Ampacities

A

Alwayslearningelec

Senior Member
Location
NJ
Occupation
Estimator
I have a few questions regarding protecting wire from overload of current. I know the OCPD( whether breaker, switch, fuse etc.) protects the wiring.

I get a little confused sometimes by allowable( or the correct)amperages to be used for the wire and OCPD.

  1. How does the ampacity of the load/wire and OCPD work? The wire ampacity must notbe larger than the OCPD and Load amperage?
If someone could give quick overview(shortcut thinking) of the ampacity sizes of load/wire and OCPD. Thanks.
 
Basic rule (other than 240.4(G)

Calculate the load (100% non-conspicuous + 15% continuous)
Select the conductor with that ampacity or greater (after deratmg when required and based on termination ratings )
Select an overcurrent device to protect that conductor


Example: 47 amn load requires a conductor with 47 amp adjusted ampacity.
Verify conductor ampacity to termination ratings 10.14(C) 60 or 75°
Select next size breaker to protect conductor (50 amnp)
 
The wire can be as large as you need and more as long as the wire has the ability to handle the load needed. Take a 200 amp panel that has a load of only 50 amps. We may have a 3/0 copper conductor for the 50 amps.

The answer to your question will also vary based of the type of load. For instance, a motor my be fed with a breaker that is larger than the ampacity of the wire as long as there is overload protection in the motor. In that case, the overcurrent protective device just protects from short circuit and ground faults.
 
For simplicity sake, wire can never have an ampacity (maximum number of amps it can safely carry) than the number of amps it is going to carry under normal circumstances. Everything else about sizing wire, overcurrent protection, etc. is there to ensure this golden rule is followed. This rule allows for Dennis and Infinity's answers, along with almost every other situation.
 
Select a wire size for the load depending on continuous load or non continuous or intermittent load etc.
adjust for ambient temperature
adjust for #of current carrying conductors

Select fuse or breaker not to exceed wire ampacity but consider next size up OCPD if the selected size is not available

Motors are different. If a VFD is used size on the basis of the VFD name plate

For motor circuits use amperage allowed in 430

Select motor overload protection based on motor name plate
For packaged equipment Chillers, HVAC equipment use the equipment name plate.
 
augie47 said:
Basic rule (other than 240.4(G)

Calculate the load (100% non-conspicuous + 15% continuous)
Select the conductor with that ampacity or greater (after deratmg when required and based on termination ratings )
Select an overcurrent device to protect that conductor


Example: 47 amn load requires a conductor with 47 amp adjusted ampacity.
Verify conductor ampacity to termination ratings 10.14(C) 60 or 75°
Select next size breaker to protect conductor (50 amnp)
Click to expand...
After calculating the true load ampacity the wire can be sized at that ampacity OR LARGER?
 
Alwayslearningelec said:
After calculating the true load ampacity the wire can be sized at that ampacity OR LARGER?
Click to expand...
Let's say you have a 52 amp load with a wire that is rated 55 amps. This is fine but you can also upsize the overcurrent protective device to 60 amps as long as the load is not greater than the wires ampacity (55 amps).
 
Alwayslearningelec said:
After calculating the true load ampacity the wire can be sized at that ampacity OR LARGER?
Click to expand...
Use your common sense one this question. I am NOT in any way being sarcastic. We often forget to take a step back and just look. As an estimator how many time have you multiplied x * z and come up with xxxxxxxxxxxxxxxxxxxxxxzzzzzzzzzzzzzzzz, then said, "no way." and looked to find out you fat fingered a number.

That said. If 20 amps has the potential to burn up the insulation on a wire, would the code EVER allow it to be used?

Alternately if #12 wire can carry 20A, why would the code protest you using #8 that carries 40 amps?
 
Just remember if you upsize a conductor, other than to the next size standard breaker, you would have to upsize the equipment grounding conductor proportionally -- 250.122(B) I think
 
Strathead said:
Use your common sense one this question. I am NOT in any way being sarcastic. We often forget to take a step back and just look. As an estimator how many time have you multiplied x * z and come up with xxxxxxxxxxxxxxxxxxxxxxzzzzzzzzzzzzzzzz, then said, "no way." and looked to find out you fat fingered a number.

That said. If 20 amps has the potential to burn up the insulation on a wire, would the code EVER allow it to be used?

Alternately if #12 wire can carry 20A, why would the code protest you using #8 that carries 40 amps?
Click to expand...
Understand. For some weird reason it's the concept on this that my brain short circuits...lol

So a OCPD needs to be sized according the the wires ampacity? Meaning the OCPD can't be less than the wire ampacity?
 
Alwayslearningelec said:
Understand. For some weird reason it's the concept on this that my brain short circuits...lol

So a OCPD needs to be sized according the the wires ampacity? Meaning the OCPD can't be less than the wire ampacity?
Click to expand...
No, the overcurrent protective device in most case can't be larger than the wire ampacity. You wouldn't want a 30 amp wire with a 50 amp breaker. That would allow the wire to heat up before the breaker could trip.
 
Alwayslearningelec said:
Understand. For some weird reason it's the concept on this that my brain short circuits...lol

So a OCPD needs to be sized according the the wires ampacity? Meaning the OCPD can't be less than the wire ampacity?
Click to expand...
As Dennis said, no. Maybe this will help. The ampacity of the wire is one parameter. The ampacity of a circuit is a separate and usually different parameter. The rule I gave you is for the first parameter. The ampacity of a circuit is the size of the overcurrent protection, period.

Use the KISS principle the above two items are true. Now, there are times when the ampacity of the circuit can exceed the ampacity of the wire, but they are basically exceptions. On top of that, they are exceptions that are rarely utilized on engineered drawings.

Remember the code is inclusive, so if it doesn't say you can't do something, then you can. It says the wire's minimum ampacity is ................ then nothing says what its maximum capacity is. That means it CAN be anything larger.
 
The OCPD is a gate. It only allows so much current through.

If you have a valve that allows 1 gallon per minute to flow, then you dump that 1 gallon per minute into a swale that can handle 500 gallons per minute, that’s fine. You have a small trickle down a giant drainage swale.

If you have a valve that allows 500 gallons per minute through, and then you dump that water into a small trench that you dug with your hand, it’s going to overflow and flood that entire area.

Same principle here. That OCPD is what protects everything downstream. If you want to protect 3/0 conductors with a 20A OCPD, that is your prerogative, you’ll just have to explain to your boss why you’re spending so much money on copper.
 
Ampacity ratings are aimed at protecting the insulation from damage from overheating. That's why different insulation types have various ampacities.
 
When I install HVAC equipment, I go one step further from the Equpment Label and always follow the manufacturer's Installation Manual that might require 60-degree wiring only.
 
Dennis Alwon said:
Just remember if you upsize a conductor, other than to the next size standard breaker, you would have to upsize the equipment grounding conductor proportionally -- 250.122(B) I think
Click to expand...
Not only that, but you often have to also consider that overly large wire may not fit into a device terminal. For example you can run #8 wire on a 20A branch circuit, but just try terminating #8 wire into a 20A circuit breaker or on a duplex outlet... (without having to wire nut a pigtail to it).
 
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