Allowable ampacities Table 310-15 vs. voltage drop calculators

Status
Not open for further replies.
C

chuckfraz

Member
Location
New Jersey
Can anyone help clear up the difference between the allowable ampacities in table 310-15 and the results many online voltage drop calculators show? For instance, a 350 amp circuit, 3P 480/277 at 100 ft. Table 310-15 indicates 500 MCM copper conductor to be used at 75 degree. Table 310-15 does not take into account distance, voltage, voltage drop or phases. Voltage drop calculator results recommend 1/0 copper for the same circuit but also state they don't take into account minimum ampacities of table 310-15.
Confusing to say the least.
Thanks
 
It's not confusing at all. In fact, you explained it perfectly. There are two separate issues to be dealt with, and they are not related to each other. You need to pick a conductor that is large enough to handle the calculated load (i.e., the ampacity issue) and that is also large enough to provide a sufficient voltage at the end of the run (i.e., the voltage drop issue). So you look at both, and you pick the larger of the two results.

Welcome to the forum.
 
So does that mean you can only use the calculated result if it is higher than the table?
Is there any situation where you would be allowed to use the lower 'calculated" wire size?
Cofusing for me because I grew up in the electronics world and it makes sense to size the wire to load, distance and voltage. Table 310-15 states the wire sizes apply to circuits 0 -2000 volts. Seems like overkill to me.
Thanks
 
chuckfraz said:
So does that mean you can only use the calculated result if it is higher than the table?
Is there any situation where you would be allowed to use the lower 'calculated" wire size?
Cofusing for me because I grew up in the electronics world and it makes sense to size the wire to load, distance and voltage. Table 310-15 states the wire sizes apply to circuits 0 -2000 volts. Seems like overkill to me.
Thanks
Click to expand...

Yes, in most instances you can use the ampacity table as usually voltage drop is not an issue unless the circuit is longer than about a hundred feet. I think what you are missing here is that the ampacity table takes into account the temperature rating of the device it is connected to. For example, a voltage calc might say that a given conductor will deliver XX amps to the load at a stated voltage drop but it may run at a temperature that is above the temp rating of the terminal or device. The ampacity tables include this.
 
chuckfraz said:
Is there any situation where you would be allowed to use the lower 'calculated" wire size?
Click to expand...
I think not. Here is the general sequence of the design process:

  1. Calculate the load. Take into account any reductions for allowable demand factors, and any increases for continuous loads.
  2. Select a conductor size that has sufficient ampacity to handle the calculated load. Take into account any derating factors related to ambient temperatures being higher than 30C or related to having more than three current-carrying conductors in the same raceway.
  3. Select an overcurrent device that is capable of protecting the selected conductor.
  4. If voltage drop is a concern (generally not, for runs under 100 feet), then decide whether to select a larger conductor, in order to reduce the amount of voltage drop.
  5. If you do select a larger ungrounded conductor (due to voltage drop or for any other reason), you also need to upsize the equipment grounding conductor, per 250.122(B).


 
chuckfraz said:
Table 310-15 states the wire sizes apply to circuits 0 -2000 volts. Seems like overkill to me.
Click to expand...
Not to me. A conductor that has an insulation system rated for 5000 volts or one rated for 15000 volts is going to have a higher ampacity than a wire of the same AWG size that is rated for 600 volts. That is because its insulation system is more rugged, and better able to withstand the heat that is generated by the current flowing through it. So it makes sense to have separate ampacity tables for the conductors with lower rated insulation and for conductors with higher rated insulation.

 
Think of the wire itself as a component with a bunch of different ratings: resistance, power dissipation, temperature, etc.

In the electronics world, operating at low voltage and low currents, you are much more likely to 'bump into' the resistance of the wire as a limiting factor.

In the building wiring world, operating at higher voltages and higher currents, you are more likely to 'bump into' the power dissipation capacity of the wire as a limiting factor. If you size based on resistance only, then short conductors would be used in a fashion that would exceed their power dissipation rating. They would overheat and their insulation would fail, even though the voltage drop would be perfectly acceptable.

You have to respect _all_ of the limits of the components being used.

