Siberia Racing Tech Pages

Beware the Bear

Track Voltage Drop Testing

This is the last in a series of web pages inspired by the building of Magic Raceway.  In this page I discuss the importance of designing the track to reduce voltage drop and then how to perform a voltage drop test.  In the design of the track it is important to locate power taps within a reasonable distance from each other.  There is an old rule of thumb that indicates that power taps located ten feet apart are acceptable.  That rule of thumb is probably 50 years old.  Its was probably fine when we were racing T-Jets.  Its not so fine for today's high downforce cars.  

Ten feet of track between jumpers gives us twenty feet of steel rail to contend with. Steel has a resistance between three and 15 times that of copper.   The cross sectional area of a typical Aurora/AFX/Tomy rail is approximately equal to the cross sectional area of #16 AWG wire.   When I tested my Viper Scale Racing rails I found that thie resistance was approximately 9.7 times that of #16 AWG copper wire.  Now the resistance is dependant on the dimensions of the rail and the type of steel used.  Your rails may be different so its necessary to measure the voltage drop and back calculate resistance.  Assuming that your tracks rails have a resistance equal to ten times that of #16 AWG copper wire (i.e. 40 Ohms/1000 ft) the resultant voltage drop of ten feet of track would be 2 amps x 2 rails x 10 feet x 40 ohms / 1000 feet = 1.6 Volts.  This voltage drop is  1.6V x 100 / 18V = 8.9% of 18 V and would be excessive.  This is especially true of sectional track as each track joint adds resistance.  If we assume that the ten feet of track has 10 joints per rail and each joint has a resistance of 1/100 of an ohm the voltage drop increases by 0.4 volts and the total voltage drop is 2V x 100 / 18V  or 11.1% of 18 Volts.  This 11.1% value does not include the resistance of the under table wiring, fuses, relays, etc. 

What is a reasonable voltage drop to design for?  Unfortunately nobody really knows and there is no standard.  On my last two tracks I used a goal of 3% voltage drop assuming a power supply value of 18 V and a load of two-amps per lane.  Why does the acceptance criteria include current?  Voltage drop is based on two factors.  These are resistance and current.  Without specifying a current its impossible to calculate voltage drop.  Two amps was chosen as it is higher than the amp draw of a running car.  A car may draw more than two amps when accelerating from a stop.  However once up to speed the running current drops to one amp or less because of the voltage generated by the armature as it is spinning.  This Back EMF lowers the effective resistance of the armature.  Two amps was chosen as it bounds the cars running current..    

The track design process should include preliminary calculations to help determine wire sizes and lengths.  Certain factors were included in the design to minimize voltage drop.  For example two wires were routed to each lanes power taps and between the drivers stations and the power taps there were no shared current paths.  Likewise wire lengths are kept to a minimum.  A recommended source for wire resistance values is the National Electric Code.  For those who don't have a copy of the code a suitable source can be found on line by searching for the Okonite Cable Engineering Handbook.  I have been using this handbook at work and at home since the early 1980s.  A portion of the wire resistance table (Table 1-3) from the 2018 edition is shown below.

 

Okonite Engineering Handbook Wire Resistance Tables for 10-24 AWG Wire

Some things to know about this table are that the resistances given are in Ohms per 1000 feet of wire;  annealed uncoated copper is typical copper wire that is available at Home Depot, Lowe's and Graybar Electric.  Annealed coated copper wire is copper wire that has been tinned.  I used resistance values for uncoated stranded copper wire.  Resistance changes with temperature and I used a temperature of 20C / 68F as the track would be located in an air conditioned basement.  I assumed that the wire would remain at room temperature as the currents are low and the wire's temperature would not change due to current induced heating.   

