Safe ampacity for layout wiring

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I recognize the interest in safety, but on a model RR layout, the need to avoid voltage drop means that the gauge wire we use should never even get close to involving overheating.  There is one matter to be aware of:  a poor connection can overheat even at small amperages far below what we'd fuse or breaker the circuits for.   For a large layout, to me this means 14-gauge stranded for single feeds and 12-gauge for busses that carry several crcuit grounds.  If building a layout with one loco and no feeds longer than, say, 4-feet, I have used 16-gauge

And I would not use a breaker or fuse greater than 10 amps, (largest on my layout is 7) primarily because that is the maximum output of model train transformers )I am led to understand that this is a UL mandate), but also because I know that under certain circumstances, even that amperage is overloading in car circuits and derailed axles..

I use this attached file.

A big issue at low voltages is the fact that a voltage drop of a couple of volts is significant when you're talking 18 volts, not so much when you're talking 120 volts.

For an 8' loop of #12 wire operating at 20 amps:

The heating wattage in the wire loop is about 10 watts,

The voltage drop is .5V, which I use as the threshold for affecting performance, especially when running in conventional mode.

If you have multiple drops from the track to this wire loop, the track will also contribute to the conduction path, lowering the resistance further, depending upon the type of track and the "goodness" of the rail joints.

I also prefer 7-10 amp breakers on track feeds, which would double or triple the permissible loop length regarding voltage drop.

I watch what I use because I run large consists in G scale right under my O. For anyone who hasn't checked it out, the OGR wire is pretty amazing to me. When you look at how the wire is stranded it reminds me of Monster speaker cable. It solders very easy too. I used outdoor low voltage 12v cable for years. I actually got better results with MTH's signal on 14 ga versus 12. Now I'm using the OGR cable as I build. I've got ten amp pre-fused channels going to my TIU.

RJR posted:

There is one matter to be aware of:  a poor connection can overheat even at small amperages far below what we'd fuse or breaker the circuits for.

This is an excellent and often overlooked point!  This issue is probably way more responsible for fires than the actual ampacity of the wiring in general!

Wire capacity depends on several factors including 1) conductor material, 2) insulation temperature rating, 3) conductor size, and 4) ability of wire to dicipate heat, ie. run in free air or conduit and if in conduit percentage of conduit fill.  If you are going to use the National Electric Code standards, you need to use NEC specified wire. A standard I have used in the past is "Power Cable Ampacities, Copper and Aluminum Conductors". This is IEEE standard S-135 and IPCEA standard P-46-426.  It has 319 pages of tables in small print.  I used this document for my job, not to build a train layout.

Voltage drop is usually more of a performance issue than a safety one. I once saw a door closed summary circuit that was miswired as a dead short. As I recall it was a very high performance insulation 16 gauge wire that ran the length of the car eight times.  There was so much voltage drop that it could not trip the breaker.  The wire got hot, hot enough to burn yourself, but it was 140 degree C insulation. It could boil water without exceeding its rating. 

In my layouts I use 14 gage wire with plastic insulation (TW or THHN) protected with 10 amp circuit protection.  I prefer magnetic rather than thermal protection.   I also always use a GFI on the transformer's input and have a master power off switch for everything.

I believe the greatest danger of a electrical fire on a layout is from the leads and internal wiring in the trains and accessories, not the wire I install. To compensate for this there is a fire extinguisher in the train room and I never leave the room unoccupied without turning the master power switch off. 

gunrunnerjohn posted:

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 Got it.  If the images have vanished, all please use the link for each image to find the original.

Don

Many interesting points.  The diversity of responses is expected.  The "code" standards are designed to be relatively idiot-proof with a wide safety margin, and since I've been an "idiot" once or twice in my life (at least) I don't feel immune to that despite a very technical life and cautious nature.  Re: layout wire and whether we "ever" get close to fire danger given our concern about voltage drop and using "heavy enough" wire: point taken, but there are folks with situations where say 18 or 20 gauge wire might be overloaded inadvertently just with quantity of either bulbs or other "small" loads.  Everyone may do as he/she sees fit and seems safe to you.

Circuit protection using "code" rules is one way we catch our mistakes the safe way.  Although Bill Bramlage's part of the video initiated my concern because the wire table is not from a source relevant to our work, his stated principle is untarnished.  As he said, for a 4 amp building in his Tower City, he provides a 5 amp breaker.  If the building goes dark, he finds out why before replacing the fuse. Besides that table bugging me, the wide diversity of views I had already read on this forum about wiring safety were another reason I posted this morning.

Don

A couple of points. 

