What Gauge Wire Do I Need?

What did your electrician say you needed? minimum would be #4cu.

He said 6 gauge. My heating contractor says 4 gauge. What is cu?

I looked it up. Copper.

I always error on the conservative side. Especially if there is a compressor or other motor load required to start. The lower the voltage drop the smoother it runs. (And the lights stay bright.)

Sorry. I read this links but it's over my head. Mike

63A is approaching the limit of a domestic supply in the UK. Most homes have a 100A supply fuse. The tails from the connection unit through the meters to the dis board in most homes are 25mm².

British standard 7671, as always, would provide the full algorithm for working it out.

I would go with the heating guy, he does this for a living and should know what this system needs. The electrician is going by the code for the lode that you are working with but may not know the quirks of the geothermal system. Even if #6 is adequate for this operation #4 is safer.

Check the installation manual.

Further, if the installation is connected to something that is outside in the UK, it needs to be inspected and tested in accordance with Part P of the building regulations and an installation certificate issued. <...not sure if the electrician really knows what he's doing given some of his other antics...> Consult a qualified electrician for the testing task. If the individual currently doing the job is not appropriately qualified and the installation is not subsequently tested by one that is, then the risks are:

• Invalidating the home fire insurance.
• Risking a fine from the local authority under the Building Regulations.
• Not being able to sell the property until the faulty installation has been corrected.

Since, you don't have all other data, these are the assumptions for the calculation:

1. The ambient temperature 40 °C

2. Cable run stand alone in the raceway

3. Cable Type: Heat Resistant thermoplastic (THHN), 90 °C temperature rating

4. Load is continuous (operates more than 3 hours at a time)

Per NEC Table 310.16, you can select #4 copper conductor cable which has the ampacity 95 A. With temperature de-rating, the ampacity is 95 x 0.91 = 86 A.

Because of the continuous load, 80% ampacity is considered, so 80% of 86 A is 68 A which is good enough for your load current 63 A.

Per NEC Table 8, cable resistance is 0.31 Ohms / 1000 Feet

Voltage drop % = (2 x 63 x 0.00031 x 45) x 100 / 240 = 0.73% which is permissible (below 3%)

So, AWG #4 cable should be fine for this load. However, for more precise selection, you have to do the coordination between short circuit current and cable damage curve.

Size 6 should be ok if load is not continuous, but better to go for size 4.

-MS

Electrician relented and has agreed to #4. Thanks. Mike

Mike,

If it took this much prodding to convince your electrician to use the correct wire then perhaps you should get a new electrician. I hope he isn't a relative.

You are absolutely correct. It gets worse. Now he claims to have put the #4cu in so I took a sample to the electric supply store (the wire isn't marked) and it was actually #6cu. Now I'm wondering where I can file a complaint. I'm finished with this guy. Mike

Have you paid him for the job?

Now I'm wondering where I can file a complaint.

you could start with the city inspection dept, Electricians Union(probably not much help), City License Dept, Better Business Bureau

Hopefully you can stop payment on the check or even file a complaint with the local cod department. If he signed off on the work permit, then he will be in violation.

Just make sure you did not take the ground conductor in for a sample. The ground should be allowed to be one gauge thinner.

This all assumes you hired a licensed electrical contractor and he obtained a legal permit for the work. An inspector would have a lot of problems with this, did one see this and sign off on it?

If there was no inspector and/or the guy is not a licensed electrical contractor and/or did it without a permit, welcome to the woes of trying to do things on the cheap! You will have no recourse other than stopping payment on a check, and believe it or not, even as a non-licensed contractor he can put a mechanics lien on your house and end up owning it. If you sue him, you will likely be fined for installing without a permit (and he knows this).

Hello mikesneill,

This Electrician is only looking out for your safety! If you can find a long enough piece of cable you may find some standards printed on one of the flat sides?

We don't know where you live. That helps in determining who to speak to.

Unfortunately guys like him make other hard working electricians look bad. It also causes distrust by those who would hire us.

That's part of the issue. The remainder is to get the correct size installed.

Without knowing where the installation is, in the UK, the local Trading Standards office can be contacted for advice. If necessary, they will launch a formal investigation with a view to criminal proceedings. The local Trading Standards office can be found in all telephone directories, among other places.

