Help with 80% rule

Status
Not open for further replies.
S

sparrott4

Member
I'm trying to find the part of the code that sets the allowable wire load to 80% of the rated capacity under certain conditions - also relating that to operation time.

Can anyone point me to the code number(s)?

It's also possibe that I'm confusing this with a UL standard and it doesn't exist in NEC.
 
210.19,215.2, 430.22,430.62,440.32

There is probably plenty more

whenever ocpds and conductors supply loads that will operate for three hours or more the Code generaly limits that continuious load (or that portion of) to 80% of the the conductors capacity. the OCPD is only tested at 80% of its rating under continuious load applications.
 
cpal said:
the OCPD is only tested at 80% of its rating under continuious load applications.
Click to expand...

OCPD are tested at 100% of their rating in OPEN AIR, the derating comes from a combination of other factors, primarily of which is the fact they are mounted in enclosures.

The NEC requires conductors to be applied at 125% of the load (in previous codes it was worded that the load could not be more than 80% of the conductor rating). Because OCPD protect conductors they are designed to have an enclosed rating of 80%.
 
Thanks guys but I guess I need to get more specific about which of the codes apply to the following:

Double-stranded conductors connecting outdoor lighting fixtures to secondary taps on a 12V transformer.

Where does the 125% figure in? And what specific code applies.
 
sparrott4 said:
Thanks guys but I guess I need to get more specific about which of the codes apply to the following:

Double-stranded conductors connecting outdoor lighting fixtures to secondary taps on a 12V transformer.

Where does the 125% figure in? And what specific code applies.
Click to expand...


You will have a voltage drop issue first. If you are running 12v landscape lights by the time you need to worry about "80%" your voltage drop will hit a point that the lights won't be at full brightness. More likely they will be very dim. The only way to off set this issue is to up size the conductors for voltage drop. You need to calculate the load for voltage drop not 80% continuous load. you are better off running multiple conductors off the XFMR to the lighting loads. Use at least #10 conductors if you are a 200watt XFMR.
 
Another way of thinking about this:

The number on the handle of the OCPD is its trip rating. This number is not the current that the OCPD is rated to carry on a continuous basis; it simply indicates (roughly) what current will cause the OCPD to open. The current that an OCPD can be expected to safely carry on a continuous basis (its 'ampacity' if you will) is an entirely different number which could be greater or less than the trip rating.

Most OCPD devices are actually _rated_ to carry 80% of their trip rating on a continuous basis. _Some_ OCPD devices are rated to carry 100% of their trip rating on a continuous basis; to use these at that current level, both the device and its enclosure must be so rated. Presumably you could have an OCPD device that could safely carry more than 100% of its trip rating on a continuos basis, but that capacity would never be used because the device would trip :)

Most or all of the '80%' rules have exceptions for 100% rated OCPD devices.

-Jon
 
I don't think my question applies to the OCPD which, in this case, is the 25A magnetic breaker on a secondary common. There can be several wires connected to a common.

My question is concerned with the amperage rating on the wire and if a % max. for continuous use applies.

Thanks.
 
sparrott4 said:
I don't think my question applies to the OCPD which, in this case, is the 25A magnetic breaker on a secondary common. There can be several wires connected to a common.

My question is concerned with the amperage rating on the wire and if a % max. for continuous use applies.

Thanks.
Click to expand...


Are these in "free air" or inside conduit, or buried in the dirt, or stapled to a tree, or running trough an interior wall, Etc. What is the listing on the wire. specifically What is written on the cable ? How far do you plan on running the cable "length in ft." How many watts of light are you trying to get on this xfmr?

A/C or D/C ?
You profile says Occupation:Manufacturer is this question related to something you are manufacturing or something you are installing?
 
Last edited:
The cable is direct burial listed and has all the ratings to comply with this use. I'm not asking about a specific project, but rather would like to know the NEC or UL code/standards that restrict the amperage each wire can carry under continuous use.

My understanding is that there used to be an 80% of capacity restriction for such wires when they are operating for three hours or longer. I think this 80% value has changed in the code, but can't find where the new info is.
 
I realize that you are asking about the _conductors_. However the reason for the 80%/125% rules is the OCPD. If you have OCPD rated for 100% use, then you can use the conductors at 100% of their calculated ampacity on a continuous basis.

As Charlie noted above, see for example 210.19 and then look at the Exception. This did not change in the 2002 or 2005 codes. 210.19 talks about the branch circuit _conductors_, but the exception is invoked if you use appropriate OCPD. Charlie listed other code references that speak to the same issue, eg. for feeders.

-Jon
 
Status
Not open for further replies.
Top