Breaker sizing

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#1

Originally Posted By: jmyers
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I got a question for all those that are electrically inclined.


You have a 20 amp breaker in the electric panel being fed with #12 wire. Outside the #12 goes to a junction box with three #14 wires. Is that installation alright?

Joe Myers

#2

Originally Posted By: Bob Badger
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No the breaker needs to be changed to a 15 amp or the wire to 12 awg.


I am guessing this is for outside lights?

--
Bob (AKA iwire)
ECN Discussion Forums
Mike Holt Code Forum

#3

Originally Posted By: jmyers
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Bob,


How about when you connect several 15 amp receptacles using #12 wire, then run one #14 off one receptacle for a light. Would you still need the 20 amp breaker?

Joe Myers

#4

Originally Posted By: Bob Badger
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Hi Joe, for general purpose circuits you can not use 14 AWG as branch circuit conductors with a 20 amp breaker.


Not even to feed one small light fixture, or one outlet.

But you can plug a lamp into that receptical that has 18 AWG lamp cord.

Just one of those code rules.

Bob

--
Bob (AKA iwire)
ECN Discussion Forums
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#5

Originally Posted By: jmyers
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Bob B.


Code reference please!

I was doing one of those technical thinking days. Today was the day, now I am done with it for a few years! ![icon_biggrin.gif](upload://iKNGSw3qcRIEmXySa8gItY6Gczg.gif)

Honestly, I could not think of anything wrong with it. I was thinking of it like this. The #12 wire was going from receptacle to receptacle for a total of five receptacles. The maximum amperage on those receptacles is 20 amps, which the 12 wire is rated for. Now if you take a 14 wire off the last receptacle which services a light with the maximum 100 watt bulb, that is less than the 15 amps the wire is capable of, meaning it is well within its amperage rating at less than 1 amp.

Ok, now hit me with your thoughts on that scenario.

Joe Myers

#6

Originally Posted By: roconnor
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What if a new home owner doesn’t have a light there, and plugs in an extension cord to a heavy piece of equipment that draws like 25 A. That breaker may not trip (usually up to 125% of rating before it trips is typical/acceptable). What happens to that #14 wire … icon_eek.gif

–
Robert O’Connor, PE


Eagle Engineering ?


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I am absolutely amazed sometimes by how much thought goes into doing things wrong
#7

Originally Posted By: jfarsetta
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It gets tuned for maximum smoke…

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Joe Farsetta


Illigitimi Non Carborundum
"Dont let the bastards grind you down..."
#8

Originally Posted By: jmyers
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Bob O,


I meant more of a dedicated light in a room, or such. I still can not think of anything wrong with that scenario. The 20 amp breaker would still maintain the 20 amp rating with the 12 wire. The 14 wire would still be within the limitations of the 15 amp rating, given a dedicated room light was placed on the circuit, the circuit would not exceed two or three amps, tops.

Given the fact that someone was smart enough to place a 25 amp load on a 20 amp circuit, it should trip anyway, correct?

Joe Myers

#9

Originally Posted By: roconnor
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jmyers wrote:
Given the fact that someone was smart enough to place a 25 amp load on a 20 amp circuit, it should trip anyway, correct?

Actually breakers are not as reliable as fuses, and there is a range where it will trip ... usually between 100% and 125% of the rating. Then a 20A breaker should trip if the load exceeds a range of anywhere between 20A and 25A (discussed in the "Fused Service Panels" topic).

So a 25A load on a 20A breaker just might not trip the breaker. One of the reasons why having an oversized breaker with the smaller wires in most situations is not a good thing.

jmyers wrote:
I meant more of a dedicated light in a room, or such.

Lets say there is a problem with the device or wiring ... due to a bad connection or whatever (say 5 ohms resistance) ... so that the draw is 25A. That 20A breaker may or may not trip. If the breaker doesnt trip at a 25A draw, what happens to that #14 wire?

--
Robert O'Connor, PE
Eagle Engineering ?
Eagle Eye Inspections ?
NACHI Education Committee

I am absolutely amazed sometimes by how much thought goes into doing things wrong
#10

Originally Posted By: Bob Badger
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jmyers wrote:
I meant more of a dedicated light in a room, or such. I still can not think of anything wrong with that scenario. The 20 amp breaker would still maintain the 20 amp rating with the 12 wire. The 14 wire would still be within the limitations of the 15 amp rating, given a dedicated room light was placed on the circuit, the circuit would not exceed two or three amps, tops.


Joe I agree with you, given that scenario it should be fine, the 14 AWG could not be overloaded and if a line to line or line to ground fault happens it will still operate the breaker.

That said it is a violation.

Quote:
240.4(D) Small Conductors. Unless specifically permitted in 240.4(E) through (G), the overcurrent protection shall not exceed 15 amperes for 14 AWG, 20 amperes for 12 AWG, and 30 amperes for 10 AWG copper; or 15 amperes for 12 AWG and 25 amperes for 10 AWG aluminum and copper-clad aluminum after any correction factors for ambient temperature and number of conductors have been applied.


And while I agree with you, I must follow the code and I can see no legitimate reason for not using the correct size wire.

To further your point lets look at 240.5(B)(2) it seems in other situations the code agrees with you too.

Quote:
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


NM (Romex) is not fixture wire, some commercial lighting fixtures come with a 6' whip of flexible metal conduit with fixture wires inside to feed that one fixture.

