Circuit Breaker Ampacity Order in Panelboard

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paulgarett

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San Rafael, CA
Should the largest ampacity circuit breakers (such as range, hot tub, dryer) be installed closest to the feeder lugs and the smallest ampacity circuit breakers (lighting, computer) be installed furthest from the feeder lugs? If high ampacity circuit breakers are installed further away from the feeder lugs, will this cause the lower ampacity circuit breakers, which are installed closest to the feeder lugs, effect the load or dim the lights?

It seems to be common sense to install the largest ampacity circuit breakers closest to the feeder lugs. Are there any specific requirements or codes that specify where the circuit breakers shall be installed in a panel, besides load balancing between phases?
 
The bus in the panel is a conductor and will develop voltage drop from one end to the other. The amount of voltage drop across that bus is going to be so insignificant and maybe even unmeasurable with most typical meters used in the field.

Have you ever seen a panel with main breaker or main lug at one end of the bus and sub feed lugs at the other end? They are designed to carry at least the rating of the panel for their entire length. There may be a maximum branch size that is allowed to be connected to the point where breakers connect.
 
Thanks for your imput. I had a feeling voltage drop may have to do with the placement of breakers in a panel, but I understand how insignificant it would be. I will pass off your guys comments to my co-worker electrical instructor.
 
paulgarett said:
Should the largest ampacity circuit breakers (such as range, hot tub, dryer) be installed closest to the feeder lugs and the smallest ampacity circuit breakers (lighting, computer) be installed furthest from the feeder lugs? If high ampacity circuit breakers are installed further away from the feeder lugs, will this cause the lower ampacity circuit breakers, which are installed closest to the feeder lugs, effect the load or dim the lights?

It seems to be common sense to install the largest ampacity circuit breakers closest to the feeder lugs. Are there any specific requirements or codes that specify where the circuit breakers shall be installed in a panel, besides load balancing between phases?
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I design my MV/LV Switchgear and MCC layouts based on the principle you describe. The reasoning is that the most heat generated by the highest current, which would be the sum of all downstream breakers, will be limited therefore the overall heat generated in the MCC is the smallest possible minimum. Heat is the enemy of insulation life and also the bus splices and stabs contact resistance.

No Code rules dictate any of this, it is just good practice. It optimizes equipment life an reduces trouble potential.
 
weressl said:
I design my MV/LV Switchgear and MCC layouts based on the principle you describe. The reasoning is that the most heat generated by the highest current, which would be the sum of all downstream breakers, will be limited therefore the overall heat generated in the MCC is the smallest possible minimum. Heat is the enemy of insulation life and also the bus splices and stabs contact resistance.

No Code rules dictate any of this, it is just good practice. It optimizes equipment life an reduces trouble potential.
Click to expand...

I am not following you, the current is the current regardless of position.:confused:
 
iwire said:
I am not following you, the current is the current regardless of position.:confused:
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The heat produced by the current is not linear. It is a square function. (I?R) If you limit the greatest amount of current to the shortest length of bus, you can reduce the heat.
 
But the majority of heat is produced in the breaker and not the bus isn't it? A thermal protective device has to have heat created to perform its job.

With that in mind if you are placing breakers according to how much heat may be produced you should maybe place the higher loads toward the top of the panel even if the main lugs/breaker is at the bottom. But no matter where you place them the same heat is still produced within the panel just more of it is in certain breakers.
 
iwire said:
I am not following you, the current is the current regardless of position.:confused:
Click to expand...
I think he is talking about current along the vertical bus. The top two inches of a vertical bus carries all the current from all breakers connected to the bus, whereas the bottom two inches carries only the current flowing out the bottom pair of breakers.
 
kwired said:
But the majority of heat is produced in the breaker and not the bus isn't it? A thermal protective device has to have heat created to perform its job.

With that in mind if you are placing breakers according to how much heat may be produced you should maybe place the higher loads toward the top of the panel even if the main lugs/breaker is at the bottom. But no matter where you place them the same heat is still produced within the panel just more of it is in certain breakers.
Click to expand...

You are correct, if the main is at the bottom the rising heat from the individual thermal elements would be cumulative and have more detrimental effect than the buss heating.

This will of course become less significant as electronic trip units take over even small breakers. Like GFC's and AFCI's do not need to have a separate thermal element since the current is 'monitored' electronically already.
 
kwired said:
But the majority of heat is produced in the breaker and not the bus isn't it? A thermal protective device has to have heat created to perform its job.
Click to expand...
Yes, you are right.

With that in mind if you are placing breakers according to how much heat may be produced you should maybe place the higher loads toward the top of the panel even if the main lugs/breaker is at the bottom. But no matter where you place them the same heat is still produced within the panel just more of it is in certain breakers.
Click to expand...
Right again. Heat Stacking is something to think about, but it really is only a concern on large switchboards and switchgear. All gear is tested based on the worst case scenario of course, but on LV panelboards and load centers, the additional heat stacking is relatively insignificant. The only time it becomes an issue is on "1/2 space" (twin) breakers, that's why some mfrs limit the number of 1/2 space slots, and they are usually on the top of the bus. But for the most part if your breakers are being affected, something else is wrong, i.e. the panel is probably overloaded.

Still, there is nothing wrong with doing it that way, I always do.
 
charlie b said:
I think he is talking about current along the vertical bus. The top two inches of a vertical bus carries all the current from all breakers connected to the bus, whereas the bottom two inches carries only the current flowing out the bottom pair of breakers.
Click to expand...

The bottom carries all the current if bottom fed panel.

weressl said:
You are correct, if the main is at the bottom the rising heat from the individual thermal elements would be cumulative and have more detrimental effect than the buss heating.

This will of course become less significant as electronic trip units take over even small breakers. Like GFC's and AFCI's do not need to have a separate thermal element since the current is 'monitored' electronically already.
Click to expand...

Do GFCI breakers have electronic current monitoring? Or just the GFCI components along with same current monitoring equipment in a standard breaker. Same question for AFCI?

I have never given that possibility any thought before on GFCI's. AFCI's are still relatively new to me and I have not had to deal with very many of them yet. The less residential work I do the better off. I like residential work but between codes and budgets they are not that appealing to do anymore. (I can handle the codes - at least they are black and white);)
 
paulgarett said:
Should the largest ampacity circuit breakers (such as range, hot tub, dryer) be installed closest to the feeder lugs and the smallest ampacity circuit breakers (lighting, computer) be installed furthest from the feeder lugs? If high ampacity circuit breakers are installed further away from the feeder lugs, will this cause the lower ampacity circuit breakers, which are installed closest to the feeder lugs, effect the load or dim the lights?

It seems to be common sense to install the largest ampacity circuit breakers closest to the feeder lugs. Are there any specific requirements or codes that specify where the circuit breakers shall be installed in a panel, besides load balancing between phases?
Click to expand...

As usual this thread has gone from a residential install to a 1,000,000 amp switch gear:D.
In a residential install as the OP has ask about it doesnt matter as to the arrangement. For me most of the conductors for the larger items, stove,dryer,water heater and A/C units are brought in to the panel from the bottom where the smaller conductors are coming in from the top. To make a neater install those larger amp breakers are going to be lower on the buss so there is no crossing of small conductors and larger conductors to land on the breakers.
 
Put the CB's in any order that you want, top or bottom. The ampacity of the bus is the same as the ampacity of the conductors feeding the panel. For the panel it won't matter either way.
 
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