Wire Size for my kiln, What should it be?

[Bill Kielb]

Here is an interesting thought and possible solution. From time to time we all respond that if the resistance of your kiln elements increase by 10% its likely time to replace them. And then there are the eventual questions about wire size for my kiln. Often the answer is move to the next thicker wire when in doubt because of voltage drop. Your typical electrician will pick wire limiting your voltage drop to 3% so is that ok for kilns? My thought was, for the kiln owner these two are actually related by energy and 3% voltage drop is likely way excessive for kiln owners.

Lets present it this way, a 10% increase of resistance in your elements translates to just over 9% decrease in wattage or heating energy for your kiln. When elements wear to this point its often difficult to achieve temperature in glaze firings in a reasonable time. Often kilns begin to over fire as the elements wear just because the time to fire becomes excessive and its time to replace.

So a 9% decrease in energy is usually bad, time to get new elements. So if I allow my electrician to wire for 3% voltage drop, that is a direct reduction in kiln energy of 3% or basically you are starting out with elements that act like they are 1/3 worn out. 

Maybe kiln Owners should limit their voltage drop to more like 1%, or at least as little as practical. Look at it this way, any voltage drop in the wire will produce heat in the wire, not the kiln. So if you consume 100 watts in the wire, it will only heat your pipe and walls and not get to the kiln.

I added some voltage drop tables below for given distances. Maybe we should be telling our electricians no more than ……….x% please. The rule he or she will use (3%) is likely just too much for kilns  and doesn't start until we exceed about 150 feet.

How much voltage drop is acceptable for you?

The charts below are for single phase Only (240 volts)

 

The chart has been removed, see below for a more conservative chart that includes Aluminum

 

 

 

 

[liambesaw]

Cool! Glad to see I'm right at .41% drop, I bet there are a lot of people who dont even know the amount of wire from their breaker to their kiln.  To be honest if I hadn't put it in myself I'd have no idea.

[Bill Kielb]

Thought I would post these charts to make it easy for others. Your design and install worked well and was the result of your effort and planning. 3% is just too much capacity to give up IMO. Hopefully this will help others with this question or get them thinking a bit

[Bill Kielb]

9 hours ago, Rockhopper said:

@Bill Kielb Thanks for doing the math, and sharing it!  I don't see it mentioned - but I assume your tables are based on copper wire, since that's what is used in most situations - but, especially with larger wires and long runs (ie sub-feed from house to garage or other out-building), aluminum is still used for cost savings.

A common example might be #2 aluminum SEU, running from house to garage.  Could you give us a comparison of the #2 aluminum vs #2 copper for a 100ft run ?

Brilliant!

Thanks for the catch, after reading your post I decided to include the Aluminum (Great Idea) and along with refashion the charts to include 90 C wire, bundled or in a raceway to be as conservative as practical. The Aluminum is significantly more resistive and as I recall in AWG we would simply go two sizes larger as a safe rule of thumb back in the day. Outside AWG (MCM) this did not apply and simply get out the calculator. Now I would guess get out the calculator no more rule of thumb.  For folks with kilns, 3% seems way  too much though.

So the new charts only show a multiple of ten feet and the user needs to double for twenty or triple for thirty feet and so on.

To answer your last Question: #2 Aluminum is nearly 60% more resistance than Copper.  So copper resistance per foot X 1.6 gets you close to Aluminum resistance per foot. To directly compare the two from the table pick an operating current then multiply by the number of ten foot increments.

So 50 Amps on a 100 foot run of #2 AWG

• Copper = .9%,    2.05 Volts
• Aluminum = 1.4%,    3.35 Volts

[neilestrick]

Just a safety note, since aluminum wire has entered the conversation. From the L&L Kilns website:

1. Do not use aluminum wire on the final connection to the kiln.
2. The specific reason particular to kilns is that the wire tends to get hotter near the kiln than it might going into some other types of appliance.
3. Being a resistive load, there is constant heat being generated by the conductors for quite a few hours. When aluminum wire gets hot it accelerates oxidation. Aluminum oxide is a resistor; copper oxide is not as much. If the connection at the terminal board gets oxidized it will really heat up - to the point where it could cause a fire.
4. Note: Depending on local codes it may be OK to use aluminum wire to your sub-panel - as long as that wire is not exceeding its temperature rating while kiln is firing on full power for an extended period of time.

