Wire size question

Thanks guys!! chris

Voltage drop in a wire depends only on the diameter of the copper core of the wire and the length. It is completely independent of the type of insulation on that wire, or what temperature the insulation is rated to. You can't get a longer run out of a given size of wire by using a "better" kind of insulation -- only by upsizing the core of the wire.

Ampacity is a function of insulation type and its attendant temperature rating. As you note, an ampacity listed at 90C is rarely directly useful -- because the terminals it connects to are typically rated to only 75 C.

Where that 90C rating comes in handy is in conduit fill de-rating. Now you can take that 14 AWG wire rated to 20 amps at 90 degrees, stuff 6 of them in a conduit, de-rate to 80%, and still get an effective 15 amp capacity.

Ampacity is a safety issue related to heating of the wires. For this, insulation type matters.

Voltage drop is a performance issue related to supplying enough voltage at the end of the run. For this, insulation type does not matter.

If the voltage drop is so bad that the motor will not start, and sits there and hums while it draws the locked rotor current, then there may be safety issues with both kinds of wire. Hopefully, the motor starter and/or the circuit breaker will trip in this case.

Table 310.16 of the 2002 NEC lists RHW and THWN as being rated to 75c and capable of 20 amps at 14 guage. You were probably thinking of THHN or THWN-2, which are listed at 90 degrees and 25 amps. Type TW is rated to only 60 degrees and 20 amps. There is no RH listed. But I can never remember these without the book handy either.

Voltage drop is not a mandatory code regulated issue, being covered by "Fine Point Notes" which suggest that satisfactory service can be obtained if the total voltage drop is held to 5%, and recommending a max of 3% for feeders and 3% for branch circuits.

Table 8 at the back of the book gives the direct current resistance in ohms per 1000 feet for various wire sizes at 75c. No distinction is made for insulation type, although there is a difference depending on whether or not the wire is stranded.

The simple equation for voltage drop becomes Vdrop = (R (from table) * (length of wire) / 1000 * current). This is what is most commonly used for figuring out if wiring or an extension cord will work, as it is conservative in most circumstances. Don't forget that it is the two-way length that matters -- a 200 foot extension cord is 400 feet of wire from the electron's point of view.

Table 9 of the NEC gives the AC resistance and reactance for various wire sizes and wiring methods (steel vs aluminum vs. plastic conduit). It offers the following equation for computing effective impedance.

Ze = R (from table) * (Power factor) + XL (from table) * Sin(arccos(Power factor)

From the effective impedance and knowledge of the current and power factor you can than compute AC voltage drop.

Insulation type does not impact any of these calculations.

I suppose you could take into account the skinny cross section of wires like THHN, compared to chubby wires like RHW, and argue that the conductor spacing, spacing of conductor to the conduit, the capacitance per unit length, and effective impedance might end up slightly different. But you'd be going on your own, and I don't know how you'd take into account the varying geometry as the wire is pulled through the conduit.

You are correct in noting that resistance is a function of temperature. (It goes up with higher temperatures). Since some insulation types will allow operation at higher temperatures in limited circumstances, you could argue that the voltage drop for the core of a THHN wire should be computed at 90c instead of the 75c for RHW. But then the THHN wire would have a higher voltage drop (and lower serviceability) than an RHW wire, the opposite of what you seemed to be implying above.