Why is this done? - Page 1
Forum section: electronics / general

author: Rick Law
author_rank: Super Contributor
timestamp: May 22, 2019, 03:31:52 am
content: I was doing some repair on my 240V (clothing) dryer. It was a simple thing, but something else there bugs me.The 240V coming into the machine uses a super huge plug, super thick 5-feet cable (see picture). The three conductors on this thick cable side-by-side laying flat is about 3/4 inch wide. All three internal wires this thick power cable feeds into are three thin 14 AWG wires.Ok, the machine has a 4750 watt heater element which could draw up to 19.8 amps. Together with the motor and misc, it would pull in a bit over 20 amps. Outside chassis cabling indeed would experience things like being step on and other abuses. It needs more protection and I understand that. But when I look at the inside wire vs the outside wire, the size difference is huge. (see attached photo)I am thinking these manufacturers are not likely to throw in something more costly than needed. So, I think there must be a reason behind it somewhere. Is this over-sizing just over-doing it or is it due to regulation or what? Any ideas?

author: maginnovision
author_rank: Super Contributor
timestamp: May 22, 2019, 03:44:19 am
content: I'm not sure but my old 240V A/C system used the same basic thing internally. Solid wire and up to 50A draw. To be fair though it did have insulators at the ends, haha.

author: Housedad
author_rank: Frequent Contributor
timestamp: May 22, 2019, 03:59:37 am
content: I wrote a tutorial about wiring in a car forum a few years ago that became a sticky there. It should answer your questions.https://www.ford-trucks.com/forums/1110172-how-to-safely-choose-the-rght-wire-size-and-type.html

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 22, 2019, 03:59:59 am
content: Perception? A light cable isn't going to feel right.Tim

author: Housedad
author_rank: Frequent Contributor
timestamp: May 22, 2019, 04:18:50 am
content: No, it is for protection and cost. A thick PVC insulation can provide the amount of environmental protection to those wires cheaper than something like thinner, tougher crossed link insulation. This is a high amp cable to be handled by Joe homeowner. You have to add extra extra protection for when the uninformed do something stupid to the cord. Liability is an issue, I would also assume.

author: Housedad
author_rank: Frequent Contributor
timestamp: May 22, 2019, 04:36:05 am
content: As for the sizing of the wires, electrical codes for house wiring are very conservative. A 14 gauge wire rated for 15 amps can easily carry 20 amps all day with proper insulation. Standard house wiring is derated down a full size gauge for pure outright safety margin. However, appliance cord (or anything else that plugs into an outlet) follow their own special rules and can be sized more in line with the non derated capabilities. The cord is considered part of the appliance, not house wiring.

author: james_s
author_rank: Super Contributor
timestamp: May 22, 2019, 05:23:46 am
content: It comes down to run length and temperature for the most part. Wires run in walls can be 100' or more depending on the size and layout of the house, and they're buried in walls, often fully surrounded by insulation. The cord on an appliance is a few feet long at most and normally exposed to air.

author: SparkyFX
author_rank: Frequent Contributor
timestamp: May 22, 2019, 08:41:38 am
content: A thicker wire/insulation will have these properties (compared to a thinner one with the same material and gauge):- larger minimum bending radius- higher thermal capacity- higher insulation voltage- higher mechanical stabilityI reckon the bending radius and vibration resistance is key for a dryer. They all shake when in operation.Inside the device that might not be that much of an issue, as they are attached to the frame in a fixed location.Thermal capacity might affect inside and outside cabling similarly, but your local code might demand thicker wiring for a heat appliance supply.Wires get derated basically for temperature, if the back of the frame exceeds some temperature and contact can not be excluded that might directly affect the cable rating - or calls for a more temperature stable material/thicker insulation.It´s probably a combination of those.

author: GlennSprigg
author_rank: Super Contributor
timestamp: May 22, 2019, 10:14:09 am
content: I think 'SparkyFX' said it all, except maybe for a few other considerations.I notice that YOU are in the USA, where 110v is the 'norm', but this device is 240v ?Obviously, 110v needs about twice the current, so they were catering to multiple markets/models ??

