Picking Nits on Wire & Breaker Sizes

[rob123]

I'd like to ask a hypothetical question that seems not to have a simple answer.

A circuit is protected at a some amperage and feeds multiple fixed & variable loads. The loads are wired and/or rated by their various manufacturers without consideration of the circuit OC protection. Won't the "weakest link" render all but the simplest circuits a hazard?

For example, if a 15A breaker is wired with #14 copper and connected to standard switches & receptacles, it is considered safe. Adding any load rated/wired below 15A creates a weak link, rendering the OC protection ineffective. A small load wired with #16 copper, for instance, can overload without ever tripping the primary OC protection.

I've heard dozens of opinions about this over the years, but no definitive answer. Thanks for your comments.

 

[hurk27]

Re: Picking Nits on Wire & Breaker Sizes

Rob What I think your asking is why are manufactures allowed to use smaller wire than we are?
Well first they have an engineer that does load calculation's to make sure that there wire will pass the UL listing for there product. we will see many products that will have 16awg in them and be rated for 15 and even 20 amp circuits. There are two reasons for this one is they have installed an overload in the unit that protects there wires from over loading and our circuit breaker only has to protect for ground faults and short circuits. And two their engineer has done the load calculation to make sure there load on these wire won't over load them. The NEC allows us to do this on motor loads but with branch circuit wiring for receptacles or lighting, it would be impossible to control what the home owner plugs in so we are limited to using 14AWG for 15 amp circuits and 12AWG for 20 amp circuits or 10Awg for 30 amps (unless it's a motor load). And you will notice that cables above 10 awg will still only have a # 10 grounding conductor even a #6 NM this is because a 10 Awg will still handle the extra current in the event of a ground fault because the breaker will open faster than the wire has a chance to heat up. and since the grounding conductor should only be used for fault current it will never be overloaded. Maybe Charlie can correct me if this is wrong but I was told once why we had to have the same size grounding conductor as the circuit conductors with size 10AWG and below is because of many receptacles accidentally being miss-wired and load current being placed upon the grounding conductor. The first grounding conductors were smaller in the first NM that had a grounding conductor. And to top it off they were aluminum to boot. But after a few receptacles with boot legged grounds and burning them open the code was changed.
Well hope this helps and maybe others will jump in here.

 

[rob123]

Re: Picking Nits on Wire & Breaker Sizes

Thanks Wayne. That's part of the reason for my question (re: mfg's. design responsiblities). But I see other examples quite often also. Your point about not being able to control what a person plugs in maybe hits it on the head. That's a perfect example of what I generalized into uncontrolled loads being placed on an OC protected circuit.

Example: A fixture rated at 60W that the user fits with a 150W bulb. The fixture mfg. has designed wires, contacts, switches, etc. to meet the 60W load (plus perhaps a safety factor). UL approves it at 60W. The electrician installs it in a 15A breakered #14 copper circuit. The end user overloads it with a high-wattage bulb, and a failure occurs. So we have a good circuit from the panel to the fixture, and we have a good fixture, but when they meet under certain circumstances, we get a failure.

Granted, I've assumed an "abuse" outside of design parameters, but such abuses occur every day. I'm an engineer, and typically work from the product end back towards the user. I also get to see the electrician work from the power end towards the user. And then I see the two collide in a sense, where two rights make a wrong.

In another example the question became what could safely be connected to a 20A breaker/#12 copper circuit. Would adding a small load defeat the main circuit protection, since the breaker & #12 would never fail while the small load overheated or shorted? Or, would a standard 15A duplex receptacle defeat a 20A protected circuit?

I realize I'm creating alot of scenarios, and some may not even meet code, but I seem to see them occur regularly. (One poster here closes each post with a quote about "every element of the code can create a trail to a fire hazard", which seems fitting).

In any case, thanks for the input. I welcome all the comments, as I'm no expert at deciphering the NEC.

 

[iwire]

Re: Picking Nits on Wire & Breaker Sizes

In my opinion between UL and the NEC these issues are addressed. Also overloading a conductor is different than a short circuit on a conductor.

Take a medium base lamp socket, the largest lamp available that will fit is 300 watts. The conductors used for medium base sockets will always handle 300 watts they can not be "overloaded". If one of these 18 or 16 AWG conductors has a short circuit or ground fault they are able to pass enough current to trip the 15 or 20 amp overcurrent device.

