Solid or Stranded?

[handyguy]

Hi,
I am going to be installing a sub panel in my attached garage. I'll be running two 220 circuits, one for a wall A/C unit and one for a dryer. I have a 50amp breaker in the main service box and will be using two 20amp doubles in the sub panel. The wire, 8 gauge, will be in metal conduit on the outside of the building, approx.70', then through the wall at the garage. My question. Does it make a difference solid or stranded? Stranded might be easier to pull. Any code on what size conduit for 4 wires?
Thanks in advance, this is my first post. I've read with interest some of the threads posted here. I liked the simple yet very informative responses offered.

[Nestor]

For an electric dryer, you should be counting on 30 amps for the dryer alone.

I don't know how much current your dryer's heating element actually draws, but the cord and receptacle for an electric dryer will be rated at 220 Volts, 30 amps. So, if it wuz me, I'd use double 30 amp breakers in the sub-panel and run cable rated at 30 amps as well.

In the little bit of wiring I did to put range cords on my 21 electric stoves and 3 electric dryers, ALL of the wiring I was dealing with was stranded. That's because if it were solid, it would be far too stiff to bend in the confines of an electrical panel or receptacle box.

[pushkins]

30 amp will be the rating on the dryer, without a doubt it's much easier to pull stranded wire, I'm not even sure the last time I saw solid pulled in conduit (other than ground wire). You may have to run THWN for outdoors even though it's in conduit.
Wait for "Hayzee" to reply, he's a master electrician with years of both residential and commercial applications.

[Speedy Petey]

You'd be hard pressed to find #8 THHN solid. Stranded is a no-brainer.

Also, pretty much all THHN is dual rated these days with THWN, so for newer wiring this is almost not worth mentioning.

I will also back up that any domestic electric dryer will be a 120/240v-30A circuit, requiring three #10 circuit conductors (hot, hot, neutral) and a #10 ground.

The conduit depends on the conductors inside. What four wires do you mean? If you mean the four #8's 3/4" would be fine. You can also use three #8 and a #10 ground.
I must say, if it were me I'd go with #6, an if in conduit the whole way, a 70A breaker. For very little extra $$ you will get some (most likely) needed extra capacity.

[pushkins]

or wait for Speedy Petey, I forgot your still around as well.

[Speedbump]

I don't know much about code, but stranded wire will conduct better than solid because of skin effect. I learned that studying for my Amateur Radio license many years ago. AC current tends to run on the outside of the wire, so the more wires in a bundle, the easier it is to carry the load.
At least that's what the experts say. I always said that since we can't see electricity, how in the world does anybody actually know what goes on inside that wire.

[Nestor]

..., but stranded wire will conduct better than solid because of skin effect. I learned that studying for my Amateur Radio license many years ago. AC current tends to run on the outside of the wire, so the more wires in a bundle, the easier it is to carry the load.

What Speedbump is saying is true, but the skin effect is dependant on the frequency of the alternating current flowing through the wire. In telecommunications work, where frequencies can be in the hundreds of gigahertz, the skin effect is important. In house wiring, where the frequency we're talking about is 60 cycles per second, the frequency is so low that the skin effect in household wiring is tiny.

"Skin effect" was completely new to me, so I did a little reading on it, and thought I'd pass on the info.

Skin effect simply means that in a conductor carrying an alternating current, most of the current in the wire will be near the outer diameter of the wire, and very little current will be carried by the metal near the center of the wire.

Skin effect is caused by the nature of alternating current. Since the voltage in household power reaches a maximum, declines to a minimum and rises back up to a maximum 60 times per second, so too does the current in the wire, and as a result, the magnetic field around the wire reaches a maximum strength, decays in strength until the magnetic field reaches a maximum strength with reversed polarity, and then decays in strength until it reaches maximum strength in the original polarity again 60 times per second. Just as moving a wire through a magnetic field will cause current to flow in it, a changing magnetic field around a stationary wire due to AC current flowing in the wire causes eddy currents to form within the wire itself. These eddy currents assist the main current flow on the exterior of the wire and oppose it on the interior of the wire resulting in the "skin effect".

