Feeder Breakers in a distribution panel (Sizing Requirements)

[Sparky2791]

I understand National Electric Code. Section 210.20(A) which basically says that a circuit breaker for a branch circuit must be rated such that it can handle the noncontinuous load plus 125% of the continuous load. (A continuous load is one where the maximum current is expected to continue for 3 hours or more.) In other words, the breaker needs an extra 25% capacity of the continuous load for headroom.
How does this apply to the breaker in the distribution panel feeding the panelboard with all the branch circuits?
I always assumed that breaker to just be sized based on total load in the panel plus 125% so taking the connected load of all the branch circuits in the panel and adding 125% to come up with the breaker size to feed the panel.

For example, a 100A panelboard fed with a 100A breaker in a distribution panel should not have more than 80A of connected load.

Is this a NEC requirement for sizing this part of the distribution system or just good engineering practice.

Thanks for your response.

 

[david luchini]

For example, a 100A panelboard fed with a 100A breaker in a distribution panel should not have more than 80A of connected load.

A 100A panelboard fed with a 100A breaker can have a calculated load of 100A if the 100A load is non-continuous.

so taking the connected load of all the branch circuits in the panel

The load on the panel would be the sum of the branch circuit loads, after the application of any demand factors in Art 220, Parts III, IV and V

 

[kwired]

210 is for branch circuits, 215 if for feeders. However 215.3 says just about the same thing 100% of non continuous plus 125% of continuous load.

Can get a little more complicated with certain loads involved like those covered by 430 and 440.

 

[jap]

At least that's what the book says

In the real world, experience will teach you differently.

JAP>

 

[kwired]

At least that's what the book says

In the real world, experience will teach you differently.

JAP>

real world varies also. 100 amp load calculation in a dwelling - you seldom will ever put a clamp on meter on the supply and read 100 may not even see 60 amps.

Commercial/industrial - can be extreme one way or the other depending on situation with a 100 amp load calculation.

 

[jap]

real world varies also. 100 amp load calculation in a dwelling - you seldom will ever put a clamp on meter on the supply and read 100 may not even see 60 amps.

Commercial/industrial - can be extreme one way or the other depending on situation with a 100 amp load calculation.

Well if you have a calculated load of 100 amps and you hardly ever see 60 amps I'd say you need a better calculator.

A true 100 amp panel fed with a 100 amp breaker with a true 100 amp load for 2 hours and 59 minutes generally makes the magic start happening.

JAP>

 

[Sparky2791]

Thank you all for your replies. I guess what I am saying is Account for 25% additional in the overall panel size?

So lets say you have 120/208V 3 Phase 42 circuit panelboard with 42 circuits each with 1KW of lighting load on the circuit. All circuits are continuous load.

So individual branch circuit sized like this;

1KW x 125% = 1.25KW/120V=10.4A (15A-SP) circuit breaker

Panel board sized like this.......(?)

42KW /208V*1.73 = 116A

Or like this

42KW * 125% = 52.5KW/208V * 1.73 = 145.8

 

[david luchini]

Like this for continuous loads.

42KW * 125% = 52.5KW/(208V * 1.73) = 145.8

This would work for non-continuous loads.

42KW /(208V*1.73) = 116A

 

[charlie b]

You include the 125% for continuous loads when you calculate total load. Then you pick components (breakers, wires, panels, etc.) that can handle that calculated load. Therefore, this version is correct for your example:

42KW * 125% = 52.5KW/208V * 1.73 = 145.8

 

[david luchini]

You include the 125% for continuous loads when you calculate total load. Then you pick components (breakers, wires, panels, etc.) that can handle that calculated load. Therefore, this version is correct for your example:

Being nitpicky, but this is technically incorrect. There is no provision in Art 220 for including 125% when calculating total load.

 

[Sparky2791]

Being nitpicky, but this is technically incorrect. There is no provision in Art 220 for including 125% when calculating total load.

I agree. Although there is nothing wrong with doing it the way Charlie b indicates, technically, the 25% is for branch circuit sizing. Seems odd it would not follow through in some way as part of the NEC though. My whole reason for questioning this is due to an existing panel board being targeted by the GC for new heating loads. I am trying to determine how much load is remaining on the panel as they want to add a lot of new electric baseboards and I do not think the panel has enough spare capacity. one way of calculating it leaves more spare capacity than the other but I want to make sure to do it by the book. If this were all new I would just size with the 25% on the total load for the new panel size.

 

[david luchini]

I agree. Although there is nothing wrong with doing it the way Charlie b indicates, technically, the 25% is for branch circuit sizing. Seems odd it would not follow through in some way as part of the NEC though.

This is incorrect as well. The 25% applies to branch circuits (210.19(A)) and to feeders (215.2(A)) and to service conductors (230.42(A)).

