Is ampacity overkill by NEC standards?

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electricalist

electricalist

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dallas tx
This isnt a bash the NEC thread or pick apart personal installs,
Feel free to address any part of the electrical system.
For me what are some opinions on the sizing of sec, feeders and branch circuits.
I notice that some will up size these when it doesnt seem to be necessary..
thanks
 
NEC ampacity sizing is decidedly conservative in part because field conditions are variable and because remember, what we call the NEC is really NFPA 70, and NFPA is the National FIRE Prevention Agency. So a certain amount of "over" sizing already takes place in the tables, when compared to what may be allowable in more controlled circumstances such as internal conductors within manufactured and tested products. So aside from allowing for voltage drop and/or future upgrades, blanket over sizing beyond the NEC could be considered somewhat wasteful.

Unless you are a wire salesman...
 
electricalist said:
This isnt a bash the NEC thread or pick apart personal installs,
Feel free to address any part of the electrical system.
For me what are some opinions on the sizing of sec, feeders and branch circuits.
I notice that some will up size these when it doesnt seem to be necessary..
thanks
Click to expand...
Are you saying they run larger conductors then necessary, or that you are seeing 200 amp feeders/services where 100 amp may have been acceptable?

10 AWG for a 20 amp branch circuit (or similar situations) may be because of voltage drop concerns.

Some people that primarily run raceways may only run 3/4 inch minimum in most instances even though 1/2 would work fine, or they will only pull three circuits in a raceway even though it has room to pull nine. - Those are all design decisions.
 
People do all kinds of things that don't serve much of a useful purpose. They will run a green wire inside of a rigid conduit even though the conduit itself is a better bonding conductor than the wire.

They will add multiple ground rods thinking somehow they will get a "better" ground.

They upsize wiring where it does not appear to do all that much.

I have seen where someone decided to put a transformer in that is 3 or 4 times larger than the actual load.

Some people run rigid where EMT is perfectly fine.

These are all design decisions made for various reasons, some of which are based on experience, some of which appear to be based on myth and superstition.

I just don't care all that much, as long as it meets code and works.
 
The NEC is overall conservative and I get a kick out of it when some states "the NEC is a minimum" in a "it's borderline dangerous" tone. Of course it's a minimum, all rules are basically minimums but the NEC is well above the minimum levels for performance or safety stand points.

Roger
 
roger said:
The NEC is overall conservative and I get a kick out of it when some states "the NEC is a minimum" in a "it's borderline dangerous" tone. Of course it's a minimum, all rules are basically minimums but the NEC is well above the minimum levels for performance or safety stand points.

Roger
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I am inclined to agree, especially for circuits 30A and under. Maybe because the majority of residential circuits are in this range and that is where there is a lot of potential for trouble from DIY installs and repairs.

Higher rated circuits and circuits above 600V seem to be less conservative though.
 
IEEE 835-1994

IEEE 835-1994

If anyone has looked at the standard power cable ampacity
tables (835-1994) the NEC overall looks somewhat conservative
to very conservative (depending on installation conditions)
I have no problem with the sizing that the NEC requires in
indoor situations or the leeway we have on our distribution
lines that are covered by the NESC (which is generally more
about clearances and construction.) Ampacity and Coordination
are determined by the Trans./Dist. Engineers & Field Supervisors.

Conservative never (usually) hurt anyone, it just costs more.
And safety is a worthy investment (in my opinion)

JR
 
Sometime I think in a standard office install like 2400 sq ft there should be no reason to up size branch circuits. Yet I come across it and knows people who do it as well as pole lights that really aren't that far but it still gets done. Someone told me the wire runs cooler and that is better on the system.
I stick to the code book mainly because it tells me what to use and for me that's good enough today.
 
electricalist said:
Sometime I think in a standard office install like 2400 sq ft there should be no reason to up size branch circuits. Yet I come across it and knows people who do it as well as pole lights that really aren't that far but it still gets done. Someone told me the wire runs cooler and that is better on the system.
I stick to the code book mainly because it tells me what to use and for me that's good enough today.
Click to expand...
If conductor is running hotter - that is resistive losses, probably doesn't make much difference to the system but is making the meter tally at a higher rate then if those losses weren't there, assuming that the load doesn't increase when you eliminate those losses. So it can get kind of complicated at determining exactly what is lost, but less complicated where it is lost.
 
If the installation is large and losses over time
(cost/benefit) are high, the wattage loss per
foot can be calculated fairly easily. The wire size
gets increased until the watt density (from the wire)
is to whatever acceptable level decided on. (Like if
the wire must pass through somewhere that the heat
rejection could cause a problem, etc.) There are also
good thermodynamic programs to calculate if the site
conditions are known.

