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Old 01-12-2012, 05:01 PM   #1
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continuous load


When I was in school back in the early 90s, the continuous load allowed for a circuit per NEC was 80% duty cycle.

I've always gone by the 80% rule since, although I can't remember if it was a rule (shall) or under workmanship (should).

I've got a situation where I have two devices, both pulling 8.3 amps @ 240v and I'd love to put them on one circuit (20A double pole, 12awg), but that would put me at 83%. The total circuit length is under 75 feet.

I briefly flipped through the NEC and couldn't find it, and thought I'd ask if someone off hand recalls ?

-- Joe

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Old 01-12-2012, 05:03 PM   #2
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Quote:
Originally Posted by anesthes View Post
I've got a situation where I have two devices, both pulling 8.3 amps @ 240v
do those loads normally cycle simultaneously?

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Old 01-12-2012, 05:04 PM   #3
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continuous load


Quote:
Originally Posted by anesthes
When I was in school back in the early 90s, the continuous load allowed for a circuit per NEC was 80% duty cycle.

I've always gone by the 80% rule since, although I can't remember if it was a rule (shall) or under workmanship (should).

I've got a situation where I have two devices, both pulling 8.3 amps @ 240v and I'd love to put them on one circuit (20A double pole, 12awg), but that would put me at 83%. The total circuit length is under 75 feet.

I briefly flipped through the NEC and couldn't find it, and thought I'd ask if someone off hand recalls ?

-- Joe
What kind of load?
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Old 01-12-2012, 05:16 PM   #4
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continuous load


Not everything is a continuous load. Many people flat out put an 80% limit on a circuit, which is wasteful sometimes.
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Answers based on the 2008 & 2011 NEC.
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Old 01-12-2012, 06:56 PM   #5
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Continuous duty is over 3 hrs. Not 80%
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Old 01-12-2012, 06:57 PM   #6
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Hi Guys,

I usually size my circuits this way (80%).

In this case, it's two electric heaters with fans. Mfg. specs say 8.3 amps each. Doesn't specify if that is peak or rms.

Most likely, both units would be on at the same time. Unsure of start up surge, have not even ordered them yet.

http://www.marleymep.com/en/multimed...-11046-001.pdf

-- Joe
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Old 01-12-2012, 06:58 PM   #7
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Originally Posted by bobelectric View Post
Continuous duty is over 3 hrs. Not 80%
Eh? We were taught 80%.

Looks like this changed a number of years ago:

http://forums.mikeholt.com/showthread.php?t=135362

Hrmm..

So it's 80% of the CB, not the circuit. If the breaker is 125% (will have to look at the specs) the cont. load is actually 20amps, but obviously if it's 100% it's 16 amps.


-- Joe

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Old 01-12-2012, 07:10 PM   #8
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Come on, kids.Art.100,definitions... Continous loads.
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Old 01-12-2012, 07:23 PM   #9
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Old 01-12-2012, 07:33 PM   #10
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Fixed electric space heating is considered a continuous load. The branch circuit conductors and ocpd are sized at 125% of the total of the continuous loads plus any non continuous loads.
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Old 01-12-2012, 07:36 PM   #11
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Quote:
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Fixed electric space heating is considered a continuous load.
Right. There is no question of that. It's a continuous load.

Quote:
Originally Posted by Stubbie View Post
The branch circuit conductors and ocpd are sized at 125% of the total of the continuous loads plus any non continuous loads.

Right but CB's are rated for 125% overload, correct?

-- Joe
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Old 01-12-2012, 07:40 PM   #12
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continuous load


Quote:
Originally Posted by anesthes View Post
Hi Guys,

I usually size my circuits this way (80%).

In this case, it's two electric heaters with fans. Mfg. specs say 8.3 amps each. Doesn't specify if that is peak or rms.

Most likely, both units would be on at the same time. Unsure of start up surge, have not even ordered them yet.

http://www.marleymep.com/en/multimed...-11046-001.pdf

-- Joe
Overkill. Unless it is a pump motor, fresh air or exhaust motor that is running 24/7, you don't need to. Running your bathroom exhaust should not be on 24/7, nor is it designed for continuous use. Now of course, when determining total load, yes there is a limit that you should place on a circuit, which is clearly stated and already figured in the NEC for you.
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Old 01-12-2012, 07:43 PM   #13
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Quote:
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Right but CB's are rated for 125% overload, correct?
-- Joe
Pick one of the manufacturers and check out what their info states. Mine state that they are rated for up to a 10kva interrupt limit, so what does that tell you.
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Old 01-12-2012, 08:01 PM   #14
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Quote:
Originally Posted by anesthes View Post

Right but CB's are rated for 125% overload, correct?
What are you talking about???
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Old 01-12-2012, 08:14 PM   #15
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continuous load


General purpose circuit breakers should not be continuously subjected to more than 80% of their rated ampacity. There ARE some circuit breakers that are rated for continuous loads at rated ampacity.

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