[Bill Kielb]

@neilestrick

Yeah never a fan of aluminum wire. Still have old tubes of penetrox somewhere trying to keep the aluminum terminations from corroding and the thermal changes from affecting virtually all terminations. Just experience though. Don’t fool yourself, inductive loads are difficult as well. If you have aluminum check it as often as practical. Several insurance analysis have shown as little as 50 watts generated at a connection can start a fire.

Still 3% voltage drop even for copper significantly affects kiln power.

[CharliesMug]

question: I want to install a new circuit from the 200amp panel in my house to a kiln at 110ft away. The kiln uses 48amps. I will run about half of the circuit in the crawl space attached to framing and then go underground to kiln. I plan to use a 60amp breaker and 4/3 romex wire however that romex is expensive and shouldn't go in conduit. Should I use 2 awg aluminum UF for the whole run instead? any suggestions for making this circuit ? it is 240volt also. Thank You, Tom

[liambesaw]

5 minutes ago, CharliesMug said:

question: I want to install a new circuit from the 200amp panel in my house to a kiln at 110ft away. The kiln uses 48amps. I will run about half of the circuit in the crawl space attached to framing and then go underground to kiln. I plan to use a 60amp breaker and 4/3 romex wire however that romex is expensive and shouldn't go in conduit. Should I use 2 awg aluminum UF for the whole run instead? any suggestions for making this circuit ? it is 240volt also. Thank You, Tom

Kilns require a full copper circuit.  4 awg THHN for the hots and a 6 for the ground?

[neilestrick]

1 hour ago, CharliesMug said:

Should I use 2 awg aluminum UF for the whole run instead? any suggestions for making this circuit ?

What you should use depends entirely on the electrical code for your area. We can make basic recommendations, but ultimately it should be done to local codes. If you don't know what those codes are, then talk to your municipal building department or go onto your town's web site and see what you can find there, or hire a licensed electrician to do the work.  If you choose to do the work yourself, it would be a good idea to get a permit so someone can verify that you did it correctly and safely. I get that you're trying to save money, but it's really important that it's done correctly and up to code, both for your safety and so there are no issues with insurance claims if there's ever a problem.

I can tell you that you should not use aluminum wire for any of it.

[Bill Kielb]

2 hours ago, CharliesMug said:

Should I use 2 awg aluminum UF for the whole run instead? any suggestions for making this circuit ? it is 240volt a

I would never use aluminum, ever. Too many issues especially at points of connection. . To limit your run to 1% drop it looks like you need #2  or #1 copper. Real distances start to mean something quickly here. This will very likely be superior to code but kilns start out with about 10% excess capacity brand new so every little bit affects the useable life of the elements. We put the chart up as an aid for when your electrician uses the 3% rule of thumb, The idea was kiln owners can specify their preference for  1%. Electricians are not necessarily familiar with kiln operation or the limits of current kiln design so the chart gives the kiln owner a level of knowledge that should be used to advise your electrician of the 1% or less requirement and compare his proposed wire size.

So to Neil’s point, you should always follow your local codes which will include requirements for embedment depth, grounding at both ends, acceptable materials, mounting, circuit protection, mounting heights, underground exit protection, safety switch requirement , etc...... This is really what your local electrician is there for. 

lastly UG rated cable is generally cheaper than raceway but does not allow for future load expansion nor do we know for sure locally it is allowed.

[whocares]

charlies mug...did you ever get your kiln wired? what did you end up using?

[oldlady]

the writer of that post joined, asked the question and has not visited the forum since that  day in november.    sadly, some people do not come back after their first question.