The Electrical Code governs the size of conductors that must be used to safely carry a given amount of electrical current. These requirements take into account voltage drop, conductor temperature, inrush current to motors and other factors such as the increase in current required for fan cooled transformers. Ampacities given for cables installed underground also take into account whether they are direct buried, in duct, the number of cables, how they are laid out and depth of burial. 

At the moment no installation Code has specific requirements for energy efficiency.  Not only is conserving energy good for the environment, it makes good economic sense.  

With deregulation of the electrical energy supply, there is speculation that energy costs will rise.   We pay for all the electrical energy consumed on the customer side of any metering equipment.  All energy wasted in electrical distribution equipment down stream from the meter is paid for along with the useable energy. 

Under most conditions Code ampacities are quite conservative. This means that a cable with a 90°C rating when run at the ampacity given in Column 4 of the Code will operate at a conductor temperature lower than 90ºC.  Conductors are probably operating closer to 75°C.  In the case for NMD90 cables we chose size #12 AWG in place of 2C #14 AWG.  Ampacity for 60, 75 and 90°C rated #14 AWG copper conductors is 15 amperes, so in the case of NMD90 it's operating closer to 60°C.  This assumes that, as Tables 1,2,3 and 4 specify, the ambient air temperature is 30°C.

 Power losses in low and medium voltage cables are virtually all conductor heating, or I²R, losses. In some cases where there’s a non-ferrous metallic layer over a single conductor cable, such as with Corflex RA90 or Teck90, and this layer is grounded at both ends significant losses can be generated in this layer in larger conductor size cable.  In most cases however, this layer is grounded at one end only and losses here are minimal.

Even though the Code is conservative in it's ampacity rating, significant energy savings can be obtained by increasing a cable's conductor size over that required by the Code. This will increase initial cable and installation cost but the payback period can be quite short. This is due to energy savings obtained through reduced I²R losses in these conductors. These savings will continue for the life of the cable, which could be fifty years or more.

 We would like to show you examples of how increasing the conductor size above what’s required by the Code can save you money in the long run and the approximate payback period of the cable cost for doing this. Our examples will be for cases using single conductor cables, for 3C or 3 X 1C cable and for a residential wiring case using 2C NMD90 cable. These cases could be applied to industrial, commercial, large and small residential applications. 

In our examples we used the fictitious cost of power of 6.0 cents per kilowatt hour.  If your rate is higher than 6.0 cents per kilowatt hour then your payback period will be less than our calculated value. If it’s lower then payback will be longer.  Here are the guidelines we used in our examples :

1. There are a total of five cases using 1/C and 3/C 90°C rated copper and ACM aluminum conductor cable.  2/C NMD90 is 90°C rated copper conductor only. 1/C cases apply to single conductor cable in free air (   Table 1 or 3 ampacities ).  3 X 1/C cases apply to three single cables in conduit or three conductor cable in air ( Table 2 or 4 ampacities ). 2/C  NMD90 cables had  ampacity from Table 2.

* Based on an operating time of  50% ( 4380 hours / year ) at the current and temperature shown. For the NMD90 case, on time was 10% ( 876 hours / year ).

** Based on the energy saved / year / kilometre multiplied by the 0.060 $ / kWhr shown above. In the 3 X 1/C cases this is the cost of energy saved by one of the three conductors. In the 2/C NMD90 case the energy saved that is shown is actually twice that in one conductor. This is because all of the current that goes out on the power conductor returns on the neutral conductor that's in a 2/C NMD90 cable.      

  *** Cable cost difference is for illustration purposes only. For our examples this cost difference is for one cable in the 1/C and  3 X 1/C cases and the cost of one 2/C cable in the NMD90 case. You can substitute your own cost difference if desired.

# Payback time is simply the cable cost difference divided by the cost of energy saved per year.

 In our examples we have shown only the time required to repay the cost of cable. The cost of cable compared to the total cost of the installed circuit varies with the type of installation, but it can be seen that using a larger than required cable size has a very short payback period.  Even if you consider the total installed circuit cost and estimate that cable cost is 1/2 of the total installed cost, payback is still relatively short.

 We used a cost of power of 6.0 cents per kilowatt hour. If your cost is higher or lower than this, payback time will vary accordingly

 One benefit of using a larger size cable is that voltage drop is less.  Because of this equipment at the load end operates more efficiently, therefore requiring less current.  This further reduces the payback period.  Another is that the "cost of power saved per year", shown in the table above, continues each and every year.  In fact as the cost of power rises, this amount of money saved will increase accordingly, or another way of looking at it is that the payback period becomes even shorter.

 There are, of course, practical limits on how far you can go.  Equipment on both ends of these cables may not be designed for too large a size.  There may be space limitations along the cable route.  There may be initial / upfront cost considerations.  All of these must be taken into account, but as you can see in the examples we increased the cable size just one standard size and even then savings were quite significant, therefore the payback period was short.

 Nexans can help you with a calculation for your specific case.  Please contact us through our Sales people shown for your area at our website.

 By : D.S.Reith - Applications Specialist

                             - November 18,  2002