(Though I agree that there is nothing wrong with doing it the way Charlie indicates...you just may end up with conductors larger than what the Code requires.)

I am trying to determine how much load is remaining on the panel as they want to add a lot of new electric baseboards and I do not think the panel has enough spare capacity. one way of calculating it leaves more spare capacity than the other but I want to make sure to do it by the book.

Generally speaking, electric heat would not be considered continuous load as it is thermostatically controlled. The Code does require you to consider it a continuous load for the purposes of branch-circuit sizing, but there is no such requirement for feeder sizing.

 

[Sparky2791]

This is incorrect as well. The 25% applies to branch circuits (210.19(A)) and to feeders (215.2(A)) and to service conductors (230.42(A)).

(Though I agree that there is nothing wrong with doing it the way Charlie indicates...you just may end up with conductors larger than what the Code requires.)



Generally speaking, electric heat would not be considered continuous load as it is thermostatically controlled. The Code does require you to consider it a continuous load for the purposes of branch-circuit sizing, but there is no such requirement for feeder sizing.

If the feeder feeds the panel with the branch circuits you would still need to include it. No? Very confusing

 

[tom baker]

The reason for the 80% max loading on continuous loads is the conductor will often end up being larger, so will act as a heat sink to keep the breaker from overheating. Remember TM breakers trip on thermal and magnetic.

 

[david luchini]

If the feeder feeds the panel with the branch circuits you would still need to include it. No? Very confusing

A continuous load on a branch circuit would also be a continuous load on a feeder. If you are asking about fixed electrical space heating, then that load wouldn't be continuous, as it is thermostatically controlled.

The Code doesn't say that fixed electrical space heating is continuous load, it only says that it shall be considered continuous load for the purposes of branch circuit sizing. It doesn't say anything about feeder sizing.

 

[kwired]

Well if you have a calculated load of 100 amps and you hardly ever see 60 amps I'd say you need a better calculator.

A true 100 amp panel fed with a 100 amp breaker with a true 100 amp load for 2 hours and 59 minutes generally makes the magic start happening.

JAP>

Not necessarily. Maybe you have that one load that seldom runs but when it does you need most if not all the capacity of your calculation. NEC doesn't give us too much exception. POCO's often size transformers and conductors with more complex formulas that account for down time for cooling between cycles on certain loads that otherwise push the basic abilities of the equipment. But they also have their gear outside of buildings or in a vault and it isn't likely to burn the building down if it does have a catastrophic failure.

 

[kwired]

Thank you all for your replies. I guess what I am saying is Account for 25% additional in the overall panel size?

So lets say you have 120/208V 3 Phase 42 circuit panelboard with 42 circuits each with 1KW of lighting load on the circuit. All circuits are continuous load.

So individual branch circuit sized like this;

1KW x 125% = 1.25KW/120V=10.4A (15A-SP) circuit breaker

Panel board sized like this.......(?)

42KW /208V*1.73 = 116A

Or like this

42KW * 125% = 52.5KW/208V * 1.73 = 145.8

Your load calculation will include 125% of continuous loads, 100% of non continuous loads, as well as any allowable demand factors - which gives you a worst case expected scenario for the amount of current that will be drawn. You must have conductors and gear that have at least that calculated ampacity. Many times people do install larger to allow for future expansion. If you know there is not going to be future expansion or even don't want any future expansion to be related to a particular portion of the system, nothing wrong with sizing things close to the minimum allowed.Often you will be at least slightly above minimum allowed just because of standard sizes of items.

 

[david luchini]

Your load calculation will include 125% of continuous loads, 100% of non continuous loads, as well as any allowable demand factors - which gives you a worst case expected scenario for the amount of current that will be drawn.

Still being nitpicky, but this is still technically incorrect. There is no provision in Art 220 for including 125% when calculating total load.

 

[wwhitney]

If I can jump in with a background theoretical question, am I correct in thinking that the reason for the extra 25% adder for continuous loads is that the shape of the trip curve of a thermal-magnetic breaker is not a perfect match with the shape of the damage curve of a conductor?

In other words, if breakers were readily available that adequately protected the conductor for short time periods (~1 min) without nuisance tripping at long time periods (~3 hrs), then all the 125% continuous factors could be removed from the NEC, and we'd save on a bunch of copper?

Cheers, Wayne

 

[synchro]

From what I've gathered the 25% adder for conductor sizing with continuous loads is to provide more heat sinking to keep the temperature of breaker terminals and internal components within bounds. Using a conductor with a higher temperature rating would not eliminate this requirement, and so I don't think the intent of the 25% adder was for conductor protection only.

The following article is a bit dated but covers this issue, particularly the section under the heading Test 2:

https://www.ecmweb.com/content/sizin...rs-made-simple