NEC 250.122
Ground Wire Size Calculator
The breaker gives you the ground wire from Table 250.122. Then NEC 250.122(B) makes it bigger, whenever the conductors in the pipe are larger than the circuit actually needs. This works out both, and tells you which one you are looking at.
NEC Table 250.122 is keyed on the DEVICE. Not on the load, and not on the conductor beside it.
The size in the pipe, whoever put it there and whatever their reason. If it is larger than this circuit needs, 250.122(B) is live and this is the field that fires it.
The conditions that set the minimum (ambient, conductor count, terminations)
These four set the MINIMUM conductor this circuit requires, which is what the increase is measured from. That is also how the code’s own exclusion is honoured: 250.122(B) does not count an increase required by 310.15(B) or 310.15(C), and a conductor upsized purely for heat or conduit fill lands on this minimum, so no increase is left to scale.
One honest caveat on that. NEC 310.15(B)(1) permits two ways to correct for ambient: the banded Table 310.15(B)(1)(1), and the equation computed at your actual temperature. This tool uses the equation, which is less conservative inside a band. If you sized your conductor off the printed table you may land one size above the minimum shown here, and this tool would read that as an increase when the ambient is what required it. Compare like with like: the minimum is on screen, and if it matches what you pulled, the ratio is 1 and nothing upsizes.
Table 250.122 prints #12 at a 20 A device, and here that is the answer. #12 copper is the minimum this circuit needs in these conditions, so nothing was increased and 250.122(B) never engages.
Show the math: NEC 250.122(B)
#12 6,530 cmil / #12 6,530 cmil = 1.0000
#12 6,530 cmil × 1.0000 = 6,530 cmil → #12
The circular mils are the printed cells of NEC Chapter 9 Table 8, never the AWG geometric series. The series agrees to within 0.08 percent and is exact only at 4/0, and this is ratio arithmetic where a rounding boundary is a whole wire size.
What the code does not say. 250.122(B) requires the increase to be proportionate to “the increase in circular mil area of the ungrounded conductors” but never defines what the increase is measured FROM. This tool measures it from the smallest conductor the circuit requires in the conditions above. That is the only baseline that makes the rule computable and it is how the rule is applied in the field, but it is our reading of an undefined term rather than something printed in the code, which is why the baseline is on screen where you can check it.
Where this number comes fromIntry Verified
- Calculated from
- NEC Table 250.122 with 250.122(A) and 250.122(B), circular mils from NEC Chapter 9 Table 8, minimum conductor from Table 310.16 with Equation 310.15(B)(1) and Table 310.15(C)(1). Computed from NEC Table 250.122 at the overcurrent device, with the 250.122(B) proportional increase taken against the printed NEC Chapter 9 Table 8 circular mils and held between the 250.122(A) table floor and the circuit-conductor ceiling. See the full receipt ↓
- Run with
- Device20 A
- Ungrounded conductor#12 copper, 75°C
- Ground wire materialcopper
- Ambient86°F (30.0°C)
- Current-carrying conductors1-3 (1.00)
- Terminations75°C
- Minimum for this circuit#12
- Checked
- Every figure an Intry calculator computes comes from a locked source module, not hand-typed. 21026 automated checks re-derive and source-check the numbers against the cited NEC section before each deploy. This is our own deterministic gate, not a third-party audit.
- Final say
- Your AHJ and local amendments have final say. Confirm before rough-in.