Some places with large loads do oversize to keep their
switchgear and conduits cooler, but those decisions
are made by ownership or their rep. as a matter of
preference.

It would be kind of interesting to know if, say, 125%
equipment was installed. Then ran cooler due to the
oversizing thus lasting longer. Would it pay for the initial
install?

JR
 
It sounds like it should but to produce a product that (for example) that's made to handle 90 degrees might be hard for that same product to have a better/different property that if kept below 90 would increase its life. It might have to be organic
 
How is wire protection at all is amazing to me.

How is wire protection at all is amazing to me.

I think it is amazing how wire is protected at all.
Basically isn't the wire size selected based upon the 100% non continuous+125% continuous rule?The the wire is selected must carry that calculated load. How often do you find a wire size that is exactly that ampacity?
Then the breaker selected to protect the wire. How often do you find a breaker that is the same ass the ampacity of the wire? So you are allowed to select a breaker with the closest ampacity above the wire ampacity (up to 800a that is). How much above may that end up to be?
Then the wire is you run goes through all sorts is variable ambient temperatures, through insulation etc that will not allow the wire to dicipate heat. Who knows if or when the insulation class of the wire will be stressed.
Then you take a thermal magnetic breaker and place it is an cool environment that allows the breaker to carry more current than its rating. Breakers are calibrated at 40deg. Where does the trip curve slide to at 25degC or lower?
On top of that a TM breaker is allowed to be calibrated +-10% which are normally of the higher side to +10%.
With this in mind I think it is a miracal how wire protectecting is provided at all.
The only thing that is fairly consistent is the short (instantatious) protection which remains fairly unaffected by ambient temperature.
 
JRW 70 said:
If the installation is large and losses over time
(cost/benefit) are high, the wattage loss per
foot can be calculated fairly easily. The wire size
gets increased until the watt density (from the wire)
is to whatever acceptable level decided on. (Like if
the wire must pass through somewhere that the heat
rejection could cause a problem, etc.) There are also
good thermodynamic programs to calculate if the site
conditions are known.

Some places with large loads do oversize to keep their
switchgear and conduits cooler, but those decisions
are made by ownership or their rep. as a matter of
preference.

It would be kind of interesting to know if, say, 125%
equipment was installed. Then ran cooler due to the
oversizing thus lasting longer. Would it pay for the initial
install?

JR
Click to expand...
If losses are reduced there will be a recovery period , how much increase there is in efficiency is the key though. A 70% increase in efficiency on a two watt load takes a long time to recover the investment if the cost was $100, but a 5% increase in efficiency on a 1 megawatt load makes up ground at a much faster rate, even if $1,000,000 was spent.

templdl said:
I think it is amazing how wire is protected at all.
Basically isn't the wire size selected based upon the 100% non continuous+125% continuous rule?The the wire is selected must carry that calculated load. How often do you find a wire size that is exactly that ampacity?
Then the breaker selected to protect the wire. How often do you find a breaker that is the same ass the ampacity of the wire? So you are allowed to select a breaker with the closest ampacity above the wire ampacity (up to 800a that is). How much above may that end up to be?
Then the wire is you run goes through all sorts is variable ambient temperatures, through insulation etc that will not allow the wire to dicipate heat. Who knows if or when the insulation class of the wire will be stressed.
Then you take a thermal magnetic breaker and place it is an cool environment that allows the breaker to carry more current than its rating. Breakers are calibrated at 40deg. Where does the trip curve slide to at 25degC or lower?
On top of that a TM breaker is allowed to be calibrated +-10% which are normally of the higher side to +10%.
With this in mind I think it is a miracal how wire protectecting is provided at all.
The only thing that is fairly consistent is the short (instantatious) protection which remains fairly unaffected by ambient temperature.
Click to expand...

I think the minimum size conductors per NEC is conservative enough that in general those conductors are not operating dangerously close to the limits when it comes to protecting the insulation. They have factored in some worst case scenarios and still put in a safety margin to come up with what they have.

How many times have you run into conductors that are under more load then NEC recommends, they have maybe been there for years, and show no obvious signs of deterioration? Maybe you will see deterioration more often on continuously loaded applications, but get away from those continuous loads and you see a lot that gets by with no serious problems - not trying to promote breaking the rules just pointing out there is already some safety factor built into the requirements we do have, and it is hard to make the NEC cover every possibility and come down to extreme precision on conductor ampacity.

One could probably load a 12 AWG copper conductor to well over 20 amps and not overheat the 90 degree insulation if you were able to ensure you had a method of drawing heat away from the conductor, the more load you have the better that heat removal will need to be though. Liquid in the raceway would help - until the liquid gets heated up so next step is a flow in that liquid and some place to dump the heat and recirculate the liquid back in the raceway to remove more heat - that is the primary basics of any refrigeration system.
 
