Residential Dryer circuit

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norcal

Senior Member
Came across a house that the dryer circuit was run w/ 10/3 NM AL, but it was a low baller special, Zinsco/Sylvania service panel, & everything else as cheap as possible. It of course was inspected & passed too. :(
 

kwired

Electron manager
Location
NE Nebraska
Ive never understood that. I could understand welders because they are intermittent (duty cycle), but why are motors allowed to load conductors to the max?
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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United States
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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.

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.
 

kwired

Electron manager
Location
NE Nebraska
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.
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
Location
NE Nebraska
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.
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).
 

ActionDave

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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.
And it is still rated at 20A on the seventy five degree scale and 25A on the ninety degree.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
I have a HOA requesting validation that a cloths dryer 30 amp outlet would have had to been wired with #10 conductor during 1985 construction of the building.

The issue is, I had #12 feeding a 30A dryer outlet on 30A breaker and aware of this violation had it rewired correctly.

I need a validation that in 1985 this circuit conductor had to be #10 and that #12 would under no circumstance be proper for this 30A circuit.

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.
 

ActionDave

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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.
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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United States
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Technician
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
Location
Springfield, MA, USA
Occupation
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
 

mbrooke

Batteries Included
Location
United States
Occupation
Technician
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.
 

kwired

Electron manager
Location
NE Nebraska
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
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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