Definition of Continuous

[That Man]

Running for 3 hours or more is imprecise. Is there further guidance? I've looked at other standards and continuous is defined as something like this:

>50% operation in a 6 hour period

Would this be a reasonable interpretation for the NEC?

 

[Dennis Alwon]

Running for 3 hours or more is imprecise. Is there further guidance? I've looked at other standards and continuous is defined as something like this:

>50% operation in a 6 hour period

Would this be a reasonable interpretation for the NEC?

How is running for 3 hours or more imprecise? Sure if a continuous load is on for 3 hours a day would it need to be wired as a continuous load?

I don't see 50% In a 6 hour period as more precise than the definition now.

You can always write a proposal to make a change.

 

[suemarkp]

No matter how you define it, there will be close cases making it "imprecise". The 3 hour rule also requires that it be full load for that time and the 3 hours of use be typical usage, not one that could just happen rarely. So a pool pump that typically runs for 8 hours a day would be continuous. Lighting in a business that runs for 8 to 10 hours a day would be a continuous load. A heater or cooker that appears to run continuously, but has heating elements that cycle on and off is not continuous.

I see something have a 50% duty cycle as not being an issue at all for long term heating of conductors. It could have a 50% duty cycle for weeks and not be a problem. 100% loading is pushing the limits since that is how things are sized. There is the defined 3 hour time period which may be long enough to create a higher ambient temperature around the breaker which will make it trip more easily.

 

[That Man]

How is running for 3 hours or more imprecise? Sure if a continuous load is on for 3 hours a day would it need to be wired as a continuous load?

I don't see 50% In a 6 hour period as more precise than the definition now.

You can always write a proposal to make a change.

I'm trying to get a feel for the intent. A load that runs for two hours, then is off for 10 minutes, then runs for 2 hours again is not continuous according to the NEC definition, but is continuous according to the 50% over 6 hours definition.

 

[tortuga]

I've looked at other standards and continuous is defined as something like this:

>50% operation in a 6 hour period

Would this be a reasonable interpretation for the NEC?

What other standards?
It would be silly to apply that to the NEC.
A 16A load on a 10AWG / 30A breaker for 6 hours will not overheat terminations at breakers or fuses which is all the continuous factor is about.

 

[Dennis Alwon]

What other standards?
It would be silly to apply that to the NEC.
A 16A load on a 10AWG / 30A breaker for 6 hours will not overheat terminations at breakers or fuses which is all the continuous factor is about.

Yea but a 16 amp load is already using 125% if you run it on a #10/ 30 amps.

 

[That Man]

What other standards?
It would be silly to apply that to the NEC.
A 16A load on a 10AWG / 30A breaker for 6 hours will not overheat terminations at breakers or fuses which is all the continuous factor is about.

I'm not asking about 50% loading, I'm asking about 50% run time. I'm not going to try to assume loading %. But I can reasonably assume run time in diversity calculations.

 

[That Man]

The 3 hour rule also requires that it be full load for that time and the 3 hours of use be typical usage, not one that could just happen rarely. So a pool pump that typically runs for 8 hours a day would be continuous. Lighting in a business that runs for 8 to 10 hours a day would be a continuous load. A heater or cooker that appears to run continuously, but has heating elements that cycle on and off is not continuous.

If you have a ready source for this information, I'd be interested in it.

 

[wwhitney]

A load that runs for two hours, then is off for 10 minutes, then runs for 2 hours again is not continuous according to the NEC definition

The NEC definition is the only one that matters for applying the NEC rules.

The only reason the NEC has rules for continuous loads is to avoid nuisance tripping of OCPD. So you are correct that loads that aren't NEC continuous but are nearly continuous could also cause such an issue. You are free to apply your own voluntary nearly continuous factor to your designs to avoid nuisance tripping. You could use a factor that is less than 125%, say.

Cheers, Wayne

 

[don_resqcapt19]

If you have a ready source for this information, I'd be interested in it.

The definition in the NEC.

Continuous Load.
A load where the maximum current is expected to continue for 3 hours or more. (CMP-2)

 

[That Man]

The definition in the NEC.

Wow, you're right. There's a lot of meaning packed into that brief definition. Thanks for the insight.

I'd still like official guidance or consensus, if it exists, for the interpretation of the duration. But it sounds like no such consensus exists. I automate most of my calculations, and dealing with runtime is just one of many fuzzy edges I'm trying to nail down, since automation requires math, and math requires discrete conditions to be useful.