Kwired the last time I told you what I've seen it caused me to stop stating my opinion as fact , I don't have much to rely on any more except when you guys that know what you know are talking ,,,I'm listening. I agree
 
I'll add just how often do you see a 90C conductor operating in an 90C ambient? It isn't quite that simple though as you need to consider ambient temp plus heat that is developed within the conductor because of current flow.

Ampacity tables in art 310 are about protecting the insulation of the conductor from damages due to overheating of that insulation and is why there are separate columns in the tables for different insulation types. The conductors themselves can operate at much higher temperature before it becomes compromised.
 
NEC ampacities are calculated using safety factors. A safety factor is a very broad engineering concept. In this case it's a factor that divides the amperage at which the conductor insulation failed in lab tests. The idea is to build in extra insurance against, manufacturing variances and defects, the varying quality of field installation, varying environmental conditions. You don't want real-world installations operating anywhere close to where conductors failed in the lab.

The derations required for temperature, multiple conductors, etc., are, in essence, additional safety factors.

Put it another way: yes, the NEC is 'overkill', but for very good reasons. It takes some really complicated data analysis and boils it down to some relatively simple rules so that you, the electrician, can install conductors without being 'borderline' dangerous. If a conductor burns up, then you really effed up; probably you made multiple mistakes. And if you follow the rules then almost never should 'bad luck' cause a fire.

Upsizing conductors to account for voltage drop so that equipment will operate properly or more efficiently is really a whole other question than whether NEC ratings are 'overkill'.
 
I think the NEC is overkill, primarily to protect DIY and unqualified personnel at the sake of making profit.

For example water heaters do not need 10 gauge wire. #14 on a 20amp breaker will work ok for a 4.5kw water heater. We do not need to upsize the conductor by 80% when we do so for the breaker in a fixed in place load. We do not need continuous vs none continuous loading in residential and where a panel has load diversity. We do not need a limit on the minimum main service disconnect. We do not need a single ampacity table that is best around the worst possible case scenarios, rather several tables based around the actual installation. We do not need obscure, vague hard to understand rules either.



In other areas the NEC is dangerously weak. GFP requirements lag behind the rest of the world, testing of all new circuits should be required but is not.



I personally have no trouble using the NEC as suggestions (purely advisory) rather than following word for word.
 
mbrooke said:
I think the NEC is overkill, primarily to protect DIY and unqualified personnel at the sake of making profit.

For example water heaters do not need 10 gauge wire. #14 on a 20amp breaker will work ok for a 4.5kw water heater. We do not need to upsize the conductor by 80% when we do so for the breaker in a fixed in place load. We do not need continuous vs none continuous loading in residential and where a panel has load diversity. We do not need a limit on the minimum main service disconnect. We do not need a single ampacity table that is best around the worst possible case scenarios, rather several tables based around the actual installation. We do not need obscure, vague hard to understand rules either.



In other areas the NEC is dangerously weak. GFP requirements lag behind the rest of the world, testing of all new circuits should be required but is not.



I personally have no trouble using the NEC as suggestions (purely advisory) rather than following word for word.
Click to expand...
I agree to some extent on some of your installation examples. I do feel there are some continuous loads in some dwellings though that do need to take extra heating because of continuous load into consideration - in particlular heating or cooling loads.

I can go to some commercial/industrial sites and notice some heat in panelboards because of loading conditions, but go to a dwelling and you seldom notice that but if you do it is usually either a breaker supplying an air conditioner that has been running most of the day or on an electric heat circuit that has been running continuous or near continuous.

GFP lags the rest of the world because they (the CMP's) got stuck (more like bought by the AFCI manufacturers) on AFCI's instead.:(

Of course our society demands a lot of these requirements not only because the owners/users need a safe installation, but the installers need a standard to follow to help prevent or minimize lawsuits when something does go wrong. Prove that you followed the standard in court and it will bring your liability potential down much faster then if you didn't follow the standard - have an inspection program affirm that your install met the standard helps you even more. That leads to if you are going to have a standard, what good does it do if you continually let people modify it as they see fit? Bad enough we have AHJ's every where that make modifications to the standard as it is, just makes it harder for the installers that work in more then one AHJ area to make sure they keep up with what is essentially a new standard in every jurisdiction.
 
kwired said:
I'll add just how often do you see a 90C conductor operating in an 90C ambient? It isn't quite that simple though as you need to consider ambient temp plus heat that is developed within the conductor because of current flow.

Ampacity tables in art 310 are about protecting the insulation of the conductor from damages due to overheating of that insulation and is why there are separate columns in the tables for different insulation types. The conductors themselves can operate at much higher temperature before it becomes compromised.
Click to expand...

In a 90C ambient temperature, we'd all be dead.
 
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