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80% rule #90411
11/20/04 02:16 PM
11/20/04 02:16 PM
G
George  Offline OP
Member
Joined: Feb 2002
Posts: 375
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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Re: 80% rule #90412
11/20/04 06:37 PM
11/20/04 06:37 PM
R
russ m  Offline
Member
Joined: Feb 2003
Posts: 169
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).]

Re: 80% rule #90413
11/21/04 02:50 AM
11/21/04 02:50 AM
G
gfretwell  Online Content

Member
Joined: Jul 2004
Posts: 9,348
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
Re: 80% rule #90414
11/21/04 08:11 AM
11/21/04 08:11 AM
W
winnie  Offline
Member
Joined: Sep 2003
Posts: 649
boston, ma
And most '5 Hp' vacuum cleaners work fine on a 15A circuit, though the lights may dim a bit [Linked Image]

-Jon

Re: 80% rule #90415
11/23/04 12:37 AM
11/23/04 12:37 AM
B
Bob  Offline
Member
Joined: Feb 2002
Posts: 182
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"?

Re: 80% rule #90416
11/23/04 01:36 AM
11/23/04 01:36 AM
E
e57  Offline
Member
Joined: May 2003
Posts: 2,876
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
Re: 80% rule #90417
11/23/04 02:26 AM
11/23/04 02:26 AM
G
George  Offline OP
Member
Joined: Feb 2002
Posts: 375
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.)

Re: 80% rule #90418
11/23/04 02:50 AM
11/23/04 02:50 AM
G
gfretwell  Online Content

Member
Joined: Jul 2004
Posts: 9,348
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.


Greg Fretwell
Re: 80% rule #90419
11/23/04 07:04 PM
11/23/04 07:04 PM
T
trekkie76  Offline
Member
Joined: Jun 2004
Posts: 220
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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