Looking for wire size for 100 amp service? If you are trying to set up a 100 amp conduit circuit, you will need to make sure to choose the right wire size. 100 amp circuit at 240V can handle up to 24,000W of electricity.
Power = 100 Amps × 240V = 24,000W
In the US, we have an AWG gauge system for wire sizes. These wires range from the biggest 0000 AWG wire with 230 ampacity to the smallest 40 AWG with less than 0.1 ampacity.
With 100 amps sub panel wire size, we get a number of different questions, such as:
- What size wire for 100 amp service?
- What size wire is good for 100 amps?
- What size wire for 100 amp service 100 feet or 150 feet away?
- What is the 100 amp breaker wire size?
Many homeowners use smaller #4 AWG and #2 AWG wires for 100 amp service and wonder why they fried their sub panel. To avoid frying your circuit, it’s vital to account for two key factors that come into play here:
- 80% National Electric Code (NEC) requirement (NEC 220-2 Code).
- Distance from the sub panel due to the voltage drop (NEC 310-16 Code).
We will use both of these factors when determining what size wire you need for 100 amp service.
Here is how you adequately size a wire for 100 amp service:
First, check the wire gauge chart here. You have all the AWG copper wires and their ampacities listed (at median 75°C or 167°C temperature). From that chart, you can see that we have 4 wire sizes with amps in the vicinity of 100 amps. These are:
Now, it seems that a #3 AWG wire with 100 ampacity is the perfect 100 amp wire size. That’s a very common mistake. In fact, the most appropriate wire size for 100 amp service is the #1 AWG wire with a 130 amp median capacity.
To understand why this is, and what wire size you need for 100 amp sub panels 100 or 150 feet away, let’s look at the what the NEC code requirements are for 100 amp wire size:
100 Amp Wire Breaker Size With 80% NEC Requirement
As you can see from the chart above, these are the wire ampacities in the vicinity of 100 amps:
- #4 AWG can handle 85 amps.
- #3 AWG can handle 100 amps.
- #2 AWG can handle 115 amps.
- #1 AWG can handle 130 amps.
Here is what NEC code says about 100 amp sub panel wire sizes:
Maximum loading for any branch circuit is 80% of the rating of the circuit for ampacity of wire for any load. (NEC 220-2)
This is a safety measure. You never put the circuit under 100% ampacity; at most you wire it to hit 80% ampacity. For our wires, this is what the actual ampacity is:
- #4 AWG has 85A ampacity but should conduct at most 68 amps.
- #3 AWG has 100A ampacity but should conduct at most 80 amps.
- #2 AWG has 115A ampacity but should conduct at most 92 amps.
- #1 AWG has 130A ampacity but should conduct at most 104 amps.
As we can see, the #3 AWG wire might have 100A ampacity but it should only be used in circuits that require 80 amps or less.
With the 80% NEC rule, the best wire size for 100 amp service is #1 AWG.
Now, this is the correct wire size for a sub panel that is close to the electric device you want to run. What is the sub panel is 100 feet away? You will need a bigger wire, and here’s how you determine the 100 amp wire size for sub panel 100 feet away:
Wire Size For 100 Amp Sub Panel 100 Feet Away
With distance, inevitably the voltage in the circuit drops. This drop is insignificant at low distance (less than 3%) but at longer distances like 50, 100, or 150 feet, the voltage drop is significant enough and you will have to account for it.
To counter the 100 amp voltage drop, you have to increase the amperage. The key question here is how much you have to increase the amps to counter this voltage drop.
NEC 310-16 rule tells us that, roughly speaking, we need to increase the amps by 20% for every 100 feet of distance from the sub panel.
Here’s what that means for a 100 amp sub panel 100 feet away (we have to account for 2 things):
- First, we need to account for 80% NEC code as we did before.
- On top of that, we need to account for this 20% voltage drop due to sending electricity 100 feet away.
Here how we determine the ampacity we need from the wire:
Account for 80% NEC code: At 0 feet distance, we need 100 amps. This 100 amps should be at most 80% of the ampacity of the wire. If 100 amps is 80%, what is the 100% wire size?
Min. Wire Ampacity = 100% × 100A / 80% = 125 Amps
If the sub panel would be 0 feet away (very close) we would need a minimum 125A wire.