At Magic Raceway the wiring from the power supply to each drivers station panel is #10 AWG copper wire.  The wiring from at the drivers station panel to the track jumpers is #14 AWG copper wire. Short leads from the track to each Power Tap terminal block will be #16 AWG copper wire having a maximum length of six inches.   Because of the way the power taps are configured each connection will consist of two #16 AWG wires routed in parallel.  Two #16 AWG wires connected in parallel have a resistance slightly lower than that of #14 AWG wire and the jumper length is included in the #14 AWG wire lengths.  

The calculations were as follows

The total voltage drop = 0.122 Volts + 0.154 Volts + 0.96 Volts + 0.148 Volts = 1.384 V or 7.6 % of 18 VDC.  

I then credited the fact that the #14 AWG wire would be wired in a ring bus configuration so that there would be multiple parallel paths to any point on the track.  This would reduce the voltage drop by 0.557 VDC or 3.1% of 18 VDC.  The revised voltage drop would be 4.5% of 18 VDC.   I then reasoned that by minimizing wire lengths and doubling up some wires I could reduce the voltage drop to less than 3%. 

So much for theory.  Read on to see how things turned out.

  Real World Voltage Drop Tests

This YouTube video made by "Tossedman" shows the results of an a HO track load test including voltage drop from the driversí station, to a power tap and to the midpoint between power taps.  It should be noted that this was a routed, continuous rail track.  The voltage drop to the midpoint between jumpers would have been more severe if it would have been a semi-sectional or sectional track because of the number of joints and the resistance of each joint. No information was provided about wire sizes, lengths or expectations.  It was a voltage drop test.

The load was a single light bulb which provided a 2.5-amp load for the power supply and track wiring.  The result of the test showed a 0.5 volt drop between the driverís station and the power tap. The voltage drop between the power tap and the track at the midpoint between power taps was an additional 0.73 volts.  The total voltage drop was 1.23 volts or 6.8% of 18 volts and the voltage at the midpoint between power taps was 16.77 volts.  It should be noted that the voltage drop between the power supply and driversí station was not measured.  From the video the power supply appears to be set at 18.1 volts.  If this is so then the total voltage drop from the power supply to the load would be 1.33 volts or 7.3% of 18.1 volts.  To allow for a comparison with my calculations, which use a 2 Amp load, and the video I used a ratio to correct the voltage drop for a 2 Amp load.  If the load in the video was reduced to 2 Amps, then the total voltage drop would be reduced from 7.3% to 5.84%.  The video presented the data but made no conclusions.  In my opinion the measured voltage drop is excessive.

Like "Tossedman" I also used light bulbs as my load.  I used two bulbs for the load test.  Each light bulb fixture consisted of a 1157 Automotive Stop, Turn Signal, Tail Light Miniature Bulb. The two bulb filaments were connected in parallel.  Each bulb drew 2.5 Amps at 12VDC.  

One light bulb was connected to a drivers station panel white and red terminal points.  The second light bulb was mounted on a fiberglass skid with tinned copper "rails" underneath.  This allowed the bulb to be located at any point on the track.  The skid was weighted to assure a good electrical connection with the track rails.  The skid was located at the midpoint between two power taps.  A jumper was placed across the drivers station white and black terminals to provide maximum voltage to the skid mounted bulb.   Unlike the mockup test shown in the following photo the two bulbs were never simultaneously connected to the same lane when the test was in progress. 

Load Test Fixtures

To ensure accuracy, voltage readings were not taken at the skid but directly from the rails immediately adjacent to the skid.  The total load on the power supplies during the test was approximately 5 Amps.  The load on the lane being tested was approximately 2.5 Amps.  Voltage measurements were taken for each lane at the following points.  

A ratio was used to correct the test load to match the assumed loads used in the preliminary calculations.  The 5 Amp measured voltage drops values were increased by a factor of 6/5 = 1.20 to reflect the diffrence betwwen the measured 5 Amp total and the assumed 6 Amp load.  The 2.5 Amp measured voltage drop values were decreased by a factor of 1.0 - (2/2.5) = 0.20 to reflect the change in current from 2.5 Amps to 2 Amps.

