Correct plug wire gauge to use at a 20A receptacle

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Greetings. Given that many appliance cords, extension cords, power strip cords etc. are not #12AWG as required for 20A service, but is often found to be #16AWG and even #18AWG. How do manufacturers of electrically wired products determines what gauge to use? For example, a company I am now contracting at has their control panels protected by 15A single pole circuit breakers (CB) fed from #14AWG power cords plugged into commercial 20A receptacles.

However, since there is a "rule of thumb" that the wire is to be protected at the point of supply, should not the feeder cable from the receptacle be #12AWG to the line side of the 15A CB? Case in point here is what if a wiring fault occurs between the receptacle and the CB? The affected wire between the plug and the fault is only rated for 15A. Is this really acceptable, and if so, why?

In the above application, it is assumed that #14AWG is adequate for continuous duty without any faulting upstream of the CB. I am trying to find some guidence from the NEC on the subject, but have not been able to find anything that specifically answers this question. Is this possibly addressed by the tap rule?
 
Wire sizing on each side of an overcurrent protective device

Wire sizing on each side of an overcurrent protective device

I believe you meant to point me to NEC 240.5(B), not 250.5(B). Maybe I can clarify my point with the following example:
There is an installed power meter in a control panel circuit measuring AC voltage. The circuit is setup as follows:

120VAC wall receptacle => 6' of #18-3 power cord with standard 3-prong 120VAC plug at receptacle => Fuse holder with 2A fuse => 10' of #18AWG MTW hookup wire terminating at power meter. The reasoning to use #18AWG wire is that it is current limited to 2A by the fuse, which has this same gauge wire on the line and load side. My point is that if a wiring fault (overcurrent) occurs on the line side of the fuse, the 20A breaker in the load center will not trip until a short circuit occurs. Even if one places a 15A CB between the plug and the fuse, the same issue remains, i.e., the wire between the CB and the fuse is undersized. I dot think that NEC 240.5(B) addresses this scenario. Your thoughts please.
 

don_resqcapt19

Moderator
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Location
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Occupation
retired electrician
Yes, it should have said 240 and not 250. If the equipment in question is not "listed" equipment, then the cord has to be protected per 240.5(A), in other words at its ampacity as listed in Article 400. If the cord is part of listed equipment it is considered to be protected by the branch circuit OCPD. I agree that there is no overload protection in this case. There may be short circuit and ground fault protection if there is enough fault current to quickly open the OCPD. In some cases the impedence of the circuit may limit the short circuit or ground fault current to a level that will not quickly open the OCPD.

I think that the IEC idea of having fuses at the male cord end to protect the cord is a good idea, but that is not required here.
 
Correct wire sizing in a control panel

Correct wire sizing in a control panel

Thank you for your help with the NEC regarding flexible cords and fixture wiring. However, I am still needing guidence on correct wire sizing on the line side of a fuse or other OCPD. Perhaps there is some other reference that adresses my question, i.e., NFPA 79? Again, is it permitted to use the same wire size on the line side of an OPCD that is used on the load side, or does it have to be larger gauge based on its upstream OCPD?
 

kwired

Electron manager
Location
NE Nebraska
Thank you for your help with the NEC regarding flexible cords and fixture wiring. However, I am still needing guidence on correct wire sizing on the line side of a fuse or other OCPD. Perhaps there is some other reference that adresses my question, i.e., NFPA 79? Again, is it permitted to use the same wire size on the line side of an OPCD that is used on the load side, or does it have to be larger gauge based on its upstream OCPD?

I think Don answered your question. If it is part of listed equipment it is protected by the branch circuit OCD. If it is not part of listed equipment then the cord must be sized according to 400.5 (A) or (B).

I don't believe feeder tap rules apply to this scenario either. All the feeder tap rules require the tap conductors to be protected by a raceway or other approved means to provide physical protection.
 

LEO2854

Esteemed Member
Location
Ma
Thank you for your help with the NEC regarding flexible cords and fixture wiring. However, I am still needing guidence on correct wire sizing on the line side of a fuse or other OCPD. Perhaps there is some other reference that adresses my question, i.e., NFPA 79? Again, is it permitted to use the same wire size on the line side of an OPCD that is used on the load side, or does it have to be larger gauge based on its upstream OCPD?

The load side does not have to be larger then the line side.
 
Wire sizing in a control panel

Wire sizing in a control panel

I think I am have trouble communicating my question. Is it possible to upload a scanned image, i.e., sketch of the circuit in question?
 

kwired

Electron manager
Location
NE Nebraska
I think I am have trouble communicating my question. Is it possible to upload a scanned image, i.e., sketch of the circuit in question?

Best way is to put the image on photobucket and link to it. I think there is a thread somewhere that has a pretty detailed instructions but I am not finding it. Using the tools the forum provides to provide images does not seem to be very successful most of the time.
 