Account for 100 feet away: Our sub panel is 100 feet away. That means we need to boost the amps by an additional 20%. How much does that get us?
Min. Wire Ampacity (100 ft away) = 125A × 1.2 = 150 Amps
That means that we need a wire with at least 150 ampacity to deliver 100 amp service 100 feet away. If you check the wire size chart (you have the link in the introduction), the #0 AWG wire (also known as 1/0 AWG wire) has 150 ampacity. That means that #0 AWG wire is the perfect size wire for 100 amp service 100 feet away.
Using this kind of calculation, you can pretty much determine which breaker size you need for 100 amp service at any distance. To illustrate, let’s look at 150 feet distance:
Wire Size For 100 Amp Sub Panel 150 Feet Away
Alright, as we have calculated before, we need at least 125 ampacity wire to handle 100 amp current. Now we need to add the 30% amp boost (+20% per every 100 feet, hence +30% for 150 feet) to these 125 amps:
Min. Wire Ampacity (150 ft away) = 125A × 1.3 = 162.5 Amps
For sending 100 amps current 150 feet away, we need a wire with at least 162.5 ampacity. If you consult the wire gauge chart, you see that the #0 wire can handle 150 amps. It’s too small.
100 amp service 150 feet away from the sub panel requires #00 AWG wire (also known as 2/0 AWG wire). This wire has a median ampacity of 175 amps; more than enough than the required minimum 162.5 ampacity.
We hope all of this helps. If you haven’t found the answer to your 100 amp question, you can use the comments below and we’ll try to help you out. For 100 amp aluminum wires, you can consult the aluminum AWG wire chart.
It might also be helpful to read some of our related articles on similar topics, including:
- 20 amp wire size.
- 30 amp wire size.
- 40 amp wire size.
- 50 amp wire size + sub panel at a 100 feet distance.
- 60 amp wire size + sub panel at a 100 feet distance.
- 200 amp wire size.
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Your article helped me tremendously. Now I know exactly what I need and the article made it so easy to understand the formulas better than any other info I’ve came across.
Thank you for the knowledge;
Craig
Thanks, Craig, it means a lot! 🙂
The answer is #4 copper. 310.12 answers it. 100 amp service is not a 100 amp load. You did the math as if you were powering one big 100 amp load. Services are done differently for the math. Just go to 310.12 in the 2020 NEC. You can round down on services because they are a collection of small load that very rarely all cycle at once. I am a Master Electrician with 28 years experience.
*whack* nuff said.
Thanks for your information. I looked up the NEC code and you are correct. For a 100 amp service 4 GA copper wire, or if you use Aluminum wire then you would have to go to 2 GA wire.
yes! 310.12 B states the 83% RULE for feeders and services.
Doing the reverse math
#4 CU 75 rating= 85 Amps
85 amps X .17 (allowance for feeders/service entrance) =14 amps
85 amps + 14 += 99 AMPS
still not able to carry 100 amps so you need to refer then to 240.4 (B)(1) which states you are allowed to go up one breaker size so long as it’s not a branch circuit and doesn’t go under the OCPD ammount. BOOM
Steve can you use #4 aluminum for a 100amp service at 150ft from the house to the barn
I appreciate the easy to digest format. Making knowledge free to all is a noble pursuit, thank you for this – I’ve always been curious about wiring gauges, now I know what I’d need if I wanted to install a sub panel!
Kind regards,
George
Hello George, thank you for this, it really means a lot.
This was so helpful !!!
Thank you
going with the 2/0 copper for 100 AMP sub-panel 150 feet away, what size ground wire would i use? 4 or 6 AWG ?
Hello Steve, thank you. 2/0 copper wire is the right choice here. According to the grounding electrode conductor chart for AC systems, you would couple 2/0 copper wire with 4 AWG copper wire or 2 AWG aluminum wire. We need to explain this chart in a separate article, prompted by this question. I guess that a lot of people need to know what size ground wire goes with the main wires. Thank you again for an idea.
EXCELLANT GUIDANCE & TEACHING!
THANK YOU VERY MUCH!
Can I use #4 copper to feed a 100 amp ATS from main panel 100 amp breaker at 8 feet? Thank you.