The above measurements show that the worst case total voltage drop from the power supply panel to the track at the midpoint between a set of power taps was 0.643 VDC or 3.57% of 18VDC.  

Don't be afraid to question the results.  I was surprised at what appeared to be an excessive voltage drop from the drivers station to the power taps.  Each table had its own "Ring Bus".  Each Ring Bus consisted of two ten foot long lengths of #14 AWG wire connected in parallel.  At various points the power taps branched off of the bus.  Based on the other values a 0.220 VDC voltage drop just didn't look right.  There must be a single weak link as the measured power tap voltages were all within 0.008 volts of each other.  It turns out that I forgot to measure the voltage drop across the jumper wire that was connected between the driver's station White and Black terminal points.  I just included the voltage drop associated with that jumper along with the rest of the Ring Bus voltage drop.  That "heavy duty" alligator clip test lead was the weak link and accounted for approximately 90% of the 0.220 VDC voltage drop from the drivers station to the power taps.  

The corrected measurements reduced the total voltage drop from power supply to any point on the track to 0.442 VDC or 2.46% of 18 VDC.  The need to correct the test results because of a faulty test lead shows how important controller lead wires and alligator clip connections are in determining how much power actually gets from the power supply to the car.

Hopefully I have provided sufficient information to allow you to perform a voltage drop test on your track.  Or, if you are designing a new track, how to plan the wiring to reduce voltage drop to the absolute minimum.  As a minimum I would recommend #10 AWG from the power supplies to the drivers station panel and #14 AWG from the drivers station panel to the power taps.  I also recommend that the under table wiring be kept as short as possible while providing multiple current paths to any power tap.  I would also recommend leaving out (or removing) things you may not need.  For example my TKO track has reversing switches installed.  The previous MaxTrax layout ran well in reverse and the reversing switches were carried over from it to the TKO.  I should have removed the reversing switches after the track was tested in reverse and I decided that it would never be raced in that direction.   The TKO had a voltage drop of approximately 4.5% from power supply to the midpoint between jumpers.  The reversing switches probably contributed 1% to that value.

I would also recommend not just taking resistance measurements as volt-ohm meters are notoriously inaccurate at low ohm values.  This is true of both the older VOMs and today's Digital meters.  The most accurate way to measure resistance is to pass a known current through the item, measure the voltage drop using a DVM and calculate the resistance (Resistance = Voltage / Current).

Based on the above there is no reason that a continuous rail track should have a total voltage drop from the power supply to any point on the track of greater than 4%.   This criteria is based on testing of two continuous rail tracks that were wired to minimize voltage drop.

The benefits of testing are learning from the experience and detecting and resolving potential problems.  Your track may have a higher voltage drop than expected leading you to add power taps or to make other changes.  I took great care to minimize wire lengths and to double up wires when possible to reduce voltage drop.  The wires from the power supply panel to the drivers station panel consisted of two #10 AWG wires in parallel.  I had the wire and decided to go for it.  Doubling up these leads didn't make much of a change but every little bit helps.  Likewise, I didn't use just mechanical connections but soldered wires together where possible.  A screwed, crimped or twisted connection can still have some resistance or may develop resistance over time depending on the environment.  A good mechanical connection that is also properly soldered remains a zero ohm connection.  I also used less than 50 feet of #14 AWG wire per lane.  This 50 feet includes both the drivers station and the under table wiring.  I have seen a four lane 4 x 16 foot track that had over 100 feet of wire installed per lane.  Why?  I have no clue but it is what the builder did.  I peeked under the table and saw wire going everywhere!  Finally, my under table wiring was configured such that there were multiple parallel paths to any point on the track.  Multiple parallel paths reduce the resistance (and the voltage drop) to any point by approximately 50%.  It can result in a very big gain for a very small increase in cost.

 

Updated June XX, 2020

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