(1.) NEC code charts are base on ambient Temperature. The temperature indicated is degrees Celsius. ( 90 degree C, THHN wire,  approaches water boiling at 100 degrees C.)  ambient indicates, room temperature of 30 degrees C.   In most cases, (IMO), the charts are assuring the proper insulation for the application (IMO).  You would be using,  Copper wire third column.  You will most likely be purchasing THHN conductors. (Table 310-16) as previously linked.  Note that there are asterisk(s) associated with 14, 12, 10 gauge wire, that refer to  section 240.4-D  The values in the third column are significantly larger that the standard accepted amperages for  14 ga. 15 amps, 12 ga. 20 amps, and 10 ga 30 amps.  These values allow for standard derate-ing of the conductors for multiple conductors in a cable or conduit.  80% and 70% derate-ing of conducts are used a lot.     

(2.) Wire length.  (Surprising).   One Saturday, on a wire pull, to the remote section of the layout, connected by a drop in bridge, I figured the track feeders/accessory power/ switch power and control/spare wires, etc. to do all the wires, I would need, at once.  Distance, line of sight,  10 ft. max.  Wire, up the wall, across the ceiling, down the wall, with ample wire to terminate on either side,  45 ft.   You will use considerable more wire than you expect. IMO.

Control panel end of wires.

Track feeders.

Remote section, wires after the pull.

?14 wires pictured? X 45 ft.  630ft total

(3.) Simplification.   12 ga solid wire for track feeders.  18 ga solid thermostat wire for switch/switch control and other accessories.  Usually available at big box stores.    

(4.)  Some system of wire management and neatness is  in order. 

Have fun with your wiring.

Mike CT

Engineer-Joe posted:

I watch what I use because I run large consists in G scale right under my O. For anyone who hasn't checked it out, the OGR wire is pretty amazing to me. When you look at how the wire is stranded it reminds me of Monster speaker cable. It solders very easy too. I used outdoor low voltage 12v cable for years. I actually got better results with MTH's signal on 14 ga versus 12. Now I'm using the OGR cable as I build. I've got ten amp pre-fused channels going to my TIU.

I also use the OGR wire and I really like it. I used it for all my track power, #14 to terminal blocks and #16 for the track drops. I agree, it's good stuff.

To Mike CT's "NEC code charts are base on ambient Temperature. The temperature indicated is degrees Celsius. ( 90 degree C, THHN wire,  approaches water boiling at 100 degrees C.)  ambient indicates, room temperature of 30 degrees C.   In most cases, (IMO), the charts are assuring the proper insulation for the application (IMO).  You would be using,  Copper wire third column.  You will most likely be purchasing THHN conductors."

Response: Right. My understanding is it's important to separate (1) insulation temp rating (which is related to assuring insulation will not melt or burn under any normal condition, for that type of insulation at the specified ambient temperature limit);  from (2) the copper conductor current rating (which is related to avoiding fire by keeping copper's temperature below the level that risks igniting nearby [or accidentally touching] combustibles.  The high temp insulation can be intact, but if the wire temp exceeds safe limits one could possibly damage or ignite adjacent material despite intact insulation--given the right combination of circumstances.

And those asterisks: They tell us to use no more than the specified breaker or fuse ratings for situations like residential installations; I should have said above that those ratings (e.g. 20 amps for 12 gauge etc.) and not the higher table values (e.g. 25A for 12 Ga) match up with the overlapping ratings in the small wire table, so the small wire table seems to have the same safety margin as the residential wiring ratings (safe for 30 amps for 10 gauge all the way to 0.5 amp for 30 gauge).  

So all of those should be safe under our layouts, for not more than 3 conductors in a closed conduit or raceway (or cable) at the standard 86 deg F ambient temp.  For higher ambient temps, the correction table at the bottom of the large wire table applies.  There is also a table online from various websites, using NEC standards for downward breaker/fuse rating corrections where more than 3 conductors are tight together in a cable or conduit; I didn't copy that in the post above but it's not hard to find if you have multi-wire cables to consider.

A small layout with mostly single wires, set where surrounding heavy material like the table top serves as a heat sink, obviously lets us get away with murder to some extent in using smaller wire.  We just shouldn't do it without thought, or use too-small wire when those added safety factors are not present!

I don't consider a wooden table, or any other normally used material, to have any value as a heat sink

RJR posted:

I don't consider a wooden table, or any other normally used material, to have any value as a heat sink

RJR: I don't use plywood tabletops that way, either!  However, it's clear that a lot of layouts with wires run here and there under a flat tabletop may be using the table as a heat sink and do not know it.  I'm thinking of 20 gauge and 18 gauge wires, used by folks who just aren't aware of the load and heating concern.  All the above is information to be used in whatever way the reader wishes to.