author: amyk
author_rank: Super Contributor
timestamp: May 22, 2019, 11:43:08 am
content: I'm pretty sure there are no 120V-only countries using that voltage for things like dryers. The cable would be even thicker than that...

author: sokoloff
author_rank: Super Contributor
timestamp: May 22, 2019, 11:59:31 am
content: Quote from: Rick Law on May 22, 2019, 03:31:52 amI seem to recall having to buy a dryer appliance cord when I bought my last electric dryer. I spot-checked a few dryers for sale on the HomeDepot.com site and none of them come with bundled power cords.So, that it's a 3rd party cord and not coming out of the manufacturers' pockets might be part of the explanation.

author: rstofer
author_rank: Super Contributor
timestamp: May 22, 2019, 02:14:04 pm
content: If you go back to Home Depot and look at those Dryer Cords, you may see that there are two different plug styles. There is an old 3 prong where the ground wire was used as a neutral for the timer and light. This is frowned on these days so the newer cable has 4 prongs (2 phases, neutral and ground).The dryer manufacturer doesn't know how your house is wired so they don't even bother to guess. They leave the cord to the aftermarket folks.A 30A circuit would be rated for no more than 24A on a long term basis. Hence the wall outlet is 120/240V 30A (4 wire these days). Similarly, a 20A circuit would be rated for no more than 16A.This 80% derating is due to the circuit breaker heating effects. It has nothing to do with the wire or the wiring devices. The wire would still have to carry 30A for a fairly long time before a 30A breaker will trip at 30A.

author: duak
author_rank: Super Contributor
timestamp: May 22, 2019, 04:31:05 pm
content: I worked at a company that was big enough to have people familiar agency approvals and would work with them for various products. It was interesting to see what was allowable and what wasn't. AWG 14 for 20 amps is OK with the correct wire and insulation and that it is inside an enclosure - it wouldn't be OK if it were branch circuit wiring. I understand that in the worst case, the overall enclosure will contain any fire. I don't know what standards apply to automobiles or aircraft - they are enclosures too but they have people strapped inside whereas appliances do not. Automobile manufacturers certainly have the same cost pressures as appliance manufacturers and will try to minimize cost by using the smallest gauge wire.I remember asking one of compliance engineers about toasters. How could toasters get approval when they clearly violate accessability rules? ie., someone could contact the energized heating element with a conductive instrument. He rolled his eyes and said he was glad he didn't have to get them passed.

author: Housedad
author_rank: Frequent Contributor
timestamp: May 22, 2019, 04:39:26 pm
content: Residential Dryer circuits throughout the USA are 30 amp 240V single phase.

author: edy
author_rank: Super Contributor
timestamp: May 22, 2019, 04:57:55 pm
content: I was just fixing my dryer last night... Had to take the whole f*&king thing apart (top, door, drum) just to fix a loose "reflector" plate (for a lack of a better term) that is attached to the back of the drum and faces the back element coil which sits in it's own reflector plate. The plate is held on to the back of the drum with 3 screws... one screw popped... turned out the hole for that screw got slightly too big, screw no longer was able to hold the plate which was warping and so would bend out, rub against the coil-element/back-plate and make tons of noise. So I ended up custom-making a larger screw and enlarged the hole slightly (but had to cut the larger screw to length using my Dremel since there is no tolerance, 1-2mm extra and the screw sticks out too much and would snag the heating coil). Now the plate is held to the drum tightly.Anyways, to answer the question about the huge thick cord for 240V mains, the back of the dryer gets EXTREMELY HOT because that is where the coil element sits (on that back reflector plate mounted on little ceramic stand-offs). In my laundry room, as probably many others, the main power cord comes out the dryer in the back and snakes a few times behind there and to the plug (which in my laundry room is actually up high behind and above the top of the dryer). I wouldn't want anything other than metal (like the exhaust duct) or something able to handle heat to be anywhere back there. I leave a nice space between the dryer and the wall just to make sure (which the duct occupies anyways).

