Question about practices problem in Volume One book

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Eddy Current

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If the ungrounded conductors for a 40A circuit are increased in size from 8AWG to 6AWG the circuit equipment grounding conductor must be increased from 10AWG to ____?
 

Smart $

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If the ungrounded conductors for a 40A circuit are increased in size from 8AWG to 6AWG the circuit equipment grounding conductor must be increased from 10AWG to ____?

Where does the multiplier 1.59 come into play?
There's no need for a multiplier when proportionate upsizing is within gage size range. If you upsize 2 gage sizes, then you upsize 2 gage sizes... i.e. #8 to #6 ungrounded is two (2) AWG sizes, so you upsize #10 grounding to a #8.

When you get into kcmil wire sizes is when you need the so-called "multiplier". It is nothing more than the ratio of the upsized size in cmil to the not upsized size in cmil. For #8 required minimum upsized to #6, its 26240cmil over 16510cmil... or 1.59:1 rounded to hundredths. Multiply that times 10380cmil and you get 16497cmil, or a #8.
 
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charlie b

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There's no need for a multiplier when proportionate upsizing is within gage size range.
I would not accept that as a useful rule of thumb, without first creating a spreadsheet of possible upsizes, and proving it works in all cases. Have you already done just such an analysis? If so, I would appreciate getting a copy of the spreadsheet, so that I can put it in my binder of "technical tidbits" that I keep at the office.

 

Smart $

Esteemed Member
Location
Ohio
I would not accept that as a useful rule of thumb, without first creating a spreadsheet of possible upsizes, and proving it works in all cases. Have you already done just such an analysis? If so, I would appreciate getting a copy of the spreadsheet, so that I can put it in my binder of "technical tidbits" that I keep at the office.
Actually I do have a spreadsheet analysis. However, to accept the "rule" mentioned earlier you only have to adopt the AWG diameter formula as the basis for proportioning...


AWGupsizingratio.gif


If you use NEC table values you will get aberrant results. The table values have been rounded at least twice over. ASTM diameters are only rounded to 4 decimal places. Tabled cmil values are based on those rounded values and even rounded again. Additionally, mathematically accepted rounding standards were not used for the rounding of tabled values. Therefore, why should we use accepted math proportioning standards to do this evaluation???

Additionally, where in the requirement does it say what proportioning method is to be used??? Does it provide a tolerance? ...i.e. it doesn't even say we must use the next larger tabled value when the proportional value is not exact. If we use a similar practice to rounding values as is done on the tabled values, we can use a smaller size that is close to the proportional value... but who is to say how close it must be???
 

Smart $

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G._S._Ohm

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Therefore, why should we use accepted math proportioning standards to do this evaluation???

Does it provide a tolerance?

but who is to say how close it must be?
It's supposedly the 'framework' for all this, just like E = IR, so it's an unchanging reference point. But, it's very difficult to go the other way, from real world diameter measurements and then correctly infer the rule behind it.

The tolerance can be inferred to some degree of accuracy.
E.g., if the ratio of the CMs of conductors that are commonly available is 1.59, then the practical tolerance limit is 1 +/- (0.59/2) = 1 +/- 0.3 = plus or minus 30%, a pretty loose tolerance.
If you parallel conductors the overall tolerance tightens according to a Root Sum Square formula having to do with statistics.

There's more.
The conductor diameters are equally spaced on a log scale, not a linear scale, so a linear tolerance of +/- some number is not strictly accurate, but they use it anyway.

The people who have to work with this system will provide feedback as to usable tolerance limits. Pretty loudly, probably.
If there's a market for conductor sizes that are +/- 1%, those conductors will appear.
 
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