The NEC and the 80% rule and Homebrewing

[kenh]

You are incorrect. I feel like we've just about beat this horse dead. Continuous vs. Non- continuous and whatnot.

I suggest you study this attachment before you tell me I'm wrong. It's the trip curve of Square D's QO® and QOB Miniature Circuit Breakers, the type of "house breakers" that are in my house main panel. The curve is in multiples of rated current and it clearly shows the trip curve going to the rate current multiplier of 1, and 1 = 20 Amps, but not crossing over. In other words, at 20 Amps the breaker will eventually trip - at 19.5 Amps it will never trip.

Here is where the document came from.
http://www.schneider-electric.com/d...d-QOB-Miniature-Circuit/?reference=0730CT9801

 

[skunkfunk]

I suggest you study this attachment before you tell me I'm wrong. It's the trip curve of Square D's QO® and QOB Miniature Circuit Breakers, the type of "house breakers" that are in my house main panel. The curve is in multiples of rated current and it clearly shows the trip curve going to the rate current multiplier of 1, and 1 = 20 Amps, but not crossing over. In other words, at 20 Amps the breaker will eventually trip - at 19.5 Amps it will never trip.

Here is where the document came from.
http://www.schneider-electric.com/d...d-QOB-Miniature-Circuit/?reference=0730CT9801

The TCC is used mostly for coordinating with up-stream protective devices and calculating arc-flash hazards, though why you would ever need to do either of these things on a 20 amp circuit is beyond me. It is not used for determining the continuous rating of a circuit.

For that, you look in a code book, which clearly says that thermal-magnetic breakers that are not rated to carry 100% load shall not be loaded past 80% of their nameplate rating except intermittently, and household breakers are not rated such. It has nothing to do with the capacity of the wires - in many cases you are actually allowed to connect conductors with a lower ampacity than the breaker.

And it may not be law itself, but an electric code of some year or another has been adopted by your AHJ and it is typically mandated by law that any new work is required to conform to that authority's interpretation.

 

[mattd2]

Wow this question did bring a lot of information out.
Can I check my understanding of some comments posted already:
1) The wiring in your walls should of been installed to handle 125% of the rated current of the supplying breaker (from the minimum gauge allowed for in notes to the NEC current carrying capacity table), this therefore "de-rates" that same wiring to be able to continuously carry 100% of the breakers rated current. So we should (after inspection to confirm that the wiring was correctly installed ) be able to sleep easy that if our current draw of our equipment is below the breaker rating we should not expect a fire to start in our walls (and if the current draw is over the breaker limit the breaker will trip before any fire would start)
2) A "normal" household breaker will start to trip due to overheating if run at or near its peak for an extended period of time - I guess if this is true that is what I started this thread for. If you are saying that a breaker is designed to trip if continuously loaded above 80% of its rated limit then, again, we should not have to worry about fires, etc. as the breaker is design to cut power if it gets over its rated continues capacity.

I still don't think my original question has clearly been addressed though. Why are people advising to providing overcurrent protection and cable size of 125% of the equipment’s capacity due to the 80% de-rating for continuous loads for systems and loads that are not continuous. I can see the safety argument of "just to be safe" but it is adding unnecessary costs and not necessarily adding safety.
For most people their systems will not be drawing above 80% continuously for 3 hours due to the fact that PID are modulating the power to elements, downtime between when heating is required, etc.
From some comments in this thread (sorry if I did interpreted them wrong) even if they were over the 80% continuously the likely event would be a breaker trip.

Also my understanding is the NEC cable size table not considering cables in free air like that used for extension cords, etc.

Thanks for everyone’s comments in trying to clear this up
Cheers

I guess to summarise what I see is:
I want to run to up to 2x 4500W elements - potentially they may be on at the same time for a breif amount of time,
Therefore I have (4500 * 2) / 240 = 37.5A (gives me a little to play with for pumps, etc. if everything is on all at once!)
So I'll get a 40A breaker/GFCI and run all my cable as per the NEC requirements.
Advice - NO! You need to derate your wiring, breaker, etc. 80% so you need to run it as if it was a 50A circuit.

But 40A circuit wiring is already spece'd in the NEC to handle 40A contiously, and a 40A breaker/GFCI is fine to handle non-continous 40A loads (which brewing is).

 

[skunkfunk]

I think you are misunderstanding the 125% thing. Wiring mininum size is not based on the load for normal branch circuits. It is sized based on the overcurrent protection. In many cases, the wiring will be rated lower than the breaker's rating due to the "next standard size" exception.

#12 wire in a 20 amp circuit is NOT oversized, as there is no case in which you can use a larger breaker as overcurrent protection for that wire.

