Ampacity of 90C 10AWG copper conductors is 40 amps.
240.4(D) says other wise.
Also, there is not a circuit out there than can be figured at 90 deg C. You need to take into account all the items in a circuit, and very little is rated for 90 deg other than the wire.
The quick answer is 30A. The long answer needs a lot more info.
Originally Posted by manhattan42
If on the other hand you are concerned about voltage drop, then the voltage will be reduced by ~7.5 volts over 75 feet.
This is a voltage drop of about 6.2% if using 120volts or 3.1% if using 240volts.
May I ask what you are basing this on?
Voltage drop is directly proportionate to amperage. If you are using 40A then all these numbers are meaningless.
No. Ampacity of 90C 10AWG copper conductors is 40 amps... not 30 amps according to Table 310.16.
The maximum overcurrent protection of 10 AWG cannot exceed 30amps according to 240.4.
As far as the voltage drop calcs go, you are correct, and I should have used 30 amps not 40 amps in the math...
Using 30 amps, the voltage drop is 5.59 volts or a 4.6% drop on a 120v circuit or a 2.3% drop on a 240v circuit... and nonetheless reflects the same reality that the drop exceeds recommended voltage drop of 3% maximum for branch circuits but is less than the recommended maximum feeder voltage drop of 5% regardless...
Manhattan... exactly... we know nothing about what this poster wants to do. Yet you go on and on about your "knowledge" that doesn't answer or solve anything. The poster asked a question, albeit one that can't be answered as stated, but you go on and on and hijack the post with your paraintellect. Put me in the category with donf and stanfortyman... you are missing the point of both the original post, and the purpose of this forum. It's about helping people, not spouting off about your "knowledge"
For what it is worth, I was not jumping on your answer. It did not make sense to me because of my understanding of howthe conductors are selected from each of the temp. columns.
I know I read somewhere, I suspect that it was in a Residential wiring textbook that hiding at the moment, that although NM-B cable is made up of two 90c wires does not mean you can automatically use the 90c column.
I don't want to squabble with you. However, I do know that if we do not know the temperature of the connection device, we must use the 60c column.
I believe that it also stated that only the sheath is marked at 90c, not the conductors themselves.
I'll be straight up with you, I believe you to be incorrect and I'll see if I can find the documentation to support my position or apologize to you if I am incorrect.
Interesting discussion, however, most of it irrelevant regarding Romex cable.
The length of the circuit, as related to ampacity, is irrelevant. The maximum ampacity #10 copper wire in a Romex cable, no matter whether one foot or 1000 feet, is 30 amps.
The implied question is if #10 Romex can deliver 30 amps 75 feet.
The key word in the original question is "Romex", which is technically NM cable.
Whether this is residential or otherwise, is irrelevant.
This wiring method is covered by Article 334 of the National Electric Code. This is the first article to be reviewed, not ampacity and temperature tables.
Copied from the NFPA 70 NEC 2008 Edition:
Section 334.80 Ampacity. The ampacity of Types NM, NMC, and NMS cable shall be determined in accordance with 310.15.
The ampacity shall be in accordance with the 60°C (140°F) conductor temperature rating.
Therefore, a #10 copper wire that is part of a NM cable shall be rated according to the 60°C column of Table 310.16, at no more than 30 amps.
This is whether the installation in residential or commercial. I do not see how that made a difference in the beginning of the thread.
NM cable, both sheathing AND conductors, are rated 90°C, and is only for the purpose of the location of the cable and wiring. Has nothing to do with terminal temperature, since Romex already has the limitation of 60°C.
So, kverrett, to start with, the maximum ampacity and overcurrent protection of #10-3 Romex is 30 amps.
Next, to the implied question, is #10 wire sufficient to carry 30 amps 75 feet.
Here we need to assume the voltage is 120/240 volts, with the greater load at 240 volts. For worst case calculation purposes, we will assume 30 amps at 240 volts.
The basic voltage drop formula recommended by IEEE is:
Vd=2 x R x L x I divided by 1000
Vd = Voltage drop
2 = Number of conductors in circuit
R = DC Resistance of conductor per 1000 foot from NEC® Chapter 9 Table 8
L = Distance
I = Current in amperes (A)
Since we know Romex cable, we will assume solid copper wire, R=1.21 Ohms per 1000 feet.
Vd= 2 x 1.21 x 75 feet x 30 amps / 1000 = 5.445 volts dropped.
Next, assuming this is a branch circuit, and not a feeder, the recommended maximum voltage drop is 3%.
Assuming 240 volts and a 3% max Vd, the maximum voltage dropped allowed is the 7.2 volts, so the 5.445 volts dropped is below 3%.
Your #10-3 Romex cable will be fine to carry 30 amps 75 feet.
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