Hello Brett, #4 copper wire has a median temperature ampacity of 85A. So that would be too thin; #1 AWG wire with 130A ampacity would be the appropriate choice in this case. Hope this helps.
You have basically described how stupid the writers of the NEC are. Why make a table and then apply an 80% rule? Why not just make a table that reflects the proper values? And the writer of the above article has embraced this foolishness whole-heartedly. Where has the common sense gone?
Hello Wayne, well, the ampacity of a wire is a measured quantity. A 100A wire is a 100A wire. NEC specifies how we use it; to protect against fires, it’s advisable to load it up to 80% instead of 100%. NEC guides are more than 100 years old; maybe they will modernize in a way that they would be easier to understand for all people.
Preface: I was trained as an electrical engineering, but I am not a practicing engineer nor am I an electrician. That said, I strive to do good research and cite good sources. So please correct any mistakes you find. I’d like to share what I’ve learned and learn from others.
Preface: Engineering can be thought of applied physics. The “laws of physics” can be expressed as equations, and are tested to verify they are consistent with reality. These formulae can often be complex, so engineers sometimes use simpler versions for particular situations. Engineering as a discipline puts a large emphasis on real-world behavior, cost, efficiency, and practicality.
All materials have various failure modes. For example, excess current can cause failure of a wire. The failure can manifest in different ways, but ultimately failure is defined in some specific way to characterize when a material cannot fulfill its desired purpose.
Failure modes can be tested under controlled conditions using a defined protocol.
Once failure modes are sufficiently well understood for particular materials, some standards bodies will also share particular formulae for particular conditions. Provided that your application fits the specified criteria, you can use the formula to check to see if your application falls into a safe operating zone.
When it comes to wire ampacity ratings, there are specific definitions of testing protocols. [This National Fire Protection Association page: Determining Current Carrying Capacity of Conductors](https://www.nfpa.org/News-and-Research/Publications-and-media/Blogs-Landing-Page/NFPA-Today/Blog-Posts/2021/07/21/Determining-Current-Carrying-Capacity-of-Conductors) is a good place to start.
I’d like to add something that I don’t typically see mentioned. Understanding failure tells only part of the complete story as far as reliability goes. It is often important to consider *degradation* as well. According to [this NIST article](https://www.itl.nist.gov/div898/handbook/apr/section4/apr423.htm) “When failure can be related directly to a change over time in a measurable product parameter, it opens up the possibility of measuring degradation over time and using that data to extrapolate when failure will occur. That allows us to fit acceleration models and life distribution models without actually waiting for failures to occur.”
Hi David, thank you for your valuable input. Degradation is an interesting insight that is mostly missing because it’s really hard to measure the level of degradation over time. That’s why is usually omitted but it is still really relevant in real-life practice.
What do you do when the the neutral/ground bar in the main panel has a maximum wire size of #4? The 100A QO breaker isn’t a problem as #4 is the minimum wire size.
Hi there, well, the #4 wire won’t be able to handle 100 amps. You would need a main panel that allows for a bigger wire. 100A is quite a lot; if you run it to a 120V circuit, it would produce 12,000W of power. That’s quite a lot; not something standard-issue main panels were designed for.
With electricity rates going up sky high would under amp wiring cause more usage in watts as charged by electric co.?I have 100 amps feed on #4awg copper about 30 feet
Hi Doug, you should follow the NEC code with all the wirings. Under amp wiring will reduce the amps but the electric device will still want those amps; this can lead to a fire and is strongly advised against. Even if the amps are lower, the electric device will need the same power for the same output. The best thing here is to set the electric device to a lower setting to save electricity (you will cut the output as well, there is no cheating physics).
Installed a subpanel 5′ away from mainpanel. Had #1 wire. Bought a 100a CH breaker. Wire does not fit into breaker. Says on the breaker up to #4. What are you supposed to do in that situation? Can I up size the breaker?
Hi Mike, alright, the #4 copper wire has an 85A ampacity. That’s quite a lot of amps; even at the low 110V voltage, you can power 110V × 85A = 9,350 Watts with that. If you don’t intent to install 10kW+ devices on that subpanel, the #4 wire would be sufficient here.