 

[mattd2]

I think you are misunderstanding the 125% thing. Wiring mininum size is not based on the load for normal branch circuits. It is sized based on the overcurrent protection. In many cases, the wiring will be rated lower than the breaker's rating due to the "next standard size" exception.

#12 wire in a 20 amp circuit is NOT oversized, as there is no case in which you can use a larger breaker as overcurrent protection for that wire.

Sorry but I am still misunderstanding you
What is the current carrying capacity of a #12 AWG wire? Is it not 25A as per the NEC table linked above? In other words a #12 AWG cable can carry up to 25A if installed with 3 or less current carrying conductors. **But any circuit using #12 AWG wire can not have a CB installed of greater than 20A.
The way I read that is the wire has a greater current carrying capacity than the breaker will allow it to carry.
The reasoning for this I would guess is to do with the 80% derating in residential installations. The general public does not know that you should not load your 15/20/?? amp circuit above 12/16/??A continously (longer than 3 hours). They see a 15 amp plug and expect it to run 15 amps all day long, therefore the code has considered this by amking sure in residential (or other lower voltage installations where standard plug in equipment is used) that the wire is already derated so that it is not continously loaded over 80% its current carrying capacity.

I guess I am trying to say that the #12 AWG that is capacble of 25A is derated to 20A. If a #12 AWG wire was only able to carry 20A safely then the table would just specify 20A, not specify a higher rating and then call out a lower rating as a note.

 

[skunkfunk]

I do not know why, but previous editions have said yet other things about the capacity of #12 wire. Note that none of the larger wires have asterisks.

Guess it might have something to do with voltage drop?

As I write this, I'm wiring up my stc 1000 to a freezer and a heater using 14AWG. But it's only a few feet of it, and it's not in the wall. Not sure why the rules are different for appliances.

 

[mattd2]

I do not know why, but previous editions have said yet other things about the capacity of #12 wire. Note that none of the larger wires have asterisks.

Guess it might have something to do with voltage drop?

As I write this, I'm wiring up my stc 1000 to a freezer and a heater using 14AWG. But it's only a few feet of it, and it's not in the wall. Not sure why the rules are different for appliances.

That's my guess as why it is different. One the heat can dissipate better and two there is the likelyhood of observing a problem with the wire prior to it burning down your house.

I wouldn't expect voltage drop to be the reasoning - as i would expect that that would be covered in the tex. i.e. you still have to calculate the required guage base on voltage drop, etc. but that caluclation can not specify a smaller guage can be used than that listed in the table.

 

[mattd2]

I decided to do a bit more reading and came accross this article for sizing conductors
http://www.ecmag.com/section/codes-standards/sizing-conductors-part-xxi#sthash.pWuaPPt6.dpuf
The 6th paragraph (starts with "In accordance with 240.4(D), the maximum overcurrent protection is after the application of any correction and adjustment factors. For example, what size branch-circuit overcurrent protection is required for 14 AWG THHN copper conductors under the following conditions? This circuit will be a single-phase, 240V branch circuit with a noncontinuous 14A load..." seems to deal with what we are talking about.

My take on that is:
Take your expected current, if continous multiply by 125% and multiply also by any correction factors, and that is your design current. Then select the required wire size from the table. And check that the load is below the maximum breaker size allow for that AWG. If you find that the minimum breaker is below the design current you will need to increase the wire gauge.

Again this all comes down to the misunderstanding of what a continous load is.
in most cases homebrewing is not a continous load and the 125% factor should not be applied.

The questions that should be asked when designing a system is "(A) Is it expected that the system will run at full load for 3 hours or more?" and" (B) What is the expected load that the system will run at for 3 hours or more?"
Then design for the larger of either A (if the answer is no ofcoarse) or B * 125%

Another article - http://www.galco.com/techdoc/chgp/ehd1020_app.pdf

Again this supports my methodology, and for the vast majority of homebrew systems would be non-continous and therefore should be wire with conductors and protected by a CB rated for the load (not 125% of the load).

 

[mattd2]

Just to kick this dead horse some more - Just came across this thread which is exactly what I am talking about.
In it the OP is told that his 20A circuit is only good for 3800W and he will not be able to run the 4500W element he wants to. It is then suggested that he replace the existing breaker with a 30A and upgrade the wiring to 10AWG.

 

[kenh]

I guess I'm one of the "dead horse kickers", but....

1. The electrical wiring in your house is already derated 20%, The breakers on your panel trip at 80% of your house wire's current capacity. So, explain why are have to derate an additional 20%????

2. No homeowner except for a few anal-retentive types has a clue about the loads on the circuits in his house. In other words, even if I am wrong there is no way for the average homeowner to comply!

 

[mattd2]

I guess I'm one of the "dead horse kickers", but....

