NEC CODE - breakers in panel

[Murloc]

I'm currently working on a job where an existing panelboard has

Bus - 1200amp rated
3 - 400amp fusible switches
1 - 250amp fusible switch
2 - 100amp fusible switches

3 * 400 + 250 + 2 * 100 = 1650amps

1650 amps - 1200amps = 450amp over rating

Would this be against NEC code. I see some old projects where its common to have circuit breakers in a panel that exceed the bus bar rating.
Here are some code sections I found:

This says no but it refers to PV systems.

NEC 705.12(D) (3) Busbars. One of the methods that follow shall be used to determine the ratings of busbars in panelboards

• The sum of 125 percent of the inverter(s) output circuit current and the rating of the overcurrent device protecting the busbar shall not exceed the ampacity of the busbar.
• Where two sources, one a utility and the other an inverter, are located at opposite ends of a busbar that contains loads, the sum of 125 percent of the inverter(s) output circuit current and the rating of the overcurrent device protecting the busbar shall not exceed 120 percent of the ampacity of the busbar. The busbar shall be sized for the loads connected in accordance with Article 220. A permanent warning label shall be applied to the distribution equipment adjacent to the back-fed breaker form the inverter that display the following or equivalent wording.

Here it basically says it depends on the manufacturer
NEC 2014 408.54 Maximum Number of Overcurrent Devices. A panelboards shall be provided with physical means to prevent the installation of more overcurrent devise than that number for which the panelboard was designed, rated, and listed.

For the purposes of this section, a 2-pole circuit breaker or fusible switch shall be considered two overcurrent devices; a 3-pole circuit breaker or fusible switch shall be considered three overcurrent devices.

 

[tom baker]

the sum of the circuit breaker ampacity has no relation to the panel main OCPD. A load calc is done and a OCPD for that load with appropriate wire size is use, or a larger OCPD can be selected ----note this is a quick summary and there are other considerations.
Often loads such as lighting are divided up for convenience, say ten offices have ten circuits where 1 or 2 could be used
In your case to add a load(s) you will need to do a load calculation per Art 220.
Note that load calculations are very conservative, see 220.87 for an alternative method

 

[augie47]

It is common for the sum of your branch (feeder) breakers to exceed the buss or main rating.
It's not much different from your home loadcenter having 400 amps of branch circuit ratings (admittingly you have a main in that situation)
Prior to the recent Codes requiring a main, the "6 disconnect" rule was common. Designers & contractors held responsibility to assure the buss and service conductors were not overloaded.

(sorry tom, posted on top of you)

 

[d0nut]

Bus - 1200amp rated
3 - 400amp fusible switches
1 - 250amp fusible switch
2 - 100amp fusible switches

3 * 400 + 250 + 2 * 100 = 1650amps

1650 amps - 1200amps = 450amp over rating

This is not how a load calculation works. You can't just add up the ratings of the breakers or fuses and say that the panel is overloaded. You would have to look at what is actually being fed from those overcurrent devices to see what the actual loads are.

Also, the fuses in those switches may be smaller than the switch rating.

 

[Murloc]

This is not how a load calculation works. You can't just add up the ratings of the breakers or fuses and say that the panel is overloaded. You would have to look at what is actually being fed from those overcurrent devices to see what the actual loads are.

Also, the fuses in those switches may be smaller than the switch rating.

right so i can have larger breakers or switches over the 1200 amp bus rating as long as I don't use 1200 amp load. so breakers you can't use more than 80%.

so........ 80% * X = 1200
X = 1200/.8
X = 1500amps,

so with a 1200 amp panel, I should be able to use up to a 1500 amps worth of breakers

 

[suemarkp]

No, because the bus has that same 80% continuous limiting factor. The key is that just because there is a 400A breaker doesn't mean it is a 400A load. It could be a motor circuit or feeder which has a larger breaker to allow for startup inrush. It could be seomthing like the branch circuit needs to be rated at 125%, but the load calculation for it does not. There is also a diversity factor in load calculations -- it assumes not everything will be used at once at full power.

Bottom line, you don't add up breakers to see if a panel is overloaded. You do a load calculation and apply the allowable factors and required factors.

Does this panel have a main breaker?

Also, just because the panel has a 1200A bus rating doesn't mean the conductors feeding it are rated that high (depends on if its a service or feeder). You are limited by the lower of the conductor ampacity, feeder breaker, or the bus rating. If there is a main breaker or feeder breaker, it should be less than the conductor ampacity since this is over 800A.

 

[tom baker]

Doing a load calculation is simple if you have current panel schedules, one lines, floor plans, loads.
If not get your clipboard and camera!