The "skin depth" is the distance from the OD of the wire where the current density falls to roughly 3/8 of it's value at the OD of the wire.

Since the skin effect is caused by the varying strength of the magnetic field around a wire carrying AC power, there is no skin effect at all in wires carrying DC power, like the battery cables or spark plug wires in your car.

Skin effect - Wikipedia, the free encyclopedia

So, if we didn't consider the frequency of the AC power, what Speedbump said would be true and it would be better to use stranded wire to carry more current.

But, since the eddy currents that create the skin effect are themselves created by the rate of change in the magnetic field around the wire, the skin effect is dependant on the frequency of the AC current flowing through the wire. At higher frequencies, there's a more rapid change in the magnetic field strength and the eddy currents become stronger, resulting in a greater skin effect. That is, at higher frequencies, the skin depth becomes thinner and thinner and more and more of the current flows through a thinner and thinner layer of copper on the outside surface of the wire.

However, at 60 cycles per second, the skin depth works out to be 366 thousands of an inch, or almost 3/8 of an inch. That means you'd need to have a copper wire nearly 3/4 inch in diameter before the current density at the center of the wire was 3/8 of what it was on the outer surface of the copper.

Skin Effect Calculator

About the only place where the skin effect would be non-negligible in a house might be in the buss bars in the house's electrical panel.

With smaller and smaller diameter wires, the current at the center of the wire will be closer and closer to what it is on the outer surface of the wire.

It's just a guess, but I'd guess that at 60 Hz the difference in cross sectional area between a solid and stranded wire would partially compensate, if not more than compensate for the skin effect, so you could end up with a greater current carrying capacity with a solid core wire than a stranded one. Certainly, the difference in cross sectional area of the wires would make any skin effect in household wiring even smaller than it is already due to the low frequency.

This is a DIY forum, and a lot of newbies get their information from places like this. So, I thought it best that I follow up Speedbumps post to say that, while what he's saying is true, skin effect is dependant on frequency, and at the low 60 cycle per second frequency that household wiring runs on, the skin effect is so small as to be negligible in all but the heaviest wiring in the house.

[HayZee518]

Wiring and Power

For a sub panel I would go with a minimum of 6 gauge, only because in the future you may wish to expand. #6 thhn/thwn at 75 degrees centigrade is good for 65 amps. the same sized cable with a temperature rating of 95 degrees centigrade, amperage goes to 75 amps. You can pull #6 times four conductors into a 3/4 pipe.
Use a 60 amp breaker to feed your sub. I don't know where you'll even find a 70 amp breaker.
Rule of thumb for a dryer is to use a 30 amp double pole, unless you actually know the load amps. most of the time the amps is calculated from the machine's watts. then its up to the individual doing the calculations if he uses 240 or 230 for the voltage.
Now as far as skin effect, as a frequency goes up the voltage and current does tend to travel along the surface rather than the full cross area of the wire. stranded wire has nineteen strands carrying current.
the concept of the skin effect is the reason microwaves use a waveguide rather than a cable because the higher frequency travels along the surface. using a cable would be wasteful. a waveguide is a hollow, tuned cavity that is highly polished. microwaves bounce around inside the waveguide from the input which is a wire stub inside the cavity and out through a feedhorn at the transducer. i.e. a microwave dish.

[Speedy Petey]

For a sub panel I would go with a minimum of 6 gauge, only because in the future you may wish to expand. #6 thhn/thwn at 75 degrees centigrade is good for 65 amps. the same sized cable with a temperature rating of 95 degrees centigrade, amperage goes to 75 amps. You can pull #6 times four conductors into a 3/4 pipe.
Use a 60 amp breaker to feed your sub. I don't know where you'll even find a 70 amp breaker.

Just a quick note.
You can use the 75 deg C column in figuring amperage for new equipment since pretty much all new equipment is rated for 75 deg C. You cannot use the 90 deg C column since about the only thing rated 90 deg is the wire itself. You can only use a certain temp column if everything in the system is the same temp of higher.
You can use the 90 deg C column when figuring ampacity derating though.

Also, 70A breakers are common place in any supply house and many home centers. It is a standard breaker size.