1. The electrical wiring in your house is already derated 20%, The breakers on your panel trip at 80% of your house wire's current capacity. So, explain why are have to derate an additional 20%????

2. No homeowner except for a few anal-retentive types has a clue about the loads on the circuits in his house. In other words, even if I am wrong there is no way for the average homeowner to comply!

I am with you. The wire in his house is good for 25A which is enough for 20A continous when the 125% rule is applied.
The one question is if the breaker is rated for 20A continous - but even if it is not you can still be code compliant if you do not run above 80% of the rated current for more than 3 hours.

 

[John53189]

My head hurts after reading this....BUT REALLY Buying the cheapest spa panel or breaker and adding the minimum wire gauge or lower is going to overheat, and kill your breaker, equipment and efficiency of the equipment. Not all of these brands are the same, and lowest price may not always be the best value. Think long term and think safety...go bigger!

​

 

[skunkfunk]

I am with you. The wire in his house is good for 25A which is enough for 20A continous when the 125% rule is applied.
The one question is if the breaker is rated for 20A continous - but even if it is not you can still be code compliant if you do not run above 80% of the rated current for more than 3 hours.

#12 wire is never used for more than a 20A branch circuit. If you have to derate the wire, which you probably don't, it would be for other reasons. There is no 125% rule for wire (it depends on the load, not the wire). Wire can run at it's rated current indefinitely, and sometimes above that rating in the case of the "next standard size" rule.

The values shown in the NEC table are calculated values based on particular conditions. Which size conductor and column of which table you use depends on factors such as insulation, connectors, ambient temperature, voltage drop, raceway, number of conductors, etc. So no, the wire in a house is not necessarily "good for 25 amps" as a blanket statement.

I guess the point of this is, don't try to cut corners. You'll never burn anything down putting in too big of a wire.

 

[augiedoggy]

My head hurts after reading this....BUT REALLY Buying the cheapest spa panel or breaker and adding the minimum wire gauge or lower is going to overheat, and kill your breaker, equipment and efficiency of the equipment. Not all of these brands are the same, and lowest price may not always be the best value. Think long term and think safety...go bigger!

​

There different ways to look at it especially when your already spending $$$ on building the brew system. As far as safety the smaller or cheaper stuff has to meet the same ratings and codes.... The minimum gauge wire that's RATED for the load is just that...RATED FOR THE LOAD in question.... this isn't guesswork where you just fork out and extra $200.00 or so for wiring rated for a 50a circuit to use in a 30a application because its "safer".... for the same reason I drive a car to work and the truck sits in the driveway broke down most the time...bigger overkill doesn't always make sense. I have more important uses for that extra gas money just as I could use that extra 100-200 bucks elsewhere instead of burying it my walls and never utilizing its potential.
If you need to run two elements at once there's 8/3 for two 4500w elements and this would be the only real point where I would consider going the 6/3 and 50a for the off chance I wanted to go to 5500w elements in the future and if I was running other appliances off the same drop like pumps.

The most I ever run off my 30a circuit now is 18.2amps.... that line cost me $200.00 to run yet I had all the stuff laying around for a 20a circuit . Yeah I would like to know what a 20 amp 12guage circuit can really handle safely myself.

 

[augiedoggy]

#12 .

I guess the point of this is, don't try to cut corners. You'll never burn anything down putting in too big of a wire.

But I think the real point of this thread is will you burn something down if you don't run "too big of a wire" but instead use the wire that's rated for the next size up from the actual load your putting on it?

 

[kal]

The 3 hour rule's been mentioned a few times but not one of the sub-rules.

The code states "the wiring for a continuously loaded appliance without a motor needs to be rated at 125% of the marked rating of that appliance". In other words, do not use more than 80% capacity.

The electrical code defines a continuous load as "A load where the maximum current is expected to continue for 3 hours or more" but then goes further to state that "A fixed storage-type water heater that has a capacity of 120 gal or less shall be considered a continuous load". In other words, doesn't matter how long the water tank heats for, it falls under the 80% rule.

So the arguments made for and against having to stay under 80% on this and other forum typically revolve around whether someone thinks that the elements will be fired for more than 3 hours continuously and/or whether or not an electric brewing setup is classified as a "fixed storage-type water heater" (given that we use the electric heating elements found in hot water tanks and the functionality is somewhat similar).

That's where the confusion usually comes from. I always tell people that it's best that they ask their local electrical authority when they get their new control panel outlet(s) inspected. Tell them what you plan on doing, ask their opinion. They're there to help.

FWIW, I've never heard of anyone using my control panel designs being told that they should stick to below 80% by inspectors or electricians, but it's always best to check anyway.

Kal