Breaker Loading

[baumancl]

Is it ever permissible per code to load a breaker more than 80% (Assuming it is not a 100% rated breaker) in particular for motor feeders and services supplying multiple motor loads. Following overcurrent protection calculations per aritcle 430.62 and 430.63 I understand that the breaker can be sized larger than the ampacity of the conductors for motor feeders. But what about when you have so many motor loads that you run into a situation where the breaker is less than 1.25 times the connected load? Need some clarification.

 

[ron]

If the load doesn't operate for 3 hours or more, then it isn't continuous.

 

[baumancl]

What if you are in a situation where mutliple motors can all start and run at once....maybe not for 3 hours to be considered continuous but still should the breaker be sized to handle the load. So the situation would be having all motors and loads running at once and the connected load exceeding 80% of the breaker rating. This would lead to nuissance tripping.

 

[jwelectric]

the connected load exceeding 80% of the breaker rating. This would lead to nuissance tripping.

Why would this make the breaker open?

 

[baumancl]

I know breakers are rated 80% to account for ambient temperature fluctuations in panelboards which derate trip characteristics. Say you have a 200 amp feeder breaker that is more than 80% loaded. Assuming the ambeint temperature derates the trip setting to 80% your saying it wont trip.

 

[acrwc10]

I know breakers are rated 80% to account for ambient temperature fluctuations in panelboards which derate trip characteristics. Say you have a 200 amp feeder breaker that is more than 80% loaded. Assuming the ambeint temperature derates the trip setting to 80% your saying it wont trip.

They are more likely to run up to 125% or higher before they trip.

 

[baumancl]

Well assuming that the breaker was originally sized to carry 125% of the load (which is 80% rating of the breaker) I would agree. But I guess getting back to my initial question. Sizing motor feeder overcurrent per NEC articles 430.52, 430.62, and 430.63 and assuming all loads are continuous. What if the end breaker size calculated is not at least 125% (80% of breaker rating) of the connected load. Is this a code violation? In some calculations with only a few loads this does not happen. But when you have a lot of motor loads (again assuming them all to be continuous) this does occur after doing the calcs.

 

[jwelectric]

Where are you coming up with this phrase?

(which is 80% rating of the breaker).

Are you saying that no breaker can be loaded to more than 80% of it's rating?

 

[baumancl]

Lets go back to something more simple. "Branch-circuit OCPDs must have an ampacity not less than 125% of the continuous loads, plus 100% of the noncontinuous loads [210.20(A)]." Now assuming you have only one continuous load on the breaker this is saying you must size the circuit breaker at least 125% of the load. This is the same as saying the breaker cannot be more than 80% loaded. (1/1.25 = .80) Are we in agreement?

 

[jwelectric]

Lets go back to something more simple. "Branch-circuit OCPDs must have an ampacity not less than 125% of the continuous loads, plus 100% of the noncontinuous loads [210.20(A)]." Now assuming you have only one continuous load on the breaker this is saying you must size the circuit breaker at least 125% of the load. This is the same as saying the breaker cannot be more than 80% loaded. (1/1.25 = .80) Are we in agreement?

No we are not in agreement.

If I have a 10 amp noncontinuous load and a 8 amp continuous load which would equal 20 amps I can use a 20 amp breaker.

In my scenario the breaker would be loaded to more than 80% of its rating but still in compliance with the code.

I could install a full 20 amp noncontinuous load on this same 20 amp breaker and this would exceed what you are saying, “the breaker cannot be more than 80% loaded”

 

[baumancl]

I agree with your statement that when non-continuous loads are involved you would exceed 80 rating of the breaker rating. I was assuming that all loads were continuous and sizing the conductors and overcurrent protection per articles 430.24, 430.52,430.62, and 430.63. Then what. Is it still permissible that the overcurrent is loaded at more that 80% of its rating (125% of the continuous load)?

 

[jwelectric]

I guess you are going to have to give me some numbers to work with because I am not grasping what you are seeking.

Is it still permissible that the overcurrent is loaded at more that 80% of its rating (125% of the continuous load)?

The easy answer to this one question is yes. It is done all the time,

But this is not the sum of your complete question. If I understand what you are asking about is a motor feeder or branch circuit. I assume it is a feeder due to you continued reference to 430.62.
If your question is concerning motors then look at Table 210.2 and you will see that 430 modifies or amends article 210 thus leaving your reference to 210.20(A) out of the equation so you will have to stop making this statement;

the breaker cannot be more than 80% loaded.

Give me some numbers that I can work with and I will be glad to help you all that I can and I am sure that others will do the same.

EDITED TO ADD:

Here you go I have made up some numbers by myself.

A feeder has four single phase 240 volt 5 hp and one 3 hp motors.

The maximum breaker allowed on this feeder if all the motors are protected with breakers would be 150 amps.

28 times 250% equals 70 amps plus the other equals 166. 430.62 state that the OCPD can not be larger than this so a 150 breaker is used to protect the feeder.

The conductor would be sized by 430.24, 28*125% plus all the other motors equals 131 amps.

80% of 150 is 120 so the calculated load for the feeders exceeds the 80% of 150 by 11 although the installation is code compliant.

Is this what you are asking about?????

 

[templdl]

If a MCCB is tested in open air to carry 100% in at 40degC what happens tor the tripping characteristics when the breaker is in a significantly lower ambient temperature such as in 70-75 deg F or 20-25deg C?
The trip curve is going to shift to the right and the breaker will take longer to trip. Now, if it takes more than 100% of its rating to trip isn't it going in the opposite direction to limit the current to 80%?

I love these 80% rated breaker discussions because for the life of me I have never found 80% on the label.

