Alright, figuring out what size you need for 90 amp service is fairly certain. We will have to consult the NEC 310.16 table for copper and aluminum wire capacities. On top of that, we will have to account for voltage drop if we want to have a 90 amp service 50, 100, 150, 200 feet away, and so on.
We’ll start by looking at the typical 90 amp wire gauge. If we want to run a wire for some distance, we will account for voltage drop. Since the math here is a bit complex (we’ll show you have to do it manually), we have prepared these two vital resources that simplify things a lot:
- 90 Amp Wire Size Calculator. In this calculator, you just insert 90 amp wire length, voltage (12V, 120V, 220V, and so on), and allowable voltage drop (3%, 5%, 10%), and the calculator will automatically determine the copper wire gauge for 90 amp service.
- 90 Amp Wire Size Charts. We have calculated what size wire you need for 90 amp circuits at 12V, 120V, and 220V voltages and ranging a wire length from 0 to 500 feet. The results of wire sizes are summarized in two charts; 1st one is for a 3% voltage drop, and the 2nd one is for a less restrictive 10% voltage drop.
First of all, let’s determine the standard 90 amp wire size:
Which Wire Gauge For 90 Amp Service? (+ 90 Amp Breaker Size)
For 90 amp service, we will need a wire that can handle 90 amps. That means that it has to have a median ampacity of 90A or more. This NEC 310.16 table includes copper and aluminum wire ampacity for AWG wires. Here is the screenshot for both copper (left) and aluminum (right) wires:
From these wire ampacity charts, we can clearly see that we can either use:
- 3 AWG copper wire for 90 amp service. This copper wire has 100A ampacity; more than the minimum required 90A ampacity.
- 2 AWG aluminum wire for 90 amp service. Here is ampacity is 90A, which is the same as the minimum required 90A ampacity.
Now, we would use these wires for short circuits. What about if we have to run a wire for 100 feet, 200 feet, or even 400 feet and still want to get 90 amps at the end? We have to account for voltage drop and stick with the NEC guidelines on voltage drop.
Example: Let’s say we want to run a 90 amp 240V wire to a sub-panel 400 feet away. The specified allowable voltage drop for sub-panels is 5%. In this case, we would not use the 3 AWG copper wire. Instead, we would be using a thicker 1 AWG copper wire to minimize the voltage drop and keep it under 5%.
We will explain how this is calculated. For the automatic 90 amp wire gauge calculator, you should use the calculator with the corresponding charts further on.
Note on 90 amp breaker wire size: According to NEC 80% rule, we must not load (continuous loads) a breaker to more than 80% of its capacity. That means that we can only put 72 amps on a 90A breaker (since 80% of 90A is 72A).
That means we need a wire that can handle at least 72 amps to wire the 90 amp breaker. If you check the screenshot above, you can see that we would use 4 AWG copper wire for a 90 amp breaker (85A ampacity; more than 72A required) or 3 AWG aluminum wire for a 90 amp breaker (75A; more than 72A required.
Alright, let’s have a look at how to calculate 90 amp wire size at a distance:
How To Calculate 90 Amp Wire Gauge For 50, 100, 150, 200, 300, 400 Feet Away?
The wire itself has some resistance (depends on the wire material). That’s why we see this phenomenon called voltage drop. If we want to wire a sub-panel, feeder, or simple branch circuit at a distance, we do have to account for it. The result is that we will need thicker wires.
Here are the maximum allowable voltage drop percentages:
- 3% for branch circuits.
- 5% for sub-panels and feeders.
- 10% for non-crucial general appliances and general lighting.
Let’s look at this example. Supposedly, we have a 120V circuit and want to wire a feeder that is 200 feet away. We are using copper wires. Here is the equation for a voltage drop:
Voltage Drop = 2 × L × K × 90 Amps / Circular Mils
For our circuit, we have a feeder on 12oV. That means we have to apply an allowable voltage drop of 5%; the voltage drop in this circuit can be up to 6.0V since 120V × 0.05 = 6.0V. L in the formula is wire length, and K is the specific resistivity of the wire (12.9 ohms for copper wires and 21.2 for aluminum wires).
We are calculating the circular mils; this is the cross-section area of the wire. Why? Well, if we know how many circular mils wire we need, we can cross-reference that to AWG gauges using this AWG to circular mils chart. Let’s express circular mils in the equation above and put all the numbers in the equation like this:
Circular Mils = 2 × 200 Feet × 12.9 Ohms × 90 Amps / 6.0 Volts = 77,400 Circular Mils
We see that, to facilitate this 90 amp current, we have to pick a wire with at least 77,400 circular mils cross-section area. Now we check the AWG to CM chart; here is the screenshot:
We see the following:
- We can’t use the standard 3 AWG copper wire here because it has less than 77,400 circular mils cross-section area (it has 52,633 circular mils, to be exact).
- We can use 1 AWG, 1/0 AWG, 2/0 AWG, 3/0 AWG, 4/0 AWG wires because they have above 77,400 circular mils area.
