3).
[*] I'm all for deleting NEC 210.6(A)(2) or at least making it not apply to
single family dwelling units.
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