You also have a problem with the construction of the cable. The fact that it is not labeled, may mean that it is a sub-standard cable. By ANSI (American National Standard Institute) the cable is required to be marked with wire size, manufacture, temperature rating and wire type code. This information can also be found in the NEC (National Electric Code).

Check it out. Good luck electricmel

Its called "Shopping around"

Get quotes from several electricians for the job your trying to accomplish.

I work on about 10A per square mm of wire, so yours would be about 7mm^2 wire, next is, how much voltage drop your willing at accept for the 2 runs of wire (there and back) say 240 at your dist panel, and 235 at your termination point, thats 2.5V per length of wire, but I cannot say that would be the drop on 7mm^2 wire, could be different due to the application/wire used (Aluminium or Copper)

But as I'm not "qualified" sparky, I cannot give a accurate value of whats required.

The one thing that you need to ALWAYS keep in mind when you are installing wire to carry larger amounts of current is to be aware that efficiency is always controlled by 1) the guage ,2) the type metal of the conductor (copper always carrys electricty better than aluminium and requires a smaller guage), 3) length of conductor and most importantly, that you understand that line losses are lost as heat by the mathmatical equation: (Power losses in the form of heat (wattage) is a function of the current (amp load) squared (in amps)(Square)=Resistance .

This means, in effect that the koss in energy is a function of the line resistance and always lost to varying degrees in the form of heat. So, never install your wire size on smaller gauges and longer lengths or you will pay for it at the meter in lost energy. Generally, you should never want to see the effect of "line drop' (lights dim)when the air conditioner comes on.

Hello mikesoneill,

I do not know where you are and cannot recall at this very moment the UK standards specifics. However I know they have a cable for the 'mains' power in, and which goes to all sockets on the ring.

The second of the two type of standard cable is thinner and used only for lighting.

Take care..........

I'm in the US. I now have another electrician coming in the next couple days to check everything the first guy did and fix it. I'm ticked off but I don't need the stress of dealing with the first guy any further. I got the first guy through a professional electricians association in the Philadelphia area which I thought made it ok. I guess I was wrong. The only way to hire a contractor is through recommendations of people who have dealt with him over a long period.

Hello mikesoneill,

No name of course, but can you tell me/us how you get on with your new Electrician please?

I suspect there will be no problems.

Good luck.

Sure

Hi mikesoneill,

I thank you for the return post, and look forward to seeing what you are doing.

Good luck and thanks once again.

The cost of the 4ga wire should be roughly 25% more than the 6ga. Unless the conduit that you are pulling them through will not allow, use the 4ga.

Hi bob c,

The amount of wire talked about by the OP is hardly enough to wire a small room with a Ring-Main.................. This is to give you some idea of scale. So any mention of 'cost' whether 25% cheaper for 4ga wire or not, is irrelevant.

As you say if the conduit to be used is already fitted, it may not allow thicker than 4ga, the stuff I would use is the 6ga.

As I have said before, safety first!

Take care.

Two ways to figure from the National Electric Code (USA). I am not familiar with metric wire sizes, on this side of "the pond" we use wire gauge. Allowable ampacity is shown on tables in article 310. Table 310.16 is for up to 3 current carrying conductors on a cable or conduit. It shows that #6 copper is good for 75 amps if it is a conductor rated at 90 deg C. The catch is in the continuous load factor. According to Articles 210.19(A), 210.20(A), 215.2(A), if the load CAN exist for 3 hours or more, then the connected load must be multiplied by 1.25. That puts you at 78.75 amps. That requires #4 copper. So the question is, are there controls that will prevent the heater from running for more than 3 continuous hours? Just, "It won't run that much" does not fulfill this requirement. -- JHF

Hello Circiut Breaker,

Just read your post with interest.

Just to say that is has to be better to 'over engineer', so as to speak, and be safe, than to guess with the gauge which might be too small and cause the place to burn?

I know I would often forget and leave my water heater (2.5 KW) on for days. And there is things like doing a day of washing. At the time I had three different machines so there would be at least two on more or less full time for up to 12 hours. And not infrequently I could be machining in my shop for several days. Which is yet another draw on the power and those 'tails' can get kinda warm?