Take a look at 240.5(B)(2)

240.5(B)(2) Fixture Wire. Fixture wire shall be permitted to be tapped to the branch circuit conductor of a branch circuit of Article 210 in accordance with the following:

(1)20-ampere circuits ? 18 AWG, up to 15 m (50 ft) of run length

(2)20-ampere circuits ? 16 AWG, up to 30 m (100 ft) of run length

(3)20-ampere circuits ? 14 AWG and larger

(4)30-ampere circuits ? 14 AWG and larger

(5)40-ampere circuits ? 12 AWG and larger

(6)50-ampere circuits ? 12 AWG and larger

How about a 10 AWG range cord on a 50 amp breaker?

Same thing applies, the stove will not draw enough current to overload the 10 AWG cord and if a short circuit happens the short length of 10 AWG will open the 50 amp OCPD.

240.5(B)(1) Supply Cord of Listed Appliance or Portable Lamps. Where flexible cord or tinsel cord is approved for and used with a specific listed appliance or portable lamp, it shall be permitted to be supplied by a branch circuit of Article 210 in accordance with the following:

(1)20-ampere circuits ? tinsel cord or 18 AWG cord and larger

(2)30-ampere circuits ? 16 AWG cord and larger

(3)40-ampere circuits ? cord of 20-ampere capacity and over

(4)50-ampere circuits ? cord of 20-ampere capacity and over

When UL lists an item they do so knowing that if the item is shipped with a 15 amp cord cap it will end up protected by either a 15 or 20 amp overcurrent device.

If the item ships with a 30 amp cord cap it will be protected at 30 amps and the internal wiring will be suitable for that use.

The protection of the wiring inside appliances is dependent on the OCPD we provide for the branch circuit.

That is why we have rules like this.

422.11 Overcurrent Protection.
Appliances shall be protected against overcurrent in accordance with 422.11(A) through (G) and 422.10.

(A) Branch-Circuit Overcurrent Protection. Branch circuits shall be protected in accordance with 240.4.

If a protective device rating is marked on an appliance, the branch-circuit overcurrent device rating shall not exceed the protective device rating marked on the appliance.

Or this one

422.11 (E) Single Nonmotor-Operated Appliance. If the branch circuit supplies a single non?motor-operated appliance, the rating of overcurrent protection shall

(1)Not exceed that marked on the appliance;

(2)If the overcurrent protection rating is not marked and the appliance is rated 13.3 amperes or less, not exceed 20 amperes; or

(3)If the overcurrent protection rating is not marked and the appliance is rated over 13.3 amperes, not exceed 150 percent of the appliance rated current. Where 150 percent of the appliance rating does not correspond to a standard overcurrent device ampere rating, the next higher standard rating shall be permitted.

What I am trying to say is that if installed to code and a homeowner follows the labeling on the equipment there should be no hazards.

Of course people do stupid things the question is always how far do we go to protect them from themselves?

 

[iwire]

Re: Picking Nits on Wire & Breaker Sizes

Double Post, sorry.

[ November 07, 2004, 08:00 AM: Message edited by: iwire ]

 

[iwire]

Re: Picking Nits on Wire & Breaker Sizes

Originally posted by rob123:
A circuit is protected at a some amperage and feeds multiple fixed & variable loads. The loads are wired and/or rated by their various manufacturers without consideration of the circuit OC protection.

Actually they do consider the circuit OC protection.

The manufacturer controls the final OCPD by the rating of the cord cap supplied or by labeling the maximum overcurrent protection.

 

[charlie]

Re: Picking Nits on Wire & Breaker Sizes

Wayne, your answer was fine so there was no need for me to jump in.

This is an interesting thread and you may be interested to know (if you don't already) that 240.5(B)(1) and 240.5(B)(3) have been changed. Small cords to appliances and extension cords are not covered by the Code except to say that they shall be considered to be protected when applied within the listing requirements.

This will permit a #20 wire to be used if it is listed for that use. That is an extreme example but it is possible. Of course, it is also possible that a #14 wire be required on a lamp. It is totally up to the product standards.

 

[bphgravity]

Re: Picking Nits on Wire & Breaker Sizes

Originally posted by rob123:
(One poster here closes each post with a quote about "every element of the code can create a trail to a fire hazard", which seems fitting).

This one:

 

[rob123]

Re: Picking Nits on Wire & Breaker Sizes

More thanks to everyone commenting! I'm reading everything & absorbing (at least some of) it.

some of the key points I've picked up on so far are:

1. The fixture, load, etc. is designed & approved to standards linked to, but not necessarily defined by the NEC. (I knew, for instance, that a fixture may be UL rated/approved, but I was not so comfortable with the notion that UL had assumed certain NEC-defined conditions on the circuit side.) A "co-operative closure" of sorts.