Intry Verified · NEC 2023 · Build AE2FC52 · 2026-08-21
| Device | Table 250.122 | Minimum conductor | Increase | Ground wire to pull |
|---|---|---|---|---|
| 15 A | #14 | #14 | 1.59x | #12 |
| 20 A | #12 | #12 | 1.00x | #12 |
| 25 A | #10 | #10 | – | – |
| 30 A | #10 | #10 | – | – |
| 35 A | #10 | #8 | – | – |
| 40 A | #10 | #8 | – | – |
| 45 A | #10 | #8 | – | – |
| 50 A | #10 | #8 | – | – |
| 60 A | #10 | #6 | – | – |
| 70 A | #8 | #4 | – | – |
| 80 A | #8 | #4 | – | – |
| 90 A | #8 | #3 | – | – |
| 100 A | #8 | #3 | – | – |
| 110 A | #6 | #2 | – | – |
| 125 A | #6 | #1 | – | – |
| 150 A | #6 | #1/0 | – | – |
| 175 A | #6 | #2/0 | – | – |
| 200 A | #6 | #3/0 | – | – |
| 225 A | #4 | #4/0 | – | – |
| 250 A | #4 | 250 kcmil | – | – |
| 300 A | #4 | 350 kcmil | – | – |
| 350 A | #3 | 500 kcmil | – | – |
| 400 A | #3 | 600 kcmil | – | – |
The last column is the one a printed table cannot carry: it depends on the conductor in your pipe, the ambient and the conductor count, none of which the code book knows. Rows in red are where 250.122(B) takes the grounding conductor above what Table 250.122 prints.
This is not the grounding electrode conductor. Table 250.122 sizes the EQUIPMENT grounding conductor, the one pulled with the circuit, from the overcurrent device. The grounding ELECTRODE conductor that goes to your ground rod or your rebar is Table 250.66, sized from the service-entrance conductors, and 250.66(A) caps it at #6 copper where it runs to a rod. Sizing either one from the other’s table is the mistake this paragraph exists to stop.
250.122(B) carries an exception this tool does not apply for you: a qualified person is permitted to size the grounding conductor to provide an effective ground-fault current path under 250.4(A)(5) or (B)(4) instead of by this proportion. That is an engineered judgement rather than arithmetic, so the tool gives you the proportional answer and names the exception rather than quietly assuming it.
Intry Verified. The ground wire sizes THE CALCULATOR ON THIS PAGE RETURNS are computed from NEC Table 250.122 with 250.122(A) and 250.122(B), against the circular mils printed in NEC Chapter 9 Table 8 (NFPA 70-2023). Figures quoted in the FAQ and the surrounding copy are read from the same printed tables rather than computed. Confirm the edition your jurisdiction has adopted; the authority having jurisdiction has final say. Those figures are computed by a locked engine straight from the source it cites, not hand-typed, and that computation re-runs inside a deterministic test on every deploy. Source-checked. Locked. This is our own deterministic gate, not a third-party audit. What Intry Verified means
The Rule Everyone Knows, and the One They Miss
The first half is simple: read NEC Table 250.122 across from the rating of the overcurrent device protecting the circuit. A 50A breaker takes #10 copper even though the circuit conductors are #6, because the equipment grounding conductor tracks the breaker rather than the phase conductors. That is the part every chart on the internet prints.
The second half is NEC 250.122(B), and it is where the red tags come from. If the ungrounded conductors are increased in size for any reason other than the corrections required by 310.15(B) or 310.15(C), the grounding conductor has to be increased in the same proportion by circular mil area. It is ratio arithmetic against a table most people do not have open, on the one conductor nobody thinks about, and it is invisible the moment the cover goes on.
The reason it is missed so often is that it does not only bite on a deliberate design upsize. It bites whenever the wire in the pipe is bigger than the circuit needs, which on an existing installation is the ordinary state of the world.
Worked Example
100A sub-panel feeder, conductors upsized for a long run
A 100A feeder needs #3 copper at the table’s own conditions, and Table 250.122 prints #8 copper for the ground. Run it as designed and #8 is the whole answer.
#1/0 = 105,600 cmil / #3 = 52,620 cmil = 2.01x
#8 = 16,510 cmil x 2.01 = 33,133 cmil
Now upsize the feeder to #1/0 for voltage drop. That is 2.01 times the circular mils the circuit needed, so the ground wire goes up by the same factor. The next printed size at or above 33,133 circular mils is #4 copper, per NEC 250.122(B). Two sizes up from what the table alone would have told you.