Take care my friend..............

Take a look at NEC Article 110.14(C) The 90 degree rating for conductors is only applicable if all the termination points , etc. are rated for 90 degrees C. Since there are no 90 degree C terminations available, the 75 degree C rating shall be used. I see this mistake being made on a regular basis. Think of it as the weak link in a chain. Basically the 90 deg rating is only there for derating of conductors. And the derate cannot result in current carrying capacity which is greater than the 75 degree rating. There is no derate here because there are only two current carrying conductors. I also calculated the voltage driop with both #4 and #6 and the drop with #6 is 2.65 volts (slightly more than 1%) and for #4 it is 1.67 volts (less than 1%) both of which are acceptable per the NEC fine print note. Voltage drop one way or the other for this circuit is not a code requirement anyway. There is simply less power delivered to the load. Fire pump installations have a code required voltage drop calculation as specified in article 695. Generally speaking voltage drop is not a code requirement. My problem is if the guy said he would use # 4 AWG and didn't, thats a big problem. Find his truck and give him four flat tires and leave him a note saying , something like "I wanted four, and I got four."

Generally the conductor is sized at 125% of the load being supplied. 63A would come to a conductor rated at 79A. The amp rating for the conductor is determined by the temperature rating of the insulation. #6 Cu 90 deg C is good for 105A, and the same size with a 60 deg C rating is good for 80A. #6 Cu resistance is 0.4 ohm/1000 ft. Since your running 45 ft you would figure 2*45*0.4/1000*63 or 2.3V drop at 63A. This is within 1% of the supply voltage so #6 would be the smallest you can use. Going to #4 would give you a little less voltage drop, larger than that is just going to cost quite a bit more with little benefit.

When you go to larger wire sizes the Cu cost goes up quickly and you find more aluminum conductors offered. Personally aluminum is fine for many things but I don't like it for electrical wiring. You have to prepare the connections carefully and, after tightening the lugs, go back and retighten them again, aluminum conductors are very soft and they tend to give under compression over time so it's good to double check them after installation for a couple of days to make sure they're tight. Make sure the lugs on the breaker and the heat pump are copper/aluminum rated if you decide to go the aluminum route. Also, with aluminum you'll need to go up in wire size for the same amp load.

The second electrician came in and reviewed the work that was done. It will cost me $600 to have the first guys multiple mistakes corrected. There are actually two under sized wires, three under capacity 240v breakers, an improper connection and other things. It will all be corrected on Monday. I'm thinking about taking the first guy to small claims court to get my $600 back. Does that sound appropriate? Mike

Hello mikesoneill.

If you are not exaggerating, and your electric's are as you say, you should sue and or report the Company. Electrical stuff cannot be installed so badly.

With plumbing you may get hot instead of cold water. In Electric's you get a shock or a fire.

I wish you luck whatever you do!

Hello bob c,[p]

Very good advice! Those pictures can be used later in necessary as proof!

GA to you Sir.

Yes, it does sound appropriate.

Second electrician completed his work today. Wow. I should have been an electrician. Big bucks. This one apparently knows what he's doing as he came well recommended. UL inspection still needed. Mike

Hello mikesonneill,

I wish you luck and hope all is OK in the future with your electric's!

Thank you and everyone else who took the time to discuss this problem. Mike

Hello mikesoneill,

Cheers mike, hope all works well for a long time! ;=)

Take care

#6 WILL WORK, SHOWING ABOUT A 10VOLT DROP IN 45'. PROVIDING YOU START AT THE PANEL WITH 240VOLT. THIS WOULD OFFER YOU 228VOLTS, OR 5% ALLOWABLE WORKING VOLTAGE DROP AT THE LOAD SITE. .

Does that mean that I'm paying for power that I never get to use because of the voltage drop? Is a larger wire more efficient from a cost of electricity perspective? Mike

Does that mean that I'm paying for power that I never get to use because of the voltage drop?

You get to use that power. It is heating up the space the wire is running through. As voltage drops, amperage will rise. A larger wire is always a more efficient current carrier. But if your run is 45 feet, and you would only need full current when all of the heaters and the air conditioner are on at the same time, It might not be practical to plan for maximum current usage.