I'm installing a 100 amp service in the backyard with a separate meter on a pole next to sub-suface power tranasformer. I have a 100 amp meter box on one side of the pole and a 100 amp braker panel with a 100 amp main breaker on the other side. I'm going to place in it a 15-20 amp breaker for gfi receptacle. What kind and gauge wire should be connecting the two panels. Also wire and gauge from the pole to the transformer 4' away.
Thanks... Bruce

Either #4 copper or #2 aluminum is good for residential 100 amp service.

I want to connect a cooktop requiring 40 amps using aluminium wire. The run is 110 feet. I can replace with a new copper wire or I have two 6 gauge aluminium wires I can use.

If the cooktop is new then you must follow the manufacturer's installation instructions. If ti is not new, can you tell us what the nameplate ratings of the cooktop are? Volts, amps, Kilowatts?

Does your cooktop only require 240 volts or do you also need to supply 120 volts? This is a critical question.

The voltage drop calculations for 115' using a 3% drop will require you to use a #8 AWG conductor listed for direct burial and a 40 amp breaker in your main service panel .

If you use Aluminum conductors, at 115' with a 3% drop, then you would need a #6 AWG conductors that are listed for direct burial and a 40 amp breaker in your main service panel.

A straight 240 Volt cable need only two conductors and a ground conductor, However, if the cooktop requires the use of 120 Volts, then you must use a three wire with ground cable as you need to provide the Neutral.

FYI - There is no appreciable voltage drop for the distance you want to go. Both the #8 (Copper) and the #6 (Aluminum) conductors are the listed size conductors for 40 amp.

Voltage drop is a recommended value. It is not code enforceable but to not account for the loss is folly. You might end up with an amperage so low as to render the cooktop useless.

The cooktop is new and I have not received it yet so I cannot tell you what the nameplate says. However I have reviewed the installation instructions online and they say it requires a 40 amp circuit and 240v current.

Hi Don, excellent answer and confirms research I have done. However, why the reference to direct burial cable since this cable is obviously inside the house. Question 2, can you send me somewhere to get an understanding about the 3% voltage drop requirement? And why is the length of the wire insignificant. I would think that at some point this would come into it. One web site uses the length as a factor in a calculation for awg wire determination.. any advice on using that aluminium conductor. I cannot splice the pigtails to connect to the cooktop and must use some alternative method.

Hey Cay,

How is it obvious that this is inside the house. Your opening post says you are directly connecting to a meter (outside connection) and that the cook top will be in the back yard (which I think means outside). Any time you have cable going underground it needs to be listed for underground use. Even if you were to run conduit from the meter to the cooktop, you would still need to use cable listed for wet areas. So for example, the standard NM-B (Non Metallic -B) or Romex (trade name) cable is not listed for wet areas, therefore you cannot direct bury the cable or run it through conduit that is buried underground.

The NEC recommends a total end to end voltage drop of 5%. Usually 2% for the feeder and 3% for the branch circuit. I prefer to use a 3% value for the feeder since as in this case there is no additional branch circuit footage to add.

The fact that it will be a straight 240 V circuit reduces somewhat the effect of the distance. If this were a 120V circuit, the cable size might be higher.

As for Aluminum wiring, in general Aluminum wire is one size larger then Copper. For example a size 14 AWG (Copper) which is listed for 15 amp would need to be a size 12 AWG in Aluminum.

This is my first time to use the service so I used the first relevant post which I should not have done. Sorry! For confirmation of your info, I plan to use the #6 aluminum wire run for 112 feet which is already installed. For what ever reason I upsized the installation of the stove and only installed a thirty amp breaker on this wire since I was a little uncertain about the safety of AL wire. I plan to just install a 40 amp breaker on the same wire, but I will use no-lox on all connections, and use connectors that isolate the copper and aluminum from each other. Do I need to do anything else?

One other thing, I would like to get a tutorial on the voltage drop issue, 3%, 5. I know the voltage will drop in the wire due to the resistance of the wire. How to calculate this without a formula is a mystery to me and I would like to get knowledgeable on this. Any help would be appreciated.

VD=(2KLI)/cmil

VD = voltage drop in volts
K = K factor, this is approximate resistivity per foot, use 12.9 for copper or 21.2 for aluminum
L = distance one way
I = actual number of amps being used
cmil = wire size in circular mils, you can use chapter 9 table 8 of the NEC to find the cmil of standard wire gages.

You have the distance and K factor, all that is left is to find the cmil of the wire in the table and know how many amps.


Edit- this formula is for single phase, for 3 phase replace the constant 2 with the square root of 3



Max voltage drop is 3%