author: tkamiya
author_rank: Super Contributor
timestamp: May 22, 2019, 05:45:08 pm
content: Wire gauge requirements are different for electrical wiring in house/business and inside equipment.It has to do with safety and cooling. Notice inside equipment, those wires are NOT bundled. They are freely hanging in air. There is a reason for that. Air space around the wire provides cooling, allowing larger current to safely pass without overheating the conductor. Also, no one comes into touch. Robust casing has a lot to do with this. Those wires do get hot. But no one come into contact with them.You see same thing in Electrical stoves and water heaters. They look scary but are perfectly safe.By the way, even in household wiring, if you are to bundle them (zip tie?) or put them in conduit, appropriate de-rating has to be applied. That's because of heat build-up.

author: maginnovision
author_rank: Super Contributor
timestamp: May 22, 2019, 06:53:19 pm
content: I've never had a 240V dryer, maybe someday.

author: Red Squirrel
author_rank: Super Contributor
timestamp: May 22, 2019, 09:17:37 pm
content: 14awg is only good for 15 amps that does seem odd to me. That does seem very low if that's actually the main internal feed. Wonder if they feel they can get away with it because it's a short run. Everything up to the dryer is going to be #10 so a little bit of #14 is not that bad maybe? Or maybe they have it setup so the airflow for the intake goes through that whole compartment, so while it's on it's getting enough airflow to stay cool maybe.Mind you 15 amps at 240v is 3600w. So maybe that's actually all it draws, and the requirement for a 30 amp breaker is just to give more breathing room, knowing it's an appliance that can draw a lot for a decent amount of time. Ex: you don't want to run a 15 amp load on a 15 amp circuit for a long time.

author: rstofer
author_rank: Super Contributor
timestamp: May 22, 2019, 10:58:13 pm
content: Quote from: duak on May 22, 2019, 04:31:05 pmToaster are a specialty in terms of requirements. Ordinarily, the outer housing would have to be grounded. UL took the position that with exposed high voltage heating coils and the possibility that someone would jam a knife in the toaster to retrieve bread, it might be better if the housing wasn't grounded. Then they turned around and required GFCI protection of the outlet.Problem solved!

author: james_s
author_rank: Super Contributor
timestamp: May 23, 2019, 12:06:57 am
content: 14AWG is good for 15 amps in a wall. For chassis wiring or power cords the rating is higher.

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 23, 2019, 12:32:09 am
content: Quote from: amyk on May 22, 2019, 11:43:08 amSome modern heat pump dryers are so efficient that they run on 120V.

author: Rick Law
author_rank: Super Contributor
timestamp: May 23, 2019, 07:51:02 pm
content: Quote from: T3sl4co1l on May 22, 2019, 03:59:59 amI also think this may be part of the reason. With the USA standard 240V socket being so hefty, a smaller cable attached to it would look odd - like a very long tail for an elephant.Quote from: sokoloff on May 22, 2019, 11:59:31 amYou are probably right. Mine is a home depot purchase. I probably forgot paying separately for it. I recall installing that myself probably because the "moving it into the laundry room" was quite an adventure, but I didn't recall connecting that giant-anaconda sized cable to the machine.... Age does funny things to memory.Quote from: edy on May 22, 2019, 04:57:55 pmYeah, I was concern about the heat there as well. The inside chassis cabling was rated 105c. I could not find 105c rated cable at 14 AWG at my local store, only 90c ones. Not willing to wait a week for delivery, I use 250c/600V rated heat shrink tubing to give it an outer jacket that can handle more.Being concerned at the interior temperature at the get-go, post repair, I actually IR-gun measured during run-time. Inside the drum was reaching about 82c. Pushing the 90c that the cable is rated for. Still, I am glad I took the trouble to give it a heftier 250c rated heat shrink jacket...

author: james_s
author_rank: Super Contributor
timestamp: May 23, 2019, 09:19:12 pm
content: I would not be surprised if another factor was the cost cutting you see everywhere else. Look at inexpensive speaker cables and whatnot, it's common to see really thick insulation with tiny thin conductors. Why? Because insulation is way cheaper than copper and if you have two cables that look similar but one is half the cost which do you think the average uninformed consumer will choose? Older dryer cords may have had much thicker conductors, I don't know that I've ever cut one open to find out.