• Welcome to the new and improved Building Code Forum. We appreciate you being here and hope that you are getting the information that you need concerning all codes of the building trades. This is a free forum to the public due to the generosity of the Sawhorses, Corporate Supporters and Supporters who have upgraded their accounts. If you would like to have improved access to the forum please upgrade to Sawhorse by first logging in then clicking here: Upgrades

Minimum Feeder Size and Voltage Drop Calculation National Electrical Code (NEC)

jar546

jar546

Forum Coordinator
Joined
Oct 16, 2009
Messages
11,041
Location
Somewhere Too Hot & Humid
OK Electrical Inspectors and Electricians, is the following scenario I put together correct?

The minimum size copper wire type THHN feeder for a 125 amp sub panel 100' away from the main panel in a 120/240 volt AC system, taking into consideration the National Electrical Code and maximum voltage drop and limited to the 75 degree Celsius column, is 2/0 AWG.

According to the NEC, the maximum allowable voltage drop for feeders and branch circuits is 3% at the farthest outlet of power, distance, and load current.

The wire size selection should be based on the ampacity of the wire, which is the amount of current that a wire can safely carry. The ampacity of a wire is determined by the wire's insulation type, temperature rating, and the ambient temperature.

The ampacity of 2/0 AWG THHN wire is limited to the 75 degree Celsius column in the NEC table 310.16, which allows for a current carrying capacity of 130 amperes. This wire size is more than enough to meet the requirements of a 125 ampere sub panel located 100 feet away from the main panel, while also keeping the voltage drop within the 3% maximum allowable limit.

The voltage drop percentage for a 2/0 AWG THHN copper wire feeder for a 125 amp sub panel 100' away from the main panel in a 120/240 volt AC system, taking into consideration the National Electrical Code and limited to the 75 degree Celsius column, would be approximately 2.4%.

This can be calculated by using the formula: Voltage drop (percentage) = (2 x one-way circuit distance in feet x load current in amperes) / (circular mils of the conductor x CMIL/amp)

The wire resistance per 100ft of 2/0 AWG THHN wire is 0.082 ohms/100ft, and the circular mil area of 2/0 AWG THHN is 240,000 cmil.

Voltage drop (percentage) = (2 x 100 x 125) / (240000 x 0.082) = 2.4%

This answer is code compliant with the NEC because the voltage drop percentage is less than 3%, which is the maximum allowable voltage drop for feeders and branch circuits according to NEC standard.

It's important to note that this is a simplified calculation and does not take into account other factors such as ambient temperature, wire installation conditions, and any other load on the same circuit, which can all affect the final ampacity and voltage drop percentage.
 
What you need is more electrical engineers for such a question. My take on voltage drop works like this: The Skilsaw is ok on a 100' #12 cord.....if you don't want to smoke the motor anything over 100' is a #10 cord.

There is a savant here at the forum that can give you an answer.... but not the rest of us.
 
jar546 said:
According to the NEC, the maximum allowable voltage drop for feeders and branch circuits is 3% at the farthest outlet of power, distance, and load current.
Click to expand...
That's not actually what the NEC says. Other than Article 647, which is quite specialized, all of the references to a voltage drop specification in the NEC occur in informational notes. That means they are not enforceable limits, just design suggestions.

So the minimum conductor size for a 125A feeder with 75C terminations and no ampacity adjustment or correction required would be #1 Cu or #2/0 Al. [Unless on a residential 125A service, then the 83% rule applies and you could use #2 Cu or #1/0 Al. But I'm going to ignore that case.]

Also, as the standard breakers sizes are 110A and then 125A, you can protect #2 Cu (115A 75C ampacity) and #1/0 Al (120A 75C ampacity) with a 125A breaker, as long as the calculated load is less than 115A or 120A, respectively, per 240.4(D). So those can plausibly be used to supply a 125A subpanel.

A couple other errors: you give the Al ampacity of 2/0 but specified Cu; and your voltage drop formula gives you volts, not a percentage. To get a percentage, you'd need to divide by the system voltage, which I don't see specified.

Cheers, Wayne
 
wwhitney said:
That's not actually what the NEC says. Other than Article 647, which is quite specialized, all of the references to a voltage drop specification in the NEC occur in informational notes. That means they are not enforceable limits, just design suggestions.

So the minimum conductor size for a 125A feeder with 75C terminations and no ampacity adjustment or correction required would be #1 Cu or #2/0 Al. [Unless on a residential 125A service, then the 83% rule applies and you could use #2 Cu or #1/0 Al. But I'm going to ignore that case.]

Also, as the standard breakers sizes are 110A and then 125A, you can protect #2 Cu (115A 75C ampacity) and #1/0 Al (120A 75C ampacity) with a 125A breaker, as long as the calculated load is less than 115A or 120A, respectively, per 240.4(D). So those can plausibly be used to supply a 125A subpanel.

A couple other errors: you give the Al ampacity of 2/0 but specified Cu; and your voltage drop formula gives you volts, not a percentage. To get a percentage, you'd need to divide by the system voltage, which I don't see specified.

Cheers, Wayne
Click to expand...
Bingo
 
You must log in or register to reply here.
Top