- In practice, we will use 1 AWG copper wire because it is the next electric wire with above 77,400 circular mils (it has 83,690 circular mils).
Now, all these calculations and cross-referencing can be a bit tedious. To make all of this fast and simple, you can use the following 90 Amp Wire Gauge Calculator. Insert the wire length, voltage, and allowable voltage drop percentage; the calculator will determine the adequately sized 90 amp wire immediately (just check the note below the calculator above below 3 AWG copper wire results).
Further on, you will also find the 90 amp wire size charts for 3% and 10% voltage drop, and for 12V, 120V, and 220V voltages, and wire lengths ranging from 0 to 500 feet.
Here’s the calculator you can freely use:
90 Amp Wire Size Calculator (Accounted For Voltage Drop)
Alright, we can solve the 240V example from above to illustrate how this calculator works:
We have a 240V service at a distance of 400 feet for a feeder (5% allowable voltage drop). What size wire we should use for 90 amp service? Just slide the 1st wire length slider to ‘400’, the 2nd voltage slider to ‘240V’, and the 3rd allowable voltage drop percentage to ‘5’. Here is the result we get:
For this circuit, we would need 1 AWG copper wire. This is the same result we got with the manual calculation and cross-referencing.
Note: If the calculator shows the results below 3 AWG copper wire (like 4 AWG, 6 AWG, 8 AWG, for example), still use the 3 AWG copper wire because it satisfies the ampacity criterion (the calculator forces the wires to satisfy the voltage drop criterion).
Here are the two charts for 90 amp wire gauges at a distance 0 to 500 feet away from the source and at 12V, 120V, and 220V voltages. We will start with the 3% allowable voltage drop chart, and proceed with the 10% allowable voltage drop chart:
90 Amp Wire Gauge Source 0-500 Feet Away (3% Allowable Voltage Drop)
|Wire Length (3% Vd):||90 Amp Wire Size At 12V:||90 Amp Wire Size At 120V:||90 Amp Wire Size At 220V:|
|0 Feet||3 AWG Copper Wire||3 AWG Copper Wire||3 AWG Copper Wire|
|50 Feet||350 kcmil Copper Wire||2 AWG Copper Wire||3 AWG Copper Wire|
|100 Feet||700 kcmil Copper Wire||1/0 AWG Copper Wire||3 AWG Copper Wire|
|150 Feet||1000 kcmil Copper Wire||2/0 AWG Copper Wire||2 AWG Copper Wire|
|200 Feet||1500 kcmil Copper Wire||4/0 AWG Copper Wire||1 AWG Copper Wire|
|300 Feet||2000 kcmil Copper Wire||8 AWG Copper Wire||1/0 AWG Copper Wire|
|400 Feet||2 Or More Wires||300 kcmil Copper Wire||3/0 AWG Copper Wire|
|500 Feet||2 Or More Wires||350 kcmil Copper Wire||4/0 AWG Copper Wire|
We can see that we have to quickly pick lower wires at lower voltages (especially 12V). Here are the wire sizes for a 90 amp circuit at a distance of 100 feet away from the source and at a 3% allowable voltage drop:
- At 12V, we have to use very thick 700 kcmil copper wire.
- At 120V, we have to use thicker 1/0 AWG copper wire.
- At 220V, we can use the standard 3 AWG copper wire.
We see that the voltage drop has a lower effect on wire size at higher voltages (220V). Let’s have a look at a less restrictive 10% allowable voltage drop chart as well:
90 Amp Wire Gauge Source 0-500 Feet Away (10% Allowable Voltage Drop)
|Wire Length (10% Vd):||90 Amp Wire Size At 12V:||90 Amp Wire Size At 120V:||90 Amp Wire Size At 220V:|
|0 Feet||3 AWG Copper Wire||3 AWG Copper Wire||3 AWG Copper Wire|
|50 Feet||1/0 AWG Copper Wire||3 AWG Copper Wire||3 AWG Copper Wire|
|100 Feet||4/0 AWG Copper Wire||3 AWG Copper Wire||3 AWG Copper Wire|
|150 Feet||300 kcmil Copper Wire||3 AWG Copper Wire||3 AWG Copper Wire|
|200 Feet||250 kcmil Copper Wire||3 AWG Copper Wire||3 AWG Copper Wire|
|300 Feet||600 kcmil Copper Wire||2 AWG Copper Wire||3 AWG Copper Wire|
|400 Feet||800 kcmil Copper Wire||1 AWG Copper Wire||3 AWG Copper Wire|
|500 Feet||1000 kcmil Copper Wire||1/0 AWG Copper Wire||2 AWG Copper Wire|
Here we see that at a distance of 100 feet away from the source, we can still use the standard 3 AWG copper wire for 90 amp service at 120V and 220V. At a very low 12V DC voltage, we have to upgrade to a 4/0 gauge copper wire.
We hope that the calculator and these charts illustrate well how to go about calculating the 90 amp wire size. If you need any help with the math, you can use the comment section below, give us some numbers, and we will help you out.