Residential Dryer circuit
- Thread starter s.degidio
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kwired
Electron manager
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- NE Nebraska
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- EC
Motor conductors are sized to the full load ampacity of the motor (and usually to 125% of FLA). Then we have two types of overcurrent protection - the branch circuit device is usually the one that appears to be high - but is only intended to provide short circuit and ground fault protection, the motor overload protection not only protects the motor from overload but also protects the conductors from overload. Between the two protection types (if applied properly) that conductor is better protected from overcurrent then some general purpose circuits are.
mbrooke
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But even at 125%, conductors can still see loading over 15, 20 and 30amps. Then again you do have a point that general use circuits tend to see more over loading.
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- Licensed Electrician
14, 12, and 10 are rated higher than 15, 20, and 30 amps the small conductor rule just limits them to that size overcurrent protection.
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- Massachusetts
You seem to be convinced that the NEC limits are not already conservative.
mbrooke
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Im sure they are, but why would 240.4 (D) restrict the current rating unless a hazard existed? :?
kwired
Electron manager
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- NE Nebraska
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- EC
do they?
Take a motor with a 14 amp full load current from 430 part XIV tables.
Now multiply that 14 by 1.25 and you must have a 17.5 amp conductor for most general applications.
Chances are you may have short circuit/ground fault protection at least 20 amps or even up to 30 amps to allow for starting.
Now throw in the fact your motor overload protection in most cases will be no more then 125% of motor full load current and you are still protecting the 17.5 amp minimum conductor at 17.5 amps (or less depending on actual nameplate FLC) aren't you?
Yes this does allow a 14 AWG at 75C to ultimately be protected at 17.5 amps instead of 15, I still have no problem with it as the motor being protected is more sensitive to a small overload then the conductor supplying it is.
mbrooke
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But that's my point in your example #14 is seeing more than 15amps. I guess the tables are conservative enough, but always wondered why less so for motors. I get the short circuit protection may be 250%, but that is for faults only.
kwired
Electron manager
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- NE Nebraska
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- EC
But all that was exceeded was the 240.4(D) requirement (that doesn't apply to this install), the 14 AWG applied at 75C was still protected at less then it's capacity in T310.15(16).
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And it is still rated at 20A on the seventy five degree scale and 25A on the ninety degree.
mbrooke
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But isn't that for de-rating only? I guess Im just confused why 240.4 (D) would limit the wire if it was safe.
petersonra
Senior Member
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- Northern illinois
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- engineer
It should not be real hard to figure out what version of the code was in use in your area at the time and go back to that code and copy the appropriate language.
personally, I think the time spent doing so is billable if you had authorization to correct such violations on your own accord and the owner is demanding the documentation.
if not authorized to perform the work, I would not be paying the bill, just as a matter of principle. It would also be the last work you ever did for me.
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- Licensed Electrician
No one that I know really knows where the table values come from, we just use them in along with other code sections for conductor sizing.
Outside of NEC many conductors get along just fine. If 310.15 was based in some kind of real world rational thought we would not have had to deal with the temp. calculations that were added in the most recent cycles.......and that's just one example.
mbrooke
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Aren't they now based on some IEEE formula? At least that's what I heard in the ROP when table 310.16 was being re-worked.
winnie
Senior Member
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- Springfield, MA, USA
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- Electric motor research
My understanding of the 'small conductor' rule is that it has to do with conductor damage during fault conditions, not overload.
The _ampacity_ of a conductor is the maximum amount of current which can be carried on a continuous basis without exceeding the temperature limit of the conductor or insulation.
The ampacity of the conductor is set by a combination of the heat produced by the conductor and the rate that this heat is dissipated to the environment. Take the same conductor and bury it in thermal insulation, and it has lower ampacity then if it is in free air.
Because small conductors have lots of surface area relative to the amount of conductor, they operate at lower 'circular mils per amp' (just look at the conductor ampacity tables, smaller conductors use less copper per amp). This is the reason that you can save copper by using conductors in parallel for large installations, and also the reason that larger circuits have less voltage drop over the same distance for their 'rated current'.
During a short circuit, currents far greater than the ampacity of the conductor will be carried until the OCPD operates. Smaller conductors operating at lower circular mils per amp will heat up more rapidly during a fault. To allow standard OCPD to protect the conductors during a short circuit, the 'trip rating' of the OCPD must be less than the continuous ampacity of the conductors.
-Jon
The _ampacity_ of a conductor is the maximum amount of current which can be carried on a continuous basis without exceeding the temperature limit of the conductor or insulation.
The ampacity of the conductor is set by a combination of the heat produced by the conductor and the rate that this heat is dissipated to the environment. Take the same conductor and bury it in thermal insulation, and it has lower ampacity then if it is in free air.
Because small conductors have lots of surface area relative to the amount of conductor, they operate at lower 'circular mils per amp' (just look at the conductor ampacity tables, smaller conductors use less copper per amp). This is the reason that you can save copper by using conductors in parallel for large installations, and also the reason that larger circuits have less voltage drop over the same distance for their 'rated current'.
During a short circuit, currents far greater than the ampacity of the conductor will be carried until the OCPD operates. Smaller conductors operating at lower circular mils per amp will heat up more rapidly during a fault. To allow standard OCPD to protect the conductors during a short circuit, the 'trip rating' of the OCPD must be less than the continuous ampacity of the conductors.
-Jon
mbrooke
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I agree with everything said above, its actually excellent info, but I partly disagree with the reasoning behind 240.4 (D). If breakers did not act fast enough on small conductors the code would not allow #14 to be placed on a 40 amp breaker for protecting motor circuits.
Of note, the magnetic trip of single pole breakers has gone down considerably, with most being around 6 to 10x the handle rating today, where as in the past breakers where around 25X. In this regard 240.4(d) would be even less valid.
Of note, the magnetic trip of single pole breakers has gone down considerably, with most being around 6 to 10x the handle rating today, where as in the past breakers where around 25X. In this regard 240.4(d) would be even less valid.
kwired
Electron manager
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- NE Nebraska
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- EC
I can't say I agree otherwise we wouldn't have the exceptions we have to 240.4(D). Not to mention a motor can actually contribute even more current into a fault.
In general you are going to damage conductor insulation from heat before you damage the conductor itself. The amount of heat developed will depend on trip curve of the device and available fault current. More available current will usually mean less time to operate the device and possibly less overall heating then when available current is fairly low.
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