When we run lengths of florescent fixtures end to end we could use 16 AWG for up to 100' on a 20 amp circuit inside the fixture.

We never do that, we would use 12 AWG, if you use non-standard wiring methods you are only asking for trouble when people who are not familiar with the code look at your work and think it is wrong.

Also many of the jobs I work have 12 AWG as the minimum size to be used for line voltage wires.

So I think your right in your thoughts, ![icon_smile.gif](upload://b6iczyK1ETUUqRUc4PAkX83GF2O.gif) but it is still a NEC violation.

--
Bob (AKA iwire)
ECN Discussion Forums
Mike Holt Code Forum
#11

Originally Posted By: jmyers
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Bob B,


I know that it is a violation, I was just trying to reason with myself what was wrong with it.

For the life in me, I can not see anything wrong with it, other than it is a violation. ![icon_biggrin.gif](upload://iKNGSw3qcRIEmXySa8gItY6Gczg.gif)

Thanks for your help.

Bob O,

I believe that I am starting to see where you are going with this. If the problem would be on or connected to the 14 wire, it may never draw enough amps to trip the breaker. The 14 wire would get hot, melt and possibly start a fire without ever tripping the breaker. Is that what you are saying?


Joe Myers

#12

Originally Posted By: roconnor
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Joe,


Bob is right about being able to get more out of the better wire in conduit (not NM). But I hadn't noticed the long runs allowed for smaller fixture wire on much larger BCW/OCP, even with better wiring. Kind of "shocking" to me, so I will have to think about that ... ![icon_lol.gif](upload://zEgbBCXRskkCTwEux7Bi20ZySza.gif)

But you get the idea of where I was going for typical home wiring ...

--
Robert O'Connor, PE
Eagle Engineering ?
Eagle Eye Inspections ?
NACHI Education Committee

I am absolutely amazed sometimes by how much thought goes into doing things wrong

#13

Originally Posted By: jmyers
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Bob B,


You are talking about TFFN in relation to THHN?

Joe Myers

#14

Originally Posted By: Bob Badger
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Yes TFFN along with a bunch of others.


Quote:
FFH-2
HF
HFF
KF-1
KF-2
KFF-1
KFF-2
PAF
PAFF
PF
PFF
PGF
PGFF
PTF
PTFF
RFH-1
RFH-2
RFHH-2*
RFHH-3*
SF-1
SF-2
SFF-1
SFF-2
TF*
TFF*
TFN*
TFFN*
XF*
XFF*
ZF
ZFF
ZHF


It was a cut and past job I did not type all that out.

Article 402 covers fixture wires.

Bob

--
Bob (AKA iwire)
ECN Discussion Forums
Mike Holt Code Forum
#15

Originally Posted By: roconnor
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The maximum ampacity of a #14 THHN would be 25A (NEC 310.15). While #14 TFFN (and other types of fixture wires) would have a lower maximum ampacity of 17A (NEC 402.5).


Yet it seems #14 TFFN at 17A capacity would be allowed to have an unlimited tap length on a 30A BCW/OCP that may not trip until it reaches a 38A draw. That's what I thought was unusual, but I understand where Bob is coming from ... just didn't notice those provisions before and have to chew on that one for a bit ... ![nachi_sarcasm.gif](upload://6HQh6KbNiD73gqTNQInjrR2zeJw.gif)

But the wire usually must match the breaker, except for some very limited exceptions ... especially in a home with the lower grade wiring ... ![icon_wink.gif](upload://ssT9V5t45yjlgXqiFRXL04eXtqw.gif)

--
Robert O'Connor, PE
Eagle Engineering ?
Eagle Eye Inspections ?
NACHI Education Committee

I am absolutely amazed sometimes by how much thought goes into doing things wrong

#16

Originally Posted By: Bob Badger
This post was automatically imported from our archived forum.


roconnor wrote:

But the wire usually must match the breaker, except for some very limited exceptions ... especially in a home with the lower grade wiring ... ![icon_wink.gif](upload://ssT9V5t45yjlgXqiFRXL04eXtqw.gif)


I am with you there, we do not use these provisions.

12/2 MC or spools of 12 THHN cost us next to nothing compared to the equipment we purchase for the job, it is just not worth it to cut corners, even if it is still safe.

There is always the chance we mis apply the rules then we would have to go back and change it, not worth the chance.

--
Bob (AKA iwire)
ECN Discussion Forums
Mike Holt Code Forum
#17

Originally Posted By: jmyers
This post was automatically imported from our archived forum.


Quote:
... especially in a home with the lower grade wiring ...


I have never thought of TFFN or any other fixture wire being a better wire than a THHN. It was always my understanding those designations were solely used for the basis of their application in relation to the insulation type on the wire, such as thermoplastic, rubber....etc.

It was my understanding the same held true for cord, such as SJO, SJOO, SJOO-WA......etc. It was not that the wires were better, just rated for more, or different uses.

Joe Myers
#18

Originally Posted By: roconnor
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Joe, I am really talking about ampacity when I say “better wire”.


TFFN (#14 @ 17A) has a lower ampacity than THHN (#14 @ 25A) per NEC 402.5 and 310.15 respectively.

Household wiring (NM) is limited to the 60C ampacities, while most commercial/industrial wiring used has a better temperature/ampacity rating.

--
Robert O'Connor, PE
Eagle Engineering ?
Eagle Eye Inspections ?
NACHI Education Committee

I am absolutely amazed sometimes by how much thought goes into doing things wrong