As wwhitney said, the intent to to prevent nuisance tripping, so the 50% runtime in a 6 hour period will account for that. Here's a source that talks about this from the Canadian code, FYI.

 

[don_resqcapt19]

Wow, you're right. There's a lot of meaning packed into that brief definition. Thanks for the insight.

I'd still like official guidance or consensus, if it exists, for the interpretation of the duration. But it sounds like no such consensus exists. I automate most of my calculations, and dealing with runtime is just one of many fuzzy edges I'm trying to nail down, since automation requires math, and math requires discrete conditions to be useful.

As wwhitney said, the intent to to prevent nuisance tripping, so the 50% runtime in a 6 hour period will account for that. Here's a source that talks about this from the Canadian code, FYI.

The general consensus of most code authorities is that it means exactly what it says. Nuisance tripping because of continuous loads is not common in the real world. I have seen commercial building lighting that runs all day, every day, with 21 or 22 amp loads on 20 amp breakers.
Remember a breaker will carry its rated load forever if the ambient is 40°C or less.

 

[That Man]

The general consensus of most code authorities is that it means exactly what it says. Nuisance tripping because of continuous loads is not common in the real world. I have seen commercial building lighting that runs all day, every day, with 21 or 22 amp loads on 20 amp breakers.
Remember a breaker will carry its rated load forever if the ambient is 40°C or less.

If the definition were precise and complete, it wouldn't be different in other codes. "It means exactly what it says" is not useful. The numbers came from somewhere, for a reason. It is the reason that I'm asking about. The reason was provided by wwhitney.

 

[Joe.B]

I'd still like official guidance or consensus

Your occupations says "Electrical Designer", if you don't mind me asking, do you have a stamp that you affix to your designs for use in construction or manufacturing?

If not, does someone that you work for (or with) have a stamp they're putting on plans?

If yes to either, then that is where the "official guidance or consensus" should come from. IMHO

 

[roger]

If the definition were precise and complete, it wouldn't be different in other codes. "It means exactly what it says" is not useful. The numbers came from somewhere, for a reason. It is the reason that I'm asking about. The reason was provided by wwhitney.

Are your designs done for compliance with these other codes you keep bringing up?

 

[Little Bill]

If the definition were precise and complete, it wouldn't be different in other codes. "It means exactly what it says" is not useful. The numbers came from somewhere, for a reason. It is the reason that I'm asking about. The reason was provided by wwhitney.

The definition is precise and complete as far as the NEC is concerned. It doesn't matter where it came from, it just matters that you follow it if your area has adopted the NEC, and without amendments.

Other "codes" don't matter, you should be working under the NEC. It says what it says!

 

[That Man]

Are your designs done for compliance with these other codes you keep bringing up?

Code and design are different things. Design needs to comply with code. But code is not a design guide. Some codes, like this one, need additional information to determine WHY it is code, so scenarios that aren't precisely defined by the code can be interpreted properly.

Your occupations says "Electrical Designer", if you don't mind me asking, do you have a stamp that you affix to your designs for use in construction or manufacturing?

If not, does someone that you work for (or with) have a stamp they're putting on plans?

If yes to either, then that is where the "official guidance or consensus" should come from. IMHO

I work for myself. PE's, inspectors, etc. don't (or haven't so far, and it's been 20 years) have opinions about design practices, just code compliance. This isn't a code compliance question, it's a code intent question. And it's been answered, thank you.

 

[roger]

As a designer you should be well versed in the applicable codes you're designing to and have a little understanding of how to research those codes.
I'll give you some direction. Go to the NFPA web site and search past NEC issues until you find the one where the "continuous load" definition first appeared, then search the ROP's, you may find what you're looking for

 

[tortuga]

Are any of these loads by chance motor loads?

 

[That Man]

As a designer you should be well versed in the applicable codes you're designing to and have a little understanding of how to research those codes.
I'll give you some direction. Go to the NFPA web site and search past NEC issues until you find the one where the "continuous load" definition first appeared, then search the ROP's, you may find what you're looking for

That's excellent advice. The problem with codes is pretty well summed up with Charlie's rule. Which is why I revisit fundamentals like this one, and why I consult the forum when I want the opinion of others. I haven't explored the ROPs before, and one of these days when I find a slice of time like I have now, I'll do so.