[ptnoble]

We are running #12 Romex wire for circuits from the panel. I want some of the breakers in the panel to be 15amp and some to be 20amp. My electrician says that they should all be 20amo because #12 Romex is code for 20 amps. I said sure but it shouldn't matter is we go smaller than code (i.e. 15amp breakers running to #12 Romex) but my electrician seems to think this will bother the inspector. Does his argument hold any water? Should I really change to #14 Romex to match the 15A breakers?

Thanks!

[shuntripper]

You may need another electrician. How would he compensate for voltage drop on a very long circuit? Putting 20A wire on a 15A CB is not a problem and is actually REQUIRED in circumstances like the above. Putting a 20A receptacle on a 15A circuit is no problem either, What IS a problem though, is a lower rated wire on a higher rated CB . The NEC states that 14awg wire is not to be on a CB or fuse rated more than 15A, 12awg not more than a 20A CB, and 10awg not more than a 30A CB.:confused: But why do you want 15A Circuit Breakers anyway when you are running 12awg wire? The only reason to use a 15A breaker is because you have 14awg wire and the Code says you have to, there can't be much of a price diff between the CBs.

[jafo1666]

We are running #12 Romex wire for circuits from the panel. I want some of the breakers in the panel to be 15amp and some to be 20amp. My electrician says that they should all be 20amo because #12 Romex is code for 20 amps. I said sure but it shouldn't matter is we go smaller than code (i.e. 15amp breakers running to #12 Romex) but my electrician seems to think this will bother the inspector. Does his argument hold any water? Should I really change to #14 Romex to match the 15A breakers??

Thanks!

Your inspector will probably shake your hand and ask you to teach your electrican what really good work is.

[ptnoble]

you may need another electrician. How would he compensate for voltage drop on a very long circuit? putting 20A wire on a 15A CB is not a problem and is actually REQUIRED in circumstances like the above. Putting a 20A receptacle on a 15A circuit is no problem either, What IS a problem though, is a lower rated wire on a higher rated CB . The NEC states that 14awg wire is not to be on a CB or fuse rated more than 15A, 12awg not more than a 20A CB, and 10awg not more than a 30A CB.:confused: But why do you want 15A Circuit Breakers anyway when you are running 12awg wire? The only reason to use a 15A breaker is because you have 14awg wire and the Code says you have to, there can't be much of a price diff between the CBs.

Thanks. This electrician is driving me a bit batty but we've agreed on a price for the whole project so I'm going to tough it out for now. Since it was brought up, what distance constitutes a very long circuit?

The reason I want to keep some of my CB as 15A is because I am moving the location of my panel which currently has 15A breakers run into 14awg wire. The old panel becomes a big J-box, but if I upgraded the 15A CBs to 20A then I would have the potential to run 20A into 14awg wire, which I want to avoid. But this setup (15A CB and 12awg) means if I upgrade in the future I only have to change the old wires. Make sense?

[shuntripper]

OK, I understand now. Yes, you should and MUST use the 15A CBs as long as there is ANY 14awg wire IN the circuit. Again, 14awg is only good for 15 amps, you should probably make a permanent label inside the new panel that informs any subsequent electrician or owner of this condition and to which circuits it applies. Would be better (and inspector may require it) just to make the circuit extensions to the new panel for the old 15A circuits in 14awg. You could do it in pipe and change the entire thing when you upgrade. Remember, where there's a will there's a way. I have to make architects' and engineers' ideas work all the time.

[shuntripper]

Forgot about the very long circuit. That's a long answer. This involves ohm's law for power and Chapter 9, table 9 in the code. E=IR and P=EI are Ohm's laws for Voltage and Power respectively. Voltage=E, current=I,resistance =R,power=P. There are 12 equations from which you can figure out relationships between any of these variables in a circuit. The twelve equations are all derived from these two. Get an Ugly's book from the wholesale house and you can figure it out. Basically I don't want to see more than 3% voltage drop over the length of the circuit.When you're done with that we'll start on trigonometry, phase-time relationships and circuit variables in three phase power.

[ptnoble]

Forgot about the very long circuit. That's a long answer. this involves ohm's law for power and Chapter 9, table 9 in the code. E=IR and P=EI are Ohm's laws for Voltage and Power respectively. voltage=E, current=I,resistance =R,power=P. There are 12 equations from which you can figure out relationships between any of these variables in a circuit. the twelve equations are all derived from these two. Get an Ugly's book from the wholesale house and you can figure it out. basically I don't want to see more than 3% voltage drop over the length of the circuit.When you're done with that we'll start on trigonometry, phase-time relationships and circuit variables in three phase power.

Do the tables give you a quick and easy answer without having to summon up long-out-of-use trigonometry? Basically, what is the maximum distance you can run a 20A circuit on 12awg copper wire? (without needing to go to I bigger gauge wire). What about the max distance to run a 30A circuit on 10awg copper wire?

[tkrussell]

There is no accurate chart with all the variables considered for voltage drop, only calculations based on the following variables, length of circuit, load in amps on circuit, wire size and type.

There are several on line voltage drop calculators available, most using the following formula.

The tables referred to in Chapter #9, Table 8 only list the properties of wires, one being the resistance per 1000 foot, whch is used in the following formula to calculate voltage drop:

(2 x length of circuit x amp load x ohms /1000 foot of chosen wire) / 1000=voltage drop, or (2*L*I*R) / 1000 =Vd.

Using Table 9, #12 solid copper wire has a resisitance 1.93 ohms per 1000 feet.

Maximum recommended voltage drop is 3 %, so max Vd for a 120 volt circuit is 3.6 volts.

Maximum allowed current on a 20 amp circuit is 80%, or 16 amps.

Using some hypothetical values:

2 * 150 feet * 16 amps * 1.93 ohms / 1000 = 9.2 volts dropped. Too far of a run.

So, to get within 3% Vd, using the same wire and load:
2 * 75 * 16 * 1.93 /1000= 4.63 volts dropped.

So. Looks like 75 feet is pushing it.


For a 30 amp 120 volt circuit, the 3%, or 3.6 volts remains the same. The Ohms/1000 ft is now 1.21 ohms, and the max current allowed on a 30 amp circuit is 26 amps.

So,
2 * 100 ft * 26 amps * 1.21/1000= 6.3 volts dropped
2 * 50 ft * 26 amps * 1.21/1000= 4.7 volts dropped

Seems your 30 amp 120 volt circuit needs to be less than 50 feet to get below 3 % Vd.

Do a web search for voltage drop calculators, try plugging in a few variables to get the sense of how this works.

[shuntripper]

Thanks TK, where is a voltage drop calculator online? My company Nextel has web capability, this could be a pretty cool resource I was unaware of.

[tkrussell]

As I stated, do a web search for voltage drop calculator, such as Google:

voltage drop calculator - Google Search

You will find several that work well.

[Stratmando]

Inspector should not have a problem of #12 on a 15 amp breaker. The breaker protects the wire, and needs to be the weak link. You must not have #14 on a 20 amp circuit breaker.