110 vs 220

[Leo Graywacz]

The 80% rule is for continuous load. Like a lighting cicuit.

[Aleks Hunter]

That's not strictly correct, and a good way to haphazardly (hazard, get it?) burn out motors. YOu can have too much power available. You have to size the circuit to the equipment both high and low current wise especially when the equipment's motor loading is task dependent. A 5 hp (to keep it simple) motor draws 4kw at rated power or about 21 amps. IT is not a good idea to wire it into a 50 amp circuit. If you're running a planer or sander or gang rip saw and have the feed rate and bite too aggressive you can easily draw twice that much power. That is why they have ammeters on better high power equipment. If you don't back it off immediately you will notice a foulstench and see smoke from the motor. Its absolutely preferable to put 25-30 amp breaker and reset it that it is to tear out and replace a motor. Circuit breakers are cheap , motors aren't.

[Leo Graywacz]

Once you let the magic smoke out you can't put it back. LOL

[Jim Neeley]

That's not strictly correct, and a good way to haphazardly (hazard, get it?) burn out motors. YOu can have too much power available. You have to size the circuit to the equipment both high and low current wise especially when the equipment's motor loading is task dependent. A 5 hp (to keep it simple) motor draws 4kw at rated power or about 21 amps. IT is not a good idea to wire it into a 50 amp circuit. If you're running a planer or sander or gang rip saw and have the feed rate and bite too aggressive you can easily draw twice that much power. That is why they have ammeters on better high power equipment. If you don't back it off immediately you will notice a foulstench and see smoke from the motor. Its absolutely preferable to put 25-30 amp breaker and reset it that it is to tear out and replace a motor. Circuit breakers are cheap , motors aren't.

Aleks: If your jurisdiction falls under the NEC:

To oversize the breaker under 430 of the NEC you must comply with all of the requirements of section 430 of the NEC. That section requires, amongst other things, that to oversize the breaker the motor must contain internal thermal overload protection. The thermal overload's job is to provide this protection.

Jim

[Jim Neeley]

The 80% rule is for continuous load. Like a lighting cicuit.

All,

Please understand that my response is not one of one-ups-man-ship or intended to strike a verbal war. It's just my interpretation that this table (which effectively applies the 80% rule) applies here.

The NEC is, in general, a well written document but the nature of the beast is such that it often takes reading a number of the sections together as a whole to understand the specific issue at hand. Compounding its understanding is the generally accepted rule that you can do about anything that's not expressly prohibited,

This leads to many cases where interpretations differ either because two people read the same words differently or because one did not consider what is said in some other section.

This is why you hear so many recommendations to talk to your AHF (Authority Having Jurisdiction [inspector]) for your area,.

Leo,

Section 210.19B does differentiate between continuous (may be used at least once at this load for 3 hours) vs. discontinuous load on applying the 80% rule (they invert it and say the wiring must handle 125% of course). Assuming that it will never be used for 3 hours at at a time, Section 210.19B does not limit us to 80%. Another section however I interpret as being more restrictive.

I am referring to Section 210.21(B)(2). That refers to cord-and-plug connected load. I have pasted the all of 210.21 below for clarity.


Please understand that my response is not one of one-ups-man-ship or intended to strike a verbal war. It's just my interpretation that this table (which effectively applies the 80% rule) applies here.

210.21 Outlet Devices. Outlet devices shall have an ampere
rating that is not less than the load to be served and
shall comply with 210.21(A) and (B).
(A) Lampholders. Where connected to a branch circuit
having a rating in excess of 20 amperes, lampholders shall
be of the heavy-duty type. A heavy-duty lampholder shall
have a rating of not less than 660 watts if of the admedium
type, or not less than 750 watts if of any other type.
(B) Receptacles.
(1) Single Receptacle on an Individual Branch Circuit.
A single receptacle installed on an individual branch circuit
shall have an ampere rating not less than that of the branch
circuit.
Exception No. 1: A receptacle installed in accordance with
430.81(B).
Exception No. 2: A receptacle installed exclusively for the
use of a cord-and-plug-connected arc welder shall be permitted
to have an ampere rating not less than the minimum
branch-circuit conductor ampacity determined by 630.11(A)
for arc welders.
FPN: See the definition of receptacle in Article 100.
(2) Total Cord-and-Plug-Connected Load. Where connected
to a branch circuit supplying two or more receptacles
or outlets, a receptacle shall not supply a total cord-
and-plug-connected load in excess of the maximum
specified in Table 210.21(B)(2).


Table 210.21(B)(2) Maximum Cord-and-Plug-Connected
Load to Receptacle
Circuit Rating (Amperes) Receptacle Rating (Amperes) Maximum Load (Amperes)
15 or 20 15 12
20 20 16
30 30 24

I reiterate that I have no intent on starting a flaming war here; I am merely station my $0.02.. YMMV.

Jim