Control Panel Wiring example

Control Panel Wiring example

I have atached a sketch describing my wiring question. Please note that this is an actual case scenario. The 2A fuses were selected to protect the components, but will protect the wire as well at this value. Per NEC 240.4(D)(1) #18AWG is rated for 7A, thus 5A fuses would have been appropriate if only protecting the wire alone. Please forgive my chicken scratch. With more time, I can prepare the same diagram in AutoCAD. Thanks.
 

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hurk27

Senior Member
While I wont get into UL listing requirements or what nots:

The fact that there is a 15 amp OCPD in the control cabinet, this will protect the #14 from overload, and the 2 amp fuses are protecting the #18 from overload.

so since the 14 is protected for overload then all we have to protect for is fault current, I think this is what Don was trying to point out.

they are two things we protect for, overload and fault current.
Overload can be protected down stream but fault protection must be up stream in a circuit.

we have many appliances that might come with a 18/2 cord but the cord is sized to the load so the breaker in the panel only has to protect from a fault this is why 240.5(B) allows a #18 on a 20 amp circuit.

this is kind of like a tap, where you might have a 400 amp circuit feeding a 100 amp tap that is protected down stream with a 100 amp OCPD the 100 amp wire is protected from over loads by the down stream OCPD while the up stream OCPD will still protect it from a fault.

there is a big difference between fault current and over load current, and it all has to do with the time it takes the OCPD to open, over loads can be just over the rating of the breaker and take a long time to open the circuit but as long as the device that can cause the overload has protection ahead of it it can be down stream of the fault protection, overloads can be by inherent design lets say one lamp socket in a lamp would be imposable to over load the lamp cord feeding it, or a smaller cord feeding an appliance that is in its design not design to be added to.

so these are what manufactures go by when selecting a cord size.

We do this in the NEC for motor loads, where we might have a #14 feeding a small AC unit but have a 30 amp breaker on it, the overload device in the motor protect the circuit from overloads and the 30 amp breaker protects for fault current.

Now the one thing Don pointed out is if the circuit is long as resistance is current limiting, this can cause problems with fault current and can cause the current to be low enough to not trip the breaker fast enough to protect the wire in the event of a fault, this can cause smaller gage conductors to be damaged or cause a fire, this is where you get into trouble using long extension cords of a small gage or very long circuit runs.

I hope this clears this up for you.
 

hurk27

Senior Member
I have atached a sketch describing my wiring question. Please note that this is an actual case scenario. The 2A fuses were selected to protect the components, but will protect the wire as well at this value. Per NEC 240.4(D)(1) #18AWG is rated for 7A, thus 5A fuses would have been appropriate if only protecting the wire alone. Please forgive my chicken scratch. With more time, I can prepare the same diagram in AutoCAD. Thanks.

Looking at your diagram since all the loads pass through the 15 amp CB then the #14 ahead of the CB can't be over loaded, and since each device is landed to the neutral block each is limited by the 2 amp fuse protecting each device so the 18 awg is good to go with a load amprage of 5.6 amps continuous or 7 amps non-continuous found in 240.5(A) (by the way on your device terminals your "N" and "G" is reversed):happyyes:

Remember a fault will provide a current level of many times the rating of the circuit sometimes several hundreds of amps and in some cases even in the thousands, so this is what opens a fuse or breaker so fast that it doesn't have time to damage the wire, of course this is limited in high resistance circuits and this is where long runs get into the picture and can cause problems.
 
Can an OCPD be used to protect upstream wire?

Can an OCPD be used to protect upstream wire?

This question is the gist of my entire thread. Is this ever permissible, or does an OCPD protect only wire connected dowstream from it? Please see previous discussions and the sketch in my previous post Please note that the Neutral-Ground wiring in part of the sketch is simply a pencil error. The 120VAC line wiring is my subject in the sketch. Also, the shorting terminal blocks are really standard terminal blocks joined with jumper bars down the center. Please reply. Thanks.
 
I just noticed page 2

I just noticed page 2

I thought nobody was responding to my post, but I just saw that there is a second page of responses. I am looking at those now.
Sorry for any redundancy. Moto.
 
Authorizing document references

Authorizing document references

Thanks for clearing up the difference between short circuit and overload protection.

I understand that the short circuit protection is always provided upstream for the purpose of selective coordination with other downstream interrupting devices so that the faulty "branch" circuit is isolated without affecting the other parts of the network.

Is there a reference that calls out the principle that overload protection can be provided downstream as well as upstream? I am accustomed to designing controls with the OCPD only located upstream. Most designs I am used to are based on the upstream device providing LSI/LSIG trips as done with higher ampacities. This incorporates both short circuit and overload cases. Moto.
 

kwired

Electron manager
Location
NE Nebraska
I thought nobody was responding to my post, but I just saw that there is a second page of responses. I am looking at those now.
Sorry for any redundancy. Moto.

Default setting is 10 posts per page. You can go into settings and change that to a different number. I have mine set to display 40 posts per page.
 