If you size the conductor based upon 125% on the continuous load + 100% on the non-coniguous load, the conductor must be sized at a minimum to
carry this load then size the breaker to protect the conductor, the breaker size is based upon the conductor on not the load. Breakers are there to protect conductors.

In the odd even that all of your load is continuous your would size your breaker based upon 125%. In the odd even that the ampacity computed were exactly that of a standard rated conductor ampacity and that ampacitiy so happened to be exactly the standard rating on a MCCB then that breaker would be sized 125% more than the continuos current with is 80% less than the breaker rating.

Derate the breaker 80%??? If you apply the cable and breaker accordingly where does this 80% derating become an issue?
You could 100% of current to a breaker and it most likely will never trip unless the ambient were to increase significantly above 40degC.
What is it all about? Not the breaker but to assure that the CABLE IS PROTECTED. Pay attention to sizing the cable and then size the breaker to protect that cable.

 

[baumancl]

Thanks. That clears it up for me. That is exactly how I would interpret it and how I understand it. The fuzziness for me comes when you size a service. In my interpretation the service feeder and overcurrent per NEC should be sized the same. I have been advised to size a service for the connected load and assuming all the loads to be continuous. This I guess makes sense on taking into account the worse case scenario but when would you ever have all the loads on and running continuously.

I looking at minimum secondary conductor sizes for services I have seen Electric Utilities apparently follow the 80% derating. For the following two residential services this is minimum sizing requirements directly from the Service Manual for a major Utilty in Wisconsin:

#4 Cu conductor for a 100 Amp Service
#2/0 Cu conductor of a 200 Amp Service

For 75 deg. C cooper condcutors per NEC table 310.16
#4 Cu has an ampacity of 85 amps
#2/0 Cu has an ampacity of 175 amps

So in looking at their minimum conductor sizes for the above two services it appear they are assuming the maximun load permissible on the service would be 80% of the service size. So it looks like to me they are also assuming that the Main CB in the panelboard (If that is the configuration of the installation) is only 80% rated. Any comments?

 

[templdl]

As mentioned before where on the breaker does it state that it is rated 80%?
I believe it may be more appropriate to state that the common breaker is to be applied as such that the continuous load is no more that 80% of the breakers rating.

I find no reason for a breaker to be rated 80% because of the example that I provided in my previous post.
UL489 tests include the fact that breaker set up appropriately for testing in a 40degC ambient must be able to carry its rated current continually without tripping. At 20-25degC ambient that same breaker is capable of carrying even more current without tripping. NEMA AB4 tests are conducted at about 25deg C at 300% of the breakers rated current which trip times of the various ratings are in the ballpark 50-300sec for 250v breaker and up to 1000 sec for breaker to 600v.

I still direct your attention away from the breaker because the breaker's not the issue as it could handle 100% of its rated current of more continually like a casual walk in the park without tripping or damage from overheating.

Breakers protect cable. If you attempt to apply a standard breaker with a continuous load at more than 80% of its rating the it not an issue with the breaker but I would most certainly be concerned with how I size the cable an if it is adequately protect.

A breaker that is undersized will simply view the overcurrent as an overload and trip thermally which is a nuisance.

It?s not about the breaker, it?s an application issue. Size the cable correctly, size the breaker to protect the cable. The breaker should trip thermally to protect the cable and not as a nuisance because it has been undersized.

 

[baumancl]

In looking at minimum secondary conductor sizes for services I have seen Electric Utilities apparently follow the 80% derating. For the following two residential services this is minimum sizing requirements directly from the Service Manual for a major Utilty in Wisconsin:

#4 Cu conductor for a 100 Amp Service
#2/0 Cu conductor of a 200 Amp Service

For 75 deg. C cooper condcutors per NEC table 310.16
#4 Cu has an ampacity of 85 amps
#2/0 Cu has an ampacity of 175 amps

So in looking at their minimum conductor sizes for the above two services it appear they are assuming the maximun load permissible on the service would be 80% of the service size. So it looks like to me they are also assuming that the Main CB in the panelboard (If that is the configuration of the installation) is only 80% rated. Any comments?[/QUOTE]

What would be your take on the above??

 

[iwire]

In looking at minimum secondary conductor sizes for services I have seen Electric Utilities apparently follow the 80% derating.

What the utilities do has absolutely nothing to do with the NEC.

The utilities size their conductors on historical data and darts thrown at a board.

However those sizes are from the NEC for dwellings only, check out Table 310.15(B)(6),

 

[templdl]

I still prefer the term "applied at 80%" as there is nothing on the label the says they are "80% rated." It would imply the the breaker may be damaged or it would be unsafe to load a breaker at more than 80% continuos when it would not compromise the breaker is any way shape of form.

 

[jwelectric]

In looking at minimum secondary conductor sizes for services I have seen Electric Utilities apparently follow the 80% derating. For the following two residential services this is minimum sizing requirements directly from the Service Manual for a major Utilty in Wisconsin:

#4 Cu conductor for a 100 Amp Service
#2/0 Cu conductor of a 200 Amp Service

For 75 deg. C cooper condcutors per NEC table 310.16
#4 Cu has an ampacity of 85 amps
#2/0 Cu has an ampacity of 175 amps

So in looking at their minimum conductor sizes for the above two services it appear they are assuming the maximun load permissible on the service would be 80% of the service size. So it looks like to me they are also assuming that the Main CB in the panelboard (If that is the configuration of the installation) is only 80% rated. Any comments?

What would be your take on the above??[/quote]

Are these conductors protected by a breaker?

80% of 100 is 80 and of 200 is 160. How are you getting that these are rated at 80%?

 

[wireman71]

Motor loads were mentioned.. Breaker usually provides short circuit and ground fault protection but not overload protection. Breaker might be sized at 250% of load for inrush current while the fused motor disconnect provide overload protection for the motor and wire.