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#90411 - 11/20/04 10:16 AM 80% rule
George Offline
Member

Registered: 02/23/02
Posts: 380
A motor, Delta bandsaw, has a name plate with the following information:

120/240 volts
1.5/2.0 hp
12.8/8.6 amps

For 120 volt use it appears that a 20amp circuit is required.

In reality the motor is

120/240 volts
2.0/2.0 hp
17.2/8.6 amps

Now, a 25amp circuit is required.

What prevents manufacturers of other equipment from "derating" their products to avoid the 80% or 50% rule.

I can see manufacturers selling a suite of kitchen appliances - dishwasher, disposal, refrigerator, and freezer, that all fit on one 20amp circuit.

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#90412 - 11/20/04 02:37 PM Re: 80% rule
russ m Offline
Member

Registered: 02/22/03
Posts: 165
Loc: Burbank,IL,USA
I guess the testing lab listing would be one safe guard. You wouldn't think they would get their listing with improper labeling for the motor.

[This message has been edited by russ m (edited 11-20-2004).]

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#90413 - 11/20/04 10:50 PM Re: 80% rule
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9045
Loc: Estero,Fl,usa
From my experience they go the other way and inflate the power of their machines. The real test would be to measure the power used when the machine is loaded to a reasonable maximum load.
My "2HP" table saw will trip a 15a(120v) breaker if I am really loading it down, like ripping 2" hardwood but it never trips a 20. The motor overload will go first.
_________________________
Greg Fretwell

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#90414 - 11/21/04 04:11 AM Re: 80% rule
winnie Offline
Member

Registered: 09/15/03
Posts: 652
Loc: boston, ma
And most '5 Hp' vacuum cleaners work fine on a 15A circuit, though the lights may dim a bit

-Jon

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#90415 - 11/22/04 08:37 PM Re: 80% rule
Bob Offline
Member

Registered: 02/05/02
Posts: 182
Loc: Mobile, AL, USA
George your post
"What prevents manufacturers of other equipment from "derating" their products to avoid the 80% or 50% rule."

What do you mean by the "80% rule"?

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#90416 - 11/22/04 09:36 PM Re: 80% rule
e57 Offline
Member

Registered: 05/27/03
Posts: 2837
Loc: S.F.,CA USA
I think I remember "hearing" about this rule, and never found it anywhere (except for AC units.) But the idea was not to load a circuit past 80%. I think it is an older motor rating rule no longer in use. Which is not the same as sizing the circuit conductors, over-current and short-circuit protection in that application for bandsaw.
 Quote:
440.62 Branch-Circuit Requirments
(C) Where Lighting Units or Other Appliances Are Also Supplied. The total marked rating of a cord-and-attachment-plug-connected room air conditioner shall not exceed 50 percent of the rating of a branch circuit where lighting outlets, other appliances, or general-use receptacles are also supplied. Where the circuitry is interlocked to prevent simultaneous operation of the room air conditioner and energization of other outlets on the same branch circuit, a cord-and-attachment-plug-connected room air conditioner shall not exceed 80 percent of the branch-circuit rating.
_________________________
Mark Heller
"Well - I oughta....." -Jackie Gleason

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#90417 - 11/22/04 10:26 PM Re: 80% rule
George Offline
Member

Registered: 02/23/02
Posts: 380
e57 has the correct code section.

It seems to me that we trust equipment manufacturers to be honest about the currrent draws so that we can supply appropriate circuits.

This instance of understating the current draw raisses some issues for me. (Especially since I design close to the limits.)

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#90418 - 11/22/04 10:50 PM Re: 80% rule
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9045
Loc: Estero,Fl,usa
How about 210.23(A)(1) Cord-and-Plug-Connected Equipment. The rating of any one cord-and-plug-connected utilization equipment shall not exceed 80 percent of the branch-circuit ampere rating.
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Greg Fretwell

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#90419 - 11/23/04 03:04 PM Re: 80% rule
trekkie76 Offline
Member

Registered: 06/19/04
Posts: 219
Loc: baileyville, maine, usa
80 percent is the inverse proportion of 125percent. so, 80 percent of the rating, or 125 percent of the load should be the same.

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