Awaiting answer for question on document refeernce

Awaiting answer for question on document refeernce

"there are two things we protect for, overload and fault current.
Overload can be protected down stream but fault protection must be up stream in a circuit."

The above quote was stated in frame #11 of this discussion thread. Can you please back this statement up regarding overload protection located downstream with a textbook or codebook reference? I would appreciate any responses at all, and this will satisfy my query.

I am working as a contractor at a PLC testing facility, and need to recommend a best practice regarding their controls wiring to the department there. You guys are supposed to be the experts. Thanks.
 

kwired

Electron manager
Location
NE Nebraska
"there are two things we protect for, overload and fault current.
Overload can be protected down stream but fault protection must be up stream in a circuit."

The above quote was stated in frame #11 of this discussion thread. Can you please back this statement up regarding overload protection located downstream with a textbook or codebook reference? I would appreciate any responses at all, and this will satisfy my query.

I am working as a contractor at a PLC testing facility, and need to recommend a best practice regarding their controls wiring to the department there. You guys are supposed to be the experts. Thanks.

It is same reason why a motor is allowed to have conductors sized to current motor will draw yet branch circuit short circuit and ground fault protection is allowed to be higher than ampacity of conductor.

Short circuits and ground faults are a situation when current will rise very rapidly and even a device designed to interrupt at 10 or 20 times the conductor ampacity will open quickly.

Overload is when current is only higher than rating of conductor or protective device my a much smaller amount - usually in the case of the load demanding more current than what was designed for. Someone plugs too many appliances into an outlet circuit, a motor is working harder than it was designed for, those type of things. Overload occurs when current flows in its intended path - therefore overload protection can be anywhere in the series of a particular circuit it is protecting and it will do what it was intended to do.

Short circuit/ground fault protection and overload protection are two different features offered in one package with many fuses or circuit breakers, the reality is that only short circuit/ground fault protection needs to be in the circuit at a point ahead of where the protection is desired.

Internal motor overload protectors are a good example of overload protection in the middle of a circuit instead of at the starting point of the branch circuit. Motor is overloaded the protector opens the circuit - the motor stops. It is not necessarily completely deenergized (the protector is not a disconnect) but is protected from overload.
 

hurk27

Senior Member
"there are two things we protect for, overload and fault current.
Overload can be protected down stream but fault protection must be up stream in a circuit."

The above quote was stated in frame #11 of this discussion thread. Can you please back this statement up regarding overload protection located downstream with a textbook or codebook reference? I would appreciate any responses at all, and this will satisfy my query.

I am working as a contractor at a PLC testing facility, and need to recommend a best practice regarding their controls wiring to the department there. You guys are supposed to be the experts. Thanks.

I did some searching on the subject and about the closest I can find is this PDF by CH that gives some pointers on the difference: Overload Vs Short circuit protection

it has been an accepted method of protecting equipment for a long time and UL 1077 even allows this method.

In reality you have to look at the main goal in the protection of the conductors and or equipment that might be subject to an overload.

If we have a piece of equipment there is two things that will limit the amount of current a circuit will see, first is the amount of load that can be placed upon the circuit, this might be designing equipment so that no other loads can be added or are unnecessary to add, such as simple table lamp with only one socket, as we know the small gage lamp cord going to a table lamp will most likely never see an overload, this is called fixed loads or fixed loading.

second is when we tap a given circuit with a lower rating circuit such as plugging in a piece of equipment that might only need small gage conductors to serve its load, now if there is a case where this equipment can be overloaded such as if the user was to use a long extension cord or where the equipment has motors or devices that could cause an overload that would exceed the rating of the tap conductors feeding it (plug in cord) then we need overload protection for these tap conductors.

I have found nothing that says that the overload protection of these tap conductors are required to be the branch circuit device, in-fact article 240 clearly allows for down stream protection, as does 430, does it matter where in a circuit the overloads are placed? it has been common to find the overloads in a motor control circuit placed in the common or X2 side of a control circuit, while the NEC does want them in the ungrounded if the control circuit leaves the control cabinet but placing them just ahead of the load as you have done will also protect the conductor upstream from this point since any event of overload will be dealt with, since there is no load ahead of this protection then the only load that would be a problem is the load after the 15 amp breaker in your panel or after each 2 amp fuse.

There are many places in the NEC where over-current protection is placed downstream of the conductors they also protect. look at tap rules in 240 to see just one example.
 
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hurk27

Senior Member
Another thing to keep in mind when reading the NEC.

The NEC is not a design standard 90.1(C)
It's job is to protect against fire and shock hazards 90.1(A)
And with that to meet this requirment it might not be necessarily efficient, convenient, or adequate for good service or future expansion of electrical use Verbatium: 90.1(B)

Also with the above said the NEC is a Permissive document as most laws adopted into law are, meaning that if it dosent say you can't in most cases you can.
 
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