80% rated breakers

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steve66

steve66

Senior Member
Location
Illinois
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Engineer
If I am correct, most circuit breakers are rated at 80%. That is, they can carry their full rated current for a short time, but for continuous loads that may last for hours, they are only rated for 80% of their current rating.

210.19 (2002 NEC) makes it clear that a branch circuit has to be sized for 1.25 times the continuous load. Since a branch circuit is sized per the breaker, the breaker has to be 1.25 times the continuous load.

215.3 makes the same case for feeders. The overcurrent device shall not be less than 1.25 times the continuous load.

230.42(1) states the minimum size for service entrance conductors. Basically, 1.25 times the continuous load unless the breaker is 100% rated. For a 100% rated breaker, the conductors can be sized at 100% of the continuous load.

Question: Why doesn't 230.42 require the breaker to be sized for 1.25 times the continuous load? Also, why are smaller wires allowed for a 100% rated breaker. It seems like this article should be setting minimum sizes on the breakers instead of minimum sizes on the conductors.

Final question: Do the unit loads in table 220.3(A) already have the 1.25 factor built in, or do I need to multiply lighting loads taken from this table by 1.25?

Thanks: Steve
 
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bennie

Esteemed Member
Re: 80% rated breakers

I'll get a lot of opposition to this statement, but here goes.

Breakers are rated at 100% of amp rating. When 100% load is over three hours duration, the load must be adjusted to 80% of wire ampacity and breaker amp rating.

Compensation for continuous load can be performed by two methods. Adjusting the load to 80%, or increasing the wire to 125% ampacity of continuous load. The breaker must be increased to the ampacity of the conductor before any adjustment factors are applied.

Example: 120 volt load on 20 amp breaker, maximum 2400 watts. Non-continuous load, For continuous load. 1920 watts.

2400 watts continuous load= 3000 watts calculated load = 25 amps, requiring #10 cu with a 30 amp breaker.
 
iwire

iwire

Moderator
Staff member
Location
Massachusetts
Re: 80% rated breakers

Bennie straight from the UL white book to you.

2003 UL White Book.
MAXIMUM LOAD
Unless otherwise marked, circuit breakers should not be loaded to exceed 80 percent of their current rating, where in normal operation the load will continue for 3 hours or more.
Click to expand...
 
steve66

steve66

Senior Member
Location
Illinois
Occupation
Engineer
Re: 80% rated breakers

Bennie & Iwire: I think were all pretty much agree but: 230.43 doesn't seem to require any 80% maximum load for services even for continuous loads.

In addition, in other posts to this forum: everyone seems to be OK with placing a 190A continuous load on a 200A service with a 200A breaker. Why is it OK for a service but not a feeder or a branch circuit??

Steve
 
iwire

iwire

Moderator
Staff member
Location
Massachusetts
Re: 80% rated breakers

Steve I have never said 190 amps continuous on a 200 amp service is OK.

To me a 200 amp service is good for 200 amps less than 3 hours and 160 amps for longer than 3 hours.

If you look at some 400 amp meter sockets you will see 320 continuous 400 max stamped inside.

I do not know of any exception to load a service to 100% unless the breaker and panel is rated 100%
 
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bennie

Esteemed Member
Re: 80% rated breakers

When there is no continuous load. A 200 amp service can be loaded to 200 amps calculated load.

When there is continuous load. A 200 amp service can still be loaded to 200 amps of calculated load.
 
iwire

iwire

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Staff member
Location
Massachusetts
Re: 80% rated breakers

Originally posted by steve66:

Question: Why doesn't 230.42 require the breaker to be sized for 1.25 times the continuous load? Also, why are smaller wires allowed for a 100% rated breaker. It seems like this article should be setting minimum sizes on the breakers instead of minimum sizes on the conductors.
Click to expand...
Steve the name of the section 230.42 is in is "IV. Service-Entrance Conductors" starting with 230.40

If you want to know about overcurrent protection go to the section called "VII. Service Equipment ? Overcurrent Protection" Starts at 230.90

The basic requirement is the OCPD shall be rated not higher than then the rating of the service conductors.

But as always there are exceptions to that. ;)
 
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bennie

Esteemed Member
Re: 80% rated breakers

Bob: It does sound goofy but think of it in Watts.

Non-continuous load;

200 amps at 240 volts = 48,000 watts calculated load.

Continuous load;

160 amps X 1.25 = 48,000 Watts calculated load.
48,000 Watts calculated load = 200 amps.
 
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bennie

Esteemed Member
Re: 80% rated breakers

Notice the adjustment for continuous load is already done. If you applied 1.25 to the adjusted 200 amps it would put you out in left field.

Now you have an actual connected load of 160 amps. Do you change the overcurrent device for 175 amps or leave it at 200 amps?
 
steve66

steve66

Senior Member
Location
Illinois
Occupation
Engineer
Re: 80% rated breakers

I'm still confused on two issues.

1. Thanks Bob, I did look at 230.90 and on from there. You are right, I was looking at service conductors. But, I still don't find any code reference that prohibits putting a 200 amp continuous load on a service with a 200 amp breaker.

2. I'm still not sure if unit lighting loads (table 220.3A) need to be multiplied by 1.25. Here I am thinking of an office building where all the lights are turned on at 8:00 AM and left on unitl 5:00 PM. (If these loads already include the 1.25 factor, maybe that's why I don't see any reference to 1.25 under 230.90.)

Steve
 
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