2. NEC specifications include a much broader allowance for faults/overloads/abuses than I was aware of. (ref. all comments related to real vs. rated current capacity of a conductor). Safety factors are already included.

3. Breaker response characteristics play a role in this analysis. i.e. when OCP trips out with respect to overload/fault duration. Again, I was aware that this must be considered, but not that it was accounted for in the NEC and by breaker manufacturers.

4. Taps: NEC allowed conductor down-size taps. This is one I've heard mentioned off-the-cuff in the past, but thank you for direct reference to a NEC paragraph.

5. There are only so many scenarios you can protect against, and there are only so many people you can protect from themselves. Again, we all know this, but are constantly chasing a moving goal line (cups of coffee can be dangerously hot, they say )

Thanks again for all of the input, and I'll be checking back for additions if they occur.

(and yes, bphgravity, that's the quote. but I'm not sure I want to be reminded of it again! By the way, did you know that a great deal of off-shore-manufactured electrical products carrying the UL logos have never passed UL, and that those labels are counterfeit? It's true!)

have a great day everyone.

 

[hurk27]

Re: Picking Nits on Wire & Breaker Sizes

By Charlie: This will permit a #20 wire to be used if it is listed for that use

Now that's a scary thought.

I had a neighbor who had an unlisted 20awg extension cord (Made in China) that he was using for a lamp in his bedroom. The lamp shorted out and the trailer was wired for all 20 amp circuits. This cord just glowed like a heater element. luckily the carpet was flame resistant and it didn't cause a fire but in an older home it would have.
This is one reason I don't like using 20 amp circuits for general purpose receptacles.

 

[jimwalker]

Re: Picking Nits on Wire & Breaker Sizes

This is why the lowest size breaker is safer.I prefer OCPD to be 15 amps were possible

[ November 09, 2004, 05:54 AM: Message edited by: jimwalker ]

 

[charlie]

Re: Picking Nits on Wire & Breaker Sizes

That is also why the NRTLs will never list a #20 for an appliance cord. A 20 ampere circuit will react just fine with listed equipment.

 

[charlie b]

Re: Picking Nits on Wire & Breaker Sizes

Rob:

With regard to the title you picked for this thread: A ?nit? is the larvae stage of an insect, such as a flea. If you do not "pick the nits," then you will someday have to deal with the adult fleas.

 

[rob123]

Re: Picking Nits on Wire & Breaker Sizes

re: hurk27 unrated extensions:
We had a similar experience with name-branded (but offshore manufactured) surge suppressor outlet strips. They were UL rated, and sizzled like bacon frying. Inspection showed shoddy internal wiring & workmanship. We notified UL, who wanted samples. Turned out the units were carrying counterfeit UL labels, which they said had become quite a problem. They were in the process of converting to holographic labels at the time, which I've seen since. But I'm not sure if the problem was completely solved. (hence, I sometimes second-guess "rated" devices).

(Same sizzle with some off-shore dimmers, now that I think about it.)

re: jimwalker "This is why the lowest size breaker is safer.I prefer OCPD to be 15 amps were possible"

enter the edison base fuse!

re: charlie: I pulled some lamp cord out from behind drywall once. does that count?

re: charlie b: that one has me scratching my head

 

[hurk27]

Re: Picking Nits on Wire & Breaker Sizes

Rob
Yes counterfeiting UL labels is still a very big problem and UL has info on how to spot the counterfeit one's.

Lower sized breakers will open faster than one of a higher rating when there is a voltage drop on these little cords. if the voltage drop is a few volts at the load amperage think what it will be at the fault current level. and to add to it these lamp cords get some rough use like yanking them out of a receptacle by the cord, furniture pushed up against them Etc... this helps fray the conductor and then the 16 awg cord only has enough strands to be a 22 awg cord. I think these cords should be done away with or a small fuse installed in each plug on them (like Christmas light manufactures have now done) or a better grade of cord used instead. Until this is done I will keep using 15 amp breakers for general purpose receptacles. But I do install more circuits.

As for the lamp cords being installed in walls that's a NO NO period!

 

[jimwalker]

Re: Picking Nits on Wire & Breaker Sizes

I too have seen lamp cord in walls.We can only worry about legal installed systems and try to protect from fires on them.What homeowners do is there own stupidity.