Minimum amperage for wire on short runs

Status
Not open for further replies.
H

holthauser

New member
Location
South Dakota
Looking at the ampacity table in the NEC, a question came up on whether or not that is for short runs. Is this table measuring say off 1ft of wire? Say you had 2" of wire, does a 14AWG wire at 75deg C still have the ability to hold 20amps? At some point, it seems like a short wire may have problems dissipating the heat, or am I looking at it wrong? I guess the real question is whether or not the table is using a min(max) measurement for that current rating. When you figure out ampacity by hand, run length of the wire is one of the factors.
 
the tables are extremely conservative, especially for smaller wires.

I would not get all that worried about the wires overheating as long as the rules are all followed.

keep in mind that even though a #14 might show an ampacity of 20A, you cannot use OCPD rated more than 15 A on it.
 
the amount of heat generated in a wire is proportional to its length. You typically do not have to worry about very short wires, it is with longer wires that have more resistance and voltage drop across the circuit.

though not up to code, I have seen many a water heater wired with 14 gauge wire and never have problems, because the wire is free air and about 2 ft of length til it lands on a circuit breaker. If you tried the same thing with a hundred foot of number 14 running through insulated wall cavities, you would have problems.
 
180530-1940 EDT

Temperature rise in something is a function of the power input and the ability of the something to dissipate the power input.

If I have a long wire with a constant current in said wire and all environmental factors are constant, then the temperature rise at any one point will be the same. This excludes what happens near the end points. With constant current you have the same power dissipated per unit length of the wire independent of the length.

Holding the current constant in the wire and changing the wire length will not have any effect on the temperature rise at any point.

Now to end effects. If in the long wire setup there is no excess temperature rise at the end points when long, then shortening the wire will not produce any increase in heating in the middle of the wire or the endpoints.

Obviously if changing wire length changes the environment with respect to heat transfer, then there could be other conclusions. Not likely these would be much different.

.
 
holthauser said:
Looking at the ampacity table in the NEC, a question came up on whether or not that is for short runs. Is this table measuring say off 1ft of wire? Say you had 2" of wire, does a 14AWG wire at 75deg C still have the ability to hold 20amps? At some point, it seems like a short wire may have problems dissipating the heat, or am I looking at it wrong? I guess the real question is whether or not the table is using a min(max) measurement for that current rating. When you figure out ampacity by hand, run length of the wire is one of the factors.
Click to expand...

I think you might be misunderstanding an important aspect of this. Your concern seems to assume that there is a fixed amount of heat generated within a circuit, and that this constant quantity would be dissipated by a wire regardless of length. Not so...the resistance of the components generate heat. More components, more wire, more resistance and more heat. The wire generates heat at a fixed rate per inch, per foot, etc. So at a given voltage and current load, a given wire size will dissipate, say 12 BTU's per inch. The longer the wire, the more heat generated, but it has more length and therefore more surface area with which to dissipate that heat.
 
Current capacity is limited by conductor cross-sectional area. Voltage drop is dependent on conductor length and current vs conductor area. Power dissipation is a result of current and voltage drop.
 
The ampacity tables in 310.15 are primarily about keeping insulation temperature below it's rated temperature, second thing they do is keep termination temperatures within their range.

Outside of those two things the conductor itself can carry quite a bit more current then in those tables without being destroyed. Voltage drop can be a side effect though. But the tables aren't about voltage drop either.
 
petersonra said:
the tables are extremely conservative, especially for smaller wires.

I would not get all that worried about the wires overheating as long as the rules are all followed.

keep in mind that even though a #14 might show an ampacity of 20A, you cannot use OCPD rated more than 15 A on it.
Click to expand...

Motor loads are an exception :happyyes:
 
Status
Not open for further replies.
Top