 

[Murloc]

No, because the bus has that same 80% continuous limiting factor. The key is that just because there is a 400A breaker doesn't mean it is a 400A load. It could be a motor circuit or feeder which has a larger breaker to allow for startup inrush. It could be seomthing like the branch circuit needs to be rated at 125%, but the load calculation for it does not. There is also a diversity factor in load calculations -- it assumes not everything will be used at once at full power.

Bottom line, you don't add up breakers to see if a panel is overloaded. You do a load calculation and apply the allowable factors and required factors.

Does this panel have a main breaker?

Also, just because the panel has a 1200A bus rating doesn't mean the conductors feeding it are rated that high (depends on if its a service or feeder). You are limited by the lower of the conductor ampacity, feeder breaker, or the bus rating. If there is a main breaker or feeder breaker, it should be less than the conductor ampacity since this is over 800A.

so if you can't just use a larger amount? Right you're saying that you can't just size breakers to be 1200 amps, you can't size it to 1500 amps to meet an 80% capacity.

I don't quite understand the correct method. assuming wire size meets capacity. are you saying that you can get 1200 amps of load capacity, since you usually size loads to fill 80% of breaker capacity max breaker sizing would 1500amps worth of breakers for 1200 amp bus.

 

[winnie]

Read article 220 of the code.

You calculate the load on a circuit (any circuit, branch, feeder, or service) by 'adding' up all the loads connected to that circuit.

I put 'adding' in quotes because you need to follow the specific procedures outlined to account for loads, load diversity, general use receptacles, etc.

A circuit breaker is not a load, it is a current responsive switch that feeds a circuit which supplies loads.

That 400A circuit breaker might feed a panelboard with a total of 800A of breakers in it, but with a calculated load of 250A and a real measured load of 150A. So you have no idea how much load that 400A breaker actually represents.

Jon

 

[petersonra]

so if you can't just use a larger amount? Right you're saying that you can't just size breakers to be 1200 amps, you can't size it to 1500 amps to meet an 80% capacity.

I don't quite understand the correct method. assuming wire size meets capacity. are you saying that you can get 1200 amps of load capacity, since you usually size loads to fill 80% of breaker capacity max breaker sizing would 1500amps worth of breakers for 1200 amp bus.

you can put as many breakers downstream as you want. It could be 1,000,000 amps worth of breakers with a 1200 Amp feeder or service.

the important thing is the load calculation, as long as it comes out under 1200 Amps.

 

[Murloc]

cool thx that makes a lot more sense

 

[jaggedben]

...

This says no but it refers to PV systems.

NEC 705.12(D) (3) Busbars. One of the methods that follow shall be used to determine the ratings of busbars in panelboards

• The sum of 125 percent of the inverter(s) output circuit current and the rating of the overcurrent device protecting the busbar shall not exceed the ampacity of the busbar.
• Where two sources, one a utility and the other an inverter, are located at opposite ends of a busbar that contains loads, the sum of 125 percent of the inverter(s) output circuit current and the rating of the overcurrent device protecting the busbar shall not exceed 120 percent of the ampacity of the busbar. The busbar shall be sized for the loads connected in accordance with Article 220. A permanent warning label shall be applied to the distribution equipment adjacent to the back-fed breaker form the inverter that display the following or equivalent wording.

Neither of those sections speak to your question, although interestingly the very next section afterward does. That section limits the sum of branch and feeder breakers to the panel busbar in the case where a PV backfeed does not meet the other rules you quoted. AFIAK that is the only code section that imposes a limit like you have in mind. But it only applies in that one very narrow situation involving interactive power sources. Otherwise, the only things that rule are Article 220 and panelboard instructions, as others have already explained.

 

[infinity]

right so i can have larger breakers or switches over the 1200 amp bus rating as long as I don't use 1200 amp load. so breakers you can't use more than 80%.

so........ 80% * X = 1200
X = 1200/.8
X = 1500amps,

so with a 1200 amp panel, I should be able to use up to a 1500 amps worth of breakers

There is no 80% limit for a circuit breaker unless the load is all continuous. If your load calculation came up with 1200 amps of load you can use a 1200 amp circuit breaker.

 

[Solarbrone]

Canadian code has been updated to allow for exceeding the 120 percent if controls are in place to limit the curre to flowing to the bus bar. Very logical

 

[kwired]

If these breakers are for PV equipment, then the chance of them being loaded to near 80% of their rating and for significant time during the day is pretty great.

But otherwise as a general rule actual load calculations is what is important for the rating of the panel bus and main breaker.