Wire Size problem

[kegarc2002]

Hi guys:

Beginner in Revit. I need help with this Circuit.

1-I have a (residential) Project. and the office room, has only 7 standard outlets, 180 Va, 120 V on a 20 Amp circuit.

   Revit is Sizing the wire to #10. Never got the 80% warning. I am still working on it. Main Room has a similar       problem sized the wiring to #8.

2-My other concern is I have no idea how to represent outlets connected to switches or fed by switches. What I have done here, come from dozens of video tutorials.

I attached captures of the screen, also the whole project file. 

Please help and feel free to correct anything you find wrong. Any comment is Welcome. As I said, am a beginner in Revit

Thanks in advance.

Kevin.

[HVAC-Novice · Accepted solution]

Wire size is based on allowed voltage drop and what the minimum size is based on the breaker. Revit basically preforms two checks on wire size and uses the larger size (confusingly, that is the smaller wire size number).

You can verify/set allowed voltage drops under manage -> MEP Settings -> Electrical Settings. if voltage drop is above that value, Revit will upsize the wire. 

Minimum wire sizes can be set here:

FWIW, on our projects we specify wires to be one size larger than NEC requires. But if you have any reasonably large commercial project, you need to up-size anyway. And it is better for installer to have consistency instead of having each 20A circuit having its onw wire size (because in real life, the installers will never do that). We also require 90°C insulation (but use 60°C to determine the minimum sizes). 

I also include circuit schedules that show the wire size for each circuit. You could add them to panel schedules as well, though. 

[kegarc2002]

Thanks for your help.

I changed my Settings to 1.5 and 2 %; the Wire Size grew to #8.

I am very concern about it because, in real life; electricians install a number of 10 receptacles in a circuit. Perfectly allowed by Codes NEC. Revit don't let me have more than 7. Personally, I do not think Revit is right if NEC Codes allows.

Something I am doing, is wrong somewhere.

Thanks for your teaching. All helpful.

Regards:

Kevin

[HVAC-Novice]

A duplex receptacle is 180 VA unless you have a specific use (i.e. microwave). that's' 1260 VA or 1,575 (80% rule). Thats' 13.125 A. So 7 is about right for generic receptacles. 

I think electricians don't' always account for voltage drop. That is why we put it in the design. 

[kegarc2002]

Thanks for your help again.

Now I can see 180 Va * 7 equals 1260 Va.

1575 Va / 120 V equals 13.125 A. But I don't get what are the 1575 Va.

Could you explain?

Thanks in advance.

[HVAC-Novice · Accepted solution]

If it is a constant load, NEC requires to only use 80% of the wire capacity. This is due to more overheating and degradation of the insulation if the load is constant. 

If the loads are only temporary, you can use 15A of a 15A circuit.  If the load is longer term (i.e. lighting), you only can use 80% (12A) of the capacity. So you multiply your calculated load by 1.25. A lot of that sizing is based on insulation degrading due to heat. 

Voltage drop (length!) also reduces the actual capacity of a conductor. 

I assume many codes have similar rules. There also can be different rules in addition to code. 

[kegarc2002]

Thanks again for the fast response.

I can understand that, if the load is constant or longer term; we treat the Load as 1.25 the real calculated load; to be on the safe side.

Thanks for your help.

Thanks

Kevin

[axtonbrat]

What you’re seeing is a common issue when starting with Revit. The software doesn’t always reflect field practice, especially with branch circuit wiring. In reality, a 20A general-purpose circuit using #12 wire is standard and fully compliant under NEC, even with multiple receptacles. Electricians often install 8–10 outlets on a single 20A run without upsizing unless there’s a special load or long distance causing voltage drop.

Revit, however, is very sensitive to the voltage drop settings you’ve defined. If the percentage thresholds are too strict (say 1.5% or 2%), the program will automatically size up to #10 or even #8 wire, even though that’s rarely needed in residential work. A good approach is to set those values closer to what your local jurisdiction allows. Also, double-check how you’ve classified your receptacle loads, because incorrect assignments can inflate demand and force unnecessary upsizing.

For representing outlets tied to switches, the switch system tool is the right method. Assigning receptacles to a defined switch system ensures Revit displays them correctly.

If you want to see how field electricians balance NEC rules with practical layouts, Electrician Singapore offers helpful insights that can give context beyond the software.

[HVAC-Novice]

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@axtonbrat wrote:

What you’re seeing is a common issue when starting with Revit. The software doesn’t always reflect field practice, especially with branch circuit wiring. In reality, a 20A general-purpose circuit using #12 wire is standard and fully compliant under NEC, even with multiple receptacles. Electricians often install 8–10 outlets on a single 20A run without upsizing unless there’s a special load or long distance causing voltage drop.

Revit, however, is very sensitive to the voltage drop settings you’ve defined. If the percentage thresholds are too strict (say 1.5% or 2%), the program will automatically size up to #10 or even #8 wire, even though that’s rarely needed in residential work. A good approach is to set those values closer to what your local jurisdiction allows. Also, double-check how you’ve classified your receptacle loads, because incorrect assignments can inflate demand and force unnecessary upsizing.

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Just because people in the field violate code and get away with it, is not a reason to do the same in design. How do you determine is is "not needed to upsize wire"? NEC is a required minimum design requirement. If it is is in the code, it is needed. You don't know how a house is used in the next 30 years. Just because a breaker doesn't trip all the time doesn't mean it is a safe installation. Insulation degrades over time with heat and that can create shorts, archs etc. You won't see that the day a building is finished and handed over to the owner. 

The number (and type) of receptacles determines the breaker size. Breaker size determines the MINIMUM wire size. This wire size needs to be increased based on pressure drop (wire length). Temperature rating of the insulation and ambient conditions also plays a role. 

R2026 removed voltage drop calculation. I ended up using Electro-BIM and that sizes the breaker and wire size based on the load, type of insulation , and lengths.