-Jon
 
I think there is a third issue to treat it: short-circuit current withstanding. Nor NEC neither BS7671 (IEE Reg.) deal with this. There are other as IEC 60865, IEC60346-5-52, BS7430, IEEE-80/2000, ICEA protection chart, IEEE-242/2001 and other.
Let?s take IEC 60346-5-52
I=K*S/sqrt(t) K=176 for copper grounding cable K=143 for power cables
t=fault clearing time [sec] S-cross-sectional area of conductors [mm^2]. I=maximum withstand short-time current [A].
 
I forgot to mention it is for XLPE/EPR insulated conductor where maximum short-time temperature is 250 dgr.C.:slaphead:
 
Chuck I think Charlie summed it up pretty well. As stated there are two issues at hand but I have a different take on it. One is safety (NEC), and the second is performance (voltage drop).

I work a lot with low voltage battery systems and have to use voltage drop rather than 310-15. Here is a great example. Requirement 12 volt DC, 15 amps load current, 50 feet 1-way wire distance, voltage drop not greater than 3%. 75 degree.

If I were to use NEC the MINIMUM SIZE conductor required is #14 AWG. However if I were to do that, the circuit would never work because with 15 amps and 50-feet 1-way distance voltage drop would be 32% or 8.2 volts at the load device. To get less than 3% voltage drop would require 1 AWG. With that said NEC allows me to use 1 AWG with a 15 amp load protected with a 20 amp DC breaker. The circuit is now both safe and functional.

NEC 310 does not assume any operating voltage, or voltage drop, only currents for safety. Take that same exact circuit except now lets use 120 volts and now that 14 AWG cable works and safe.

Now here is the evil twist which Charlie is talking about. Using voltage drop can get you in big trouble when using short distance at the higher voltages. Another example will clearly demonstrate this point. Lets now say 240 VAC, 200 amps, 5 feet 1-way, 3% or less voltage drop. Using the VD method tells you all you need is 14 AWG. Does that set off alarm bells? 14 AWG with 200 amps? NEC minimum requirement is 3/0.

As Charlie points out you use the larger of the two methods. With that said my take is NEC is always the minimum, unless VD dictates a larger conductor. Most installation do not have voltage drop requirement. NEC is minimum safety and does not take operation into consideration.
 
Last edited:
Please read code section 90.1, but I'll bet one of my esteemed colleagues will cut and paste it here in a few minutes.

This is supplemental to what everyone else has been saying. The NEC does not have a voltage drop requirement in the code. (some States and municipalities do). The ampacity of a wire is important because of safety. It is based on a number of factors including, temperature and insulation type. Temperature is so dependent on installation and environment that the code does its best to compensate for it in many ways, # of conductors, buried, in conduit, in air outside temp etc. But voltage drop has no direct correlation to amperage, so the code doesn't deal with it in that section. Also, you don't have to compensate for voltage drop by changing the wire size. You can do something else, like install a voltage boosting transformer.
 
All good and helpful explanations, not scratching my head any longer. Thanks!
One site on the internet displayed table 310-15 and minimum wire size calculator below the table. I know 310-15 rules but that calculator was driving me crazy. Searched other calculators online and they were all giving the same answers. Of course I'd like to use the smaller conductor, easier on the knuckles. Happy New Years!
 
chuckfraz said:
So does that mean you can only use the calculated result if it is higher than the table?
Is there any situation where you would be allowed to use the lower 'calculated" wire size?
Cofusing for me because I grew up in the electronics world and it makes sense to size the wire to load, distance and voltage. Table 310-15 states the wire sizes apply to circuits 0 -2000 volts. Seems like overkill to me.
Thanks
Click to expand...
The ampacity tables have more to do with protecting the conductor insulation from degrading from overheating conditions than they have to do with the ability of the conductor itself to carry the load.
Can you push 40 amps through a 12 AWG 90deg conductor and not notice any extreme overheating of the conductor? probably so. But consider how much it may degrade the insulation over time if allowed to run at that kind of load, then you also have to throw in other factors that require an adjustment to the ampacity. It is all to ensure the insulation integrity lasts a long time, heat is the enemy to most any electrical insulation, those that can take more heat can not take much for physical abuse.
 
Status
Not open for further replies.
Top