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Frequently Asked Questions
What size ground wire do I need for a 100 amp breaker?
The equipment grounding conductor for a 100A circuit is #8 copper per NEC Table 250.122. In aluminum the EGC is #6. The EGC is sized from the rating of the overcurrent device protecting the circuit, not from the load. But read the next question before you pull it: if the ungrounded conductors are larger than the circuit needs, NEC 250.122(B) makes the answer larger than #8.
What size ground wire for a 50 amp breaker?
The EGC for a 50A breaker is #10 copper. NEC Table 250.122 lists 60A at #10, and any rating up to that row (30A, 40A, 50A) uses the same #10 copper. In aluminum it is #8.
What size ground wire for a 20 amp circuit?
The EGC for a 20A circuit is #12 copper per NEC 250.122, which is why 12-2 nonmetallic cable includes a #12 or bare equivalent ground. A 15A circuit uses #14 copper.
Does the ground wire size depend on the breaker or the wire size?
Both, and the second half is the part that gets missed. The EGC starts from the overcurrent device rating per NEC Table 250.122. Then NEC 250.122(B) says that if the ungrounded conductors are increased in size for any reason other than the corrections required by 310.15(B) or 310.15(C), the equipment grounding conductor is increased in size proportionately to the increase in circular mil area. So a #12 EGC on a 20A circuit becomes a much larger conductor if somebody ran #2 for that circuit. The calculator on this page computes that ratio for you.
Does 250.122(B) apply if the conductors were already oversized when I got there?
On the text as printed, yes. 250.122(B) reads "if ungrounded conductors are increased in size for any reason other than as required in 310.15(B) or 310.15(C)". It names no actor and no point in time: it does not say increased by the designer, or increased during this job. What it does not define is what the increase is measured FROM, and that is a genuine gap in the code text rather than something we can look up. This tool measures it from the smallest conductor the circuit requires in the conditions you enter, which is the only baseline that makes the rule computable and is how it is applied in the field. That baseline is printed on screen so you can check it, and your AHJ has the final say on the reading.
If I upsized the wire because of heat or conduit fill, does the ground go up too?
No, and the code is explicit about it. 250.122(B) excludes increases required by 310.15(B), the ambient temperature correction, and 310.15(C), the adjustment for more than three current-carrying conductors. A conductor that is bigger only because of heat or bundling is not "increased in size" for the purposes of this rule. The calculator handles this without asking you to declare a reason: the minimum it measures against already has the correction and adjustment applied, so a heat-driven upsize lands exactly on that minimum and produces no increase.
Is the equipment grounding conductor the same as the grounding electrode conductor?
No. The equipment grounding conductor (EGC) bonds equipment back to the source and is sized by NEC 250.122 from the breaker rating. The grounding electrode conductor (GEC) connects the system to earth (ground rod, water pipe, Ufer) and is sized by NEC 250.66 from the service conductor size, with 250.66(A) capping the run to a rod at #6 copper. They are different conductors with different tables, and sizing one from the other's table is a common and serious mistake.
Can the ground wire ever need to be bigger than the circuit conductors?
No. NEC 250.122(A) says the equipment grounding conductor is not required to be larger than the circuit conductors supplying the equipment, so the proportional increase is capped there. The calculator applies that cap and tells you when it fired.
Related Calculators
Wire Size Calculator
Size the ungrounded conductors for load, distance and derating, then bring the size it gives you back here.
Conductor Capacity Check
What the conductor already in the wall carries in your conditions, and which breaker it takes.
Voltage Drop Calculator
The usual reason a conductor gets upsized, which is the usual reason the ground has to follow it.
Upsizing for drop is what pulls the ground up with it.
Work out the drop on the run first. If it forces a larger conductor, bring that size back here and this will tell you what the ground wire becomes.
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