"Dealers" is actually breakers.
percentage of current allowed on a 20 amp breaker.
- Thread starter korthawks
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- Tennessee NEC:2017
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- Semi-Retired Electrician
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- Massachusetts
Consider that in a great many instances electricians want to know if they can add load to an exsiting circuit.
That changes how we need to go about things.
And, what about the wire's capacity to handle the additional load? If one is concerned about adding additional load and the breaker protects the wire I guess I may be missing your point.
800A and under, per 240.4(B), the wire could have an ampacity less than the breaker rating. The wire is sized to noncontinuous plus 125% continuous, but that's before adjustment and correction. After adjustment and correction, the conductor ampacity may be less (but cannot/should not be less than noncontinuous plus continuous)... but that's existing load. So all will have to be redetermined to add load.
Yes, to my point. The only way that I can think of is if you used #10 wire to supply a load where #14 would be allowed after adjustment and correction and protected by a 15at breaker. Then assuming that you have #10 wire rated at 30a you add additional load forgetting that the wire is being fed by a 15at breaker. As such you exceed the 80% of the breaker which really doesn't hurt the breaker but what you most likely will end up with is a nuisance tripping breaker.
What it comes down to is there are so many variables that it amazes my that we can provide protection. Starting out with a 15at breaker calibrated a 40?c installed in 25?c the breaker's trip curve moved to the right to 17a. The breaker thermal curve itself is +-10% and is more likely on the high side twords +10% which adds almost 2a and now you are pushing 19a. And that's only the breaker. What about adjustment and correction factors in sizing the wire which are based upon you best judgment at the time of installation. In addition you have no control over what loads will be added that may exceed the capacity of the circuit as it was originally sized for.
Isn't it amazing as that protection is provided as well as it is.
- Location
- Massachusetts
I look at an existing general purpose circuit, it has been run in 12 AWG and on a 20 amp breaker.
I know that circuit is capable of a 20 amps non-continuous or 16 amps continuous without thinking about it. All I need to consider is what the existing load is and what load I want to add to it.
There is no need for me to look up the wires capacity.
- Location
- Massachusetts
I really believe you are over thinking this.
I know I don't put that much though into adding onto a 15, or 20 amp branch circuit. The OP was talking about a 20 amp circuit.
No, I wasn't over thinking this as it is very common for one to get tunnel vision. My point being that there all too many that think that this is an exact science, that is simply black and white, like most would like it to be. In knowing that there all too many variables all that can be done is to follow the NEC.
Regarding the OP whether 15 or 20a circuit the issues are the same, whether #14 wire and a 15at breaker or #12 wire with a 20at breaker, breakers protect wire, the wire is sized to carry the load. Should the load increase it the becomes imperative that the wire can still carry that load.
jxofaltrds
Inspector Mike®
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- Mike P. Columbus Ohio
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- ESI, PI, RBO
Just to throw one more wrench into the mix, you are (I believe) permitted to load a circuit breaker to 100% even with continuous loads if the circuit breaker is rated for 100% load.I don't recall where this is written. Maybe someone can find it.
From a SD Panel.
http://download.schneider-electric.com/files?p_File_Id=80691577&p_File_Name=1830.pdf
I know that the OP's post is with reference to a residential application but 100% rated devices were brought up and needs to be clarified.
The 100% rule is often hard to understand. It should only be considered with industrial commercial breakers where 100%rated breakers are available. If applied correctly it may allow smaller cable to be used which can allow a smaller frame 100% rated breaker which could mean a lower overall cost. But for residential applications such as referenced to by the OP 100% rated devices are not available. But the following is where 100% rated devices can be benificial though:
"NEC 210.20(A): Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the rating of the overcurrent device shall not be less than the noncontinuous load plus 125% of the continuous load. Exception: Where the assembly, including the overcurrent devices protecting the branch circuit(s), is listed for operation at 100% of its rating, the ampere rating of the overcurrent device shall be permitted to be not less than the sum of the continuous load plus the noncontinuous load. otte: NA continuous load as defined by NEC Article 100 is ?a load where the maximum current is expected to continue for three hours or more.? This exception can be expressed using the following formulas: Standard 80%-rated design Noncontinuous load + 125% of continuous load = total minimum load Special 100%-rated design Noncontinuous load + continuous load = total minimum load Notice that the exception refers not only to the breaker, but also to the entire assembly. Since most manufacturers do not label their enclosures at 80% or 100%, the enclosures must either meet the minimum enclosure size and ventilation requirements spelled out by the overcurrent device manufacturer or have the system tested by UL for 100%-rated breaker operation."
Please note the wire size is chosen from the 75?C column of table 310-16 but rated wire with 90?C insulation must be used.
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