AmpSizerWhat size feeder does a sub panel need?
Sub panel feeder size
3 AWG copper
on a 100 A device · 8 AWG ground · 1" conduit at 38% fill
1.6% drop at 80 ft
- Ampacity wants3 AWG
- Voltage drop wants4 AWG
- Current the conductor has to carry
- 100 A
- Usable ampacity after derating
- 100 A (table 100)
- Longest run before the drop passes 3%
- 147 ft
- In metric sizes
- 26.7 mm²
— had the ampacity and 240.4(D) still rejected it, at 0 A.
Conduit is sized for four conductors: a sub panel without a neutral cannot supply a 120 V circuit.
A sizing aid from the published NEC tables, not a design — have it signed off by a licensed electrician against the code your inspector enforces. Calculations follow the US NEC; outside the US, follow your local electrical code.
One circuit check a month
MonthlyOne thing a month worth measuring, testing or recalculating on a home electrical system — the part of sizing that happens after the install. No product pitches, and nothing that needs a live panel.
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What size wire does a sub panel feeder need?
The AmpSizer feeder size for a 100 amp sub panel 80 feet away on 240 volts is 3 AWG copper on a 100 amp breaker, with an 8 AWG ground, in 1-inch EMT. Voltage drop over that run is 1.6%.
A feeder carries four conductors, not two: two hots, a neutral for the 120 V circuits in the panel, and an equipment grounding conductor. The conduit is sized for all four, which is why a feeder wants a bigger pipe than its ampacity suggests.
One rule is worth knowing before you buy copper. 310.12 lets a dwelling service or the main feeder to a dwelling use conductors rated at 83% of the service rating, so a 100 A dwelling feeder can be 4 AWG copper — but only where it carries the entire load of the dwelling.
The panel sizes people actually install, with the conductors that feed them at short range and the distance at which voltage drop takes over.
| Panel | Copper | Aluminium | Ground | Conduit for 4 wires | Drop takes over at |
|---|---|---|---|---|---|
| 60 A | 6 AWG | 4 AWG | 10 AWG | 3/4 in | 122 ft |
| 100 A | 3 AWG | 1 AWG | 8 AWG | 1 in | 147 ft |
| 125 A | 1 AWG | 2/0 AWG | 6 AWG | 1-1/4 in | 148 ft |
| 150 A | 1/0 AWG | 3/0 AWG | 6 AWG | 1-1/4 in | 156 ft |
| 200 A | 3/0 AWG | 250 kcmil | 6 AWG | 2 in | 187 ft |
Copper conductors at 75 °C, 240 V, 3% target. Conduit is EMT for two hots, a neutral and a ground of the sizes shown.
The detached building case
This is where feeder sizing earns its keep, and it comes with two extra rules that have nothing to do with wire size.
One feeder per structure. 225.30 allows a single feeder or branch circuit to supply a separate building, with limited exceptions. Two runs out to the same garage is not the shortcut it looks like.
A disconnect at the building. 225.31 and 225.32 require a means of disconnect at the structure it supplies, readily accessible and at the nearest point of entry. That is usually a small main-breaker panel rather than a lug-only one, and it is the part people discover after they have bought the panel.
A grounding electrode at the building. 250.32 wants an electrode system at the separate structure, and the ground and neutral stay separate in that panel just as in any sub panel.
Frequently asked questions
What size wire for a 100 amp sub panel 100 feet away?
3 AWG copper or 1 AWG aluminium at 240 V, and voltage drop is 2.0% at that distance so ampacity is still the rule that decides. It is at about 145 ft that drop takes over and asks for 2 AWG copper. Run it in 1-inch conduit for four conductors, with an 8 AWG copper ground.
Does a sub panel need a neutral?
If anything in it is 120 V, yes — and in a house that means yes. A four-wire feeder is the norm: two hots, a neutral and a ground, with the neutral and ground kept separate in the sub panel and bonded only at the service. A three-wire feeder is only for a panel feeding nothing but 240 V loads.
Can I bond the neutral and ground in a sub panel?
No. The bond happens once, at the service disconnect. Bonding again downstream puts normal neutral current onto the grounding path and onto any metal connected to it, which is exactly what the separation exists to prevent. The bonding screw or strap in a panel used as a sub panel gets removed.
Can I use aluminium for a sub panel feeder?
Yes, and for a feeder it is the normal choice — 4-conductor aluminium SER is what most 100 A sub panels are fed with. It is two sizes larger than copper for the same amps, so 1 AWG aluminium replaces 3 AWG copper, and the terminations need the right compound and torque. Aluminium branch circuits are a different matter and are generally avoided.
Does the 83% rule apply to my sub panel?
Only if that feeder carries the entire load of the dwelling — a main feeder from the meter to the panel, or a feeder to a separate structure that constitutes the whole supply. A 100 A sub panel fed from a 200 A service inside the same house is an ordinary feeder and gets ordinary sizing, which is the case this calculator assumes.
How we calculate this
2 × amps × R × one-way feet ÷ 1,000 on the Chapter 9 Table 8 resistance
figures, and the conductor returned is the larger of the two requirements. The device is the standard
rating from 240.6(A) and the equipment grounding conductor is Table 250.122. Conduit is sized from
Chapter 9, Table 4 at the 40% limit for four conductors — two ungrounded, one grounded and one grounding.
This tool applies ordinary feeder sizing; the 83% allowance of 310.12 for dwelling services and main
feeders is not applied automatically, because it only holds where the feeder carries the entire dwelling
load, and that is a judgement about the installation rather than about the arithmetic.Worked example
With Sub panel rating 100, One-way run 80, System voltage 240 V, Metal Copper, Terminal rating 75 °C — the usual breaker, Ambient 86, Current-carrying conductors in the raceway 3, this page works out 3 AWG copper. on a 100 A device · 8 AWG ground · 1" conduit at 38% fill
- Current the conductor has to carry100 A
- Usable ampacity after derating100 A (table 100 )
- Longest run before the drop passes 3%147 ft
- In metric sizes26.7 mm²
Sources:
The three things that make a feeder different
Four conductors, not two. It changes the conduit size and it changes the derating count if you ever put two feeders in one pipe.
The neutral is usually full size but does not have to be. 220.61 allows a reduced neutral where the imbalance is limited, which is why 4-conductor SER often has a smaller neutral than its hots. This calculator sizes them the same, which is the safe default.
Distance is the reason feeders exist. Nobody runs a feeder ten feet. A detached garage, a barn or a shop is 60 to 200 ft away, and that is precisely the range where voltage drop stops being academic.