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Kevin, Licensed Electrical Contractor

Category: Electrical

Satisfied Customers: 3729

Experience: 30 years Licensed Electrical Contractor in Illinois, Adjunct College Electrical Instructor, Former Electrical Inspector, Diploma: Digital Electronics, FCC Technician License

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I'm installing power and wiring to a new 24'x64' building.

Customer Question

I'm installing power and wiring to a new 24'x64' building. The longest run of new wiring will be to a workshop area which is about 85 feet from the service panel. A wiring calculator says I need #10 wire for 20 amp outlets on a wiring run of this length. An acquaintance says #12 wire is ok up to 100 feet. The calculator says the maximum length for 20 amps on #12 wire is about 50'. Which is correct and why the difference? If I need to install #10 wire, is it acceptable to run a 75' header around the ceiling then tap into this header, running ~ 6' of #12 wire to outlets on the walls? (This mixes wiring sizes on the same circuit but makes wiring connections easier to do)

Hello and welcome to Just Answer. My name is ***** ***** I will be happy to assist you with your electrical question.

1) The National Electrical Code recommends that the voltage drop be 3% or less.

2) The maximum distance for a 12 AWG copper conductor @ 20 amps/120 volts is 47 feet which results in exactly a 3% VD or a loss of 3.6 volts. Therefore, code compliant.

3) The maximum distance for a 10 AWG Copper conductor @ 20 amps/120 volts is 75 feet which results in exactly a 3% VD or a loss of 3.6 volts. Therefore, code compliant.

4) Since your distance will be 85 feet, you will need 8 AWG copper conductors for the 20 amp/120 volt circuit. This results in a 3% VD or a loss of 3.6 volts.

5) 8 AWG copper is correct. The difference is due to the distance and the voltage drop over the conductors on an 85 foot run. All conductors are rated in ohm's per foot. The longer the run, the larger the conductor size needs to be in order to account for the voltage drop.

6) A 120 volt circuit will require 1 hot, 1 neutral and 1 equipment grounding conductor to the outbuilding. Therefore, a 3 wire circuit must be installed. The receptacles at the outbuilding will also be required to be GFCI protected. I assume that all you require is (1) one 120 volt dedicated circuit?

You confirm my suspicions that lots of electricians don't understand or follow NEC code on wiring sizes for long runs!There will be 2 circuits - one 20 amp (outlets) and one 15 amp (lights and door openers). Sub panel has 220v 40 amp feed (4 conductor #8 wire ,104 feet). I'm installing one 20 amp and one 15 amp GFI breaker in the new panel.Sounds like I need to shorten the wiring run to less than 75 feet, which is doable, and use # ***** wire.What about the mixing wire sizes - going from a long #10 wire header to a short wire # ***** wire on the final 6' or so? If it needs to be all #10 wire ok but I hate working with it.

Customer:replied 1 year ago.

the question is, how long can this pigtail be - 6" or 6'?

1) I am not understanding your question and I need confirmation.

Are you installing a 240 volt sub-panel to the outbuilding or are you installing a qty of (1) one 120 volt/20 amp circuit to the outbuilding. Is the 40 amp sub-panel existing?

There is a difference between a feeder circuit and a branch circuit.

it is confusing and I'm not being precise enough.A sub panel is already installed in the new building, wired for 40 amps 240v. I'm installing the branch circuits and wiring in the new building from this existing panel. I want to install one 20 amp 120v branch circuit feeding outlets in the new building and one 15 amp 120v branch circuit feeding lights and door openers.Sounds like all the wire in a 75 foot, 20 amp 120v branch circuit needs to be #10 except where you make any pigtail connections in the outlet boxes?

1) If the maximum distance from the panel to the receptacles will be 75 feet, then 10 AWG copper conductors are required.

2) 14 AWG copper is the minimum size on a 15 amp circuit and is good up to 40 feet. If over 40 feet, then 12 AWG copper is required for the 15 amp circuit which is good up to 63 feet.

3) Pigtail connections at a minimum, must match the amperage of the circuit breaker........ie..........if a 15 amp breaker, then 14 AWG copper pigtail is used. If a 20 amp breaker, then 12 AWG copper is used.

If you have any additional questions, just let me know and I’ll be glad to answer them for you.

Otherwise, don’t forget to rate me before you log Off.

Ok, I understand about the wire size for various distances and amperages. Tks. I'll measure distances and use what wire size is appropriate. You've confirmed what I thought was required. This is a big help.But but what about mixing wire sizes in the same branch circuit? Yes, it's ok to use for #12 copper pigtails as you described. But are there other exceptions?For example, I could run #10 wire the 75 feet, as we discussed, and splice into the #10 wire, say 15 feet from the sub panel, with #12 wire and run it too an outlet 8 feet away. Total wire length is 15' + 8' = 23' so the wire length is < 47' and #12 wire is ok for this length.But I've mixed two different wire sizes in the circuit. Does NEC code allow it?I hope I'm explaining this so it's understandable.

1) Yes, the NEC allows to mix wire sizes where it is required to maintain the maximum 3% voltage drop.

For example on a 20 amp circuit breaker, inside the panel you could have 12 AWG conductors splicing to 10 AWG conductors where the 10 AWG is supporting the longer circuit run. Then at a junction box located near the receptacle area, the 10 AWG could be reduced back down to 12 AWG copper in order to terminate to the device.

2) If installing 10 AWG copper and maxing it out at 75 feet and then splicing some additional 12 AWG copper onto the 10 AWG main circuit run, lets say another 23 feet of 12 AWG, this will put the end of the run receptacles above the maximum voltage drop recommendation of 3%. The NEC does not state that the voltage drop must be 3% or less. The 3% VD is only a recommendation within the NEC and is a well known practice that is followed in the trade.

Think of it this way. Certain lighting, electronics and tools may work just fine at even a 5% voltage drop, however other connected loads may not. Once going below the 115 volt level, certain loads may not work correctly. The NEC recognizes that a single phase voltage for a dwelling is either 120 or 240 volts. These are the 2 voltages that are the nominal values used in calculations. Within the NEC, there is no such thing as 110 or 220 volts. It is either 120 or 240 volts (+ or -) 3%.

Let me know if this helps explain things..........Thanks...........Kevin:)