Ampacity?

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bennie

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I have every code book back to 1951, the year I started in the trade.

I have researched every edition on section 310, concerning adjustment factors for conductor ampacity, based on voltage drop, wire fill, and ambient temperature.

Every reference is in regards to adjustment of ampacity by reducing the load on the conductor.

The ampacity,(current) of a conductor, can only?be limited by the connected load.

I think the accepted method of smaller breakers, and larger wire, is a misunderstanding of the intent of the code.

Carefully read the definition of ampacity.
 
Re: Ampacity?

I am reading both my 2002 NEC and a 1947 NEC in the "selection of ampacity" sections.

I have no clue what you are trying to point out. I don't read anything that says reduce the load. In most cases you can't. I only read it to say reduce the current carrying capacity of the conductor. If the conductor is reduced to below the demand of the load, then your only choice is to increase conductor size.

Its the conditions of use that determines ampacity? :confused:
 
Re: Ampacity?

You are correct Justin. What is better more lightly loaded No.#12 circuits or a few heavy loaded No.#10s.

I feel lightly loaded circuits produce less heat, therefore they are safer. What do you think?
 
Re: Ampacity?

sounds good but if you had to power a piece of equipment 120 ft from the panel you probably wouldnt set another panel at the load. Wouldnt you bump up the size of the wire?
 
Re: Ampacity?

I still don't get it.

A lightly loaded #12 circuit in temperatures above 86 degrees F, or with more than three current carrying conductors, or with excessive voltage drop will still need to be derated. If that deration drops the capacity of the conductor to below the rating of the circuit, you will have to either reduce the OCD, or increase the wire size.

In most cases, reducing the circuit rating is not possible, increasing wire size is.

15, 20, and 30-ampere branch circuits are already restricted for permitted load by the code, assuming that adding circuits will decrease the load can not be established or ensured.

The homeowner may choose just one receptacle to plug several loads, now that circuit is overloaded. Load reduction can't be predicted. :confused:
 
Re: Ampacity?

Examples:

#1. 20 amp circuit breaker. #12 wire. 2 amp fixtures.
10 fixtures is 100% load.

#2. 20 amp circuit breaker. #12 wire. 2 amp fixtures.
8 fixtures is 80% load.

#3. 10 fixtures at 2 amps equal 20 amps. 125% times 20 equals 25 amps. 30 amp circuit breaker, #10 wire, for one additional 2 amp fixture.

An additional panel is not cost effective unless there is numerous circuits. One fixed load, no problem with bumped up copper.
 
Re: Ampacity?

I'm really trying to understand this and I consider myself a pretty bright guy, but I still don't see the point and the examples didn't help.

:confused: :confused: :confused:
 
Re: Ampacity?

I think what Bennie was saying is that he would rather see more circuits and lightly loaded than less circuits and heavily loaded.
 
Re: Ampacity?

Justin has got the idea. Is it better to increase the circuit size 125% or decrease the load, on the circuit, to 80%?

If you increase the circuit size, the breaker also must be increased.


When making wire adjustments for temperature, the breaker must be adjusted to the higher rating.

When adjusting for wire fill the breaker also has to be adjusted for the revised load.

There is no point for a #10 wire being on a 20 amp breaker.

[ May 16, 2004, 08:40 PM: Message edited by: bennie ]
 
Re: Ampacity?

Is it better to increase the circuit size 125% or decrease the load, on the circuit, to 80%?
Click to expand...
What you are proposing is only practical for lighting and similar (receptacle) loads.

How do you "reduce the load" on a motor branch circuit or any other non-lighting load? :roll:

Ed

[ May 16, 2004, 09:36 PM: Message edited by: Ed MacLaren ]
 
Re: Ampacity?

With due respect, that doesnt make sense either. If I want to save $ on my elec. bill, I will run a #8 or #6 to the 30A C.B. conected to my air conditioner. Not a #10.
 
Re: Ampacity?

I'm also confused as to the point of the original question, but here is what I offer:

1. Ampacity (current carrying ability) of a particular wire (say #8 AWG) is determined only by its material and insulation type.

2. The main factor that determines whether this ampacity is permitted or not, is the surrounding air tempreture. The tables in the code list ampacities based on ambient 86 deg F, if the ambient temreture is more than 86 deg, then you you are not permitted to use the ampcities listed in the tables, instead, you need to use correction factors to determine the permitted ampacity.

3. The other factor that determines the permitted ampacity is the # of wires in a raceway, since more wires produce more heat inside the raceway.

example:

Ampacity of #8 THHN is 55 amps in 86 deg F.

if the outside tempreture is 110 deg F, and you have 7 current conductors in the same raceway, then this conductor is permitted to carry: .87 x .7 x 55 amps = 33 amps only.

If your load is 50 amps for example, then you need a bigger wire, assuming you're using THHN for wire insulation, and copper for wire material.

once you've selected the right wire, now it's time to calculated voltage drop; and most likely, and once you upsize the wire, the voltage drop doesn't become an issue, if it's then you upsize the wire again to get the right voltage drop.

Voltage drop is not a concern for the NEC code, the code only recommends upsizing the wire to correct voltage drop.
 
Re: Ampacity?

A fixed load cannot be changed, a variable load can be changed. This is not the issue.

Justin, calculate your savings on larger wire, and add 3 bucks at Starbucks for coffee.

Voltage drop is a concern of an experienced electrician.

[ May 16, 2004, 09:36 PM: Message edited by: bennie ]
 
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