Load calcs for a 100A panelboard with 60A main

[SteamDonkey]

We have an existing 100A panelboard that has a 60A main breaker and I'm trying to decide how much additional load can be added before we have to replace the main breaker. The majority of the connected loads are motors and I'm not sure that 430.24 applies since it pertains to conductor ampacity. 215.3 specifies minimum OCPD size for a feeder, but does it apply to panel main breakers as well? How do I calculate the load when the panel main breaker is the limiting factor?

Thanks.

 

[Smart $]

The calculated load of that panel, once performed as proscribed under Article 220, will provide you with the best and only start. Return to ask questions related to that article if need be.

 

[kwired]

We have an existing 100A panelboard that has a 60A main breaker and I'm trying to decide how much additional load can be added before we have to replace the main breaker. The majority of the connected loads are motors and I'm not sure that 430.24 applies since it pertains to conductor ampacity. 215.3 specifies minimum OCPD size for a feeder, but does it apply to panel main breakers as well? How do I calculate the load when the panel main breaker is the limiting factor?

Thanks.

You are correct that 430.23 pertains to conductor ampacity.

430 part V is for motor feeder short circuit and ground fault protection.

You will not find anything there restricting you to a minimum setting, if the breaker holds while using these motors it is fine.
This section certainly could allow you to have a 60 amp supply conductor but a 100 amp or higher overcurrent device for the purpose of allowing motor(s) to start.

 

[Smart $]

You are correct that 430.23 pertains to conductor ampacity.

430 part V is for motor feeder short circuit and ground fault protection.

You will not find anything there restricting you to a minimum setting, if the breaker holds while using these motors it is fine.
This section certainly could allow you to have a 60 amp supply conductor but a 100 amp or higher overcurrent device for the purpose of allowing motor(s) to start.

Calculated load under 220.14 is rather specific. In general, he's looking at the sum of 1) 125% of largest motor, 2) load of other motors, 3) 125% continuous load, and 4) noncontinuous load. Demand may apply to #3 and #4, as may non-coincident load of 220.60.

 

[kwired]

Calculated load under 220.14 is rather specific. In general, he's looking at the sum of 1) 125% of largest motor, 2) load of other motors, 3) 125% continuous load, and 4) noncontinuous load.

He is asking when he has to change the breaker from 60 to 100 amps. That point can be when 125% of continuous load plus non continuous load is well below 60 amps if you can not start some or all the motors and hold the feeder breaker in the process.

He may even be able to have more then 100 amp breaker and still have same supply conductors he now has. Maximum overcurrent protection for the panelboard involved however is another issue.

 

[Smart $]

He is asking when he has to change the breaker from 60 to 100 amps. That point can be when 125% of continuous load plus non continuous load is well below 60 amps if you can not start some or all the motors and hold the feeder breaker in the process.

Correct and good point. I wasn't looking at it in that respect.

He may even be able to have more then 100 amp breaker and still have same supply conductors he now has. Maximum overcurrent protection for the panelboard involved however is another issue.

Also correct... but not with the existing panelboard. The panelboard cannot be rated less than its OCPD.

Got a combination of issues involved with ratings determination... but he must as a worst case scenario, verify his load is not greater than the feeder rating.

 

[SteamDonkey]

Calculated load under 220.14 is rather specific. In general, he's looking at the sum of 1) 125% of largest motor, 2) load of other motors, 3) 125% continuous load, and 4) noncontinuous load. Demand may apply to #3 and #4, as may non-coincident load of 220.60.

220.14 determines branch circuit load for power outlets. The loads are fans (with starters) that are permanently installed and are not supplied by outlets. 220.14 and everything else I've found in 220 appears to dictate minimum ampacities for conductors. The NEC seems to have very little to say about minimum OCPD size other than 215.3. Maybe there's no code violation in running an 80A feeder to a 100A panel with a 60A main. Perhaps this is another example of why the NEC shouldn't be used as a design guide.

 

[Smart $]

220.14 determines branch circuit load for power outlets. The loads are fans (with starters) that are permanently installed and are not supplied by outlets. 220.14 and everything else I've found in 220 appears to dictate minimum ampacities for conductors. The NEC seems to have very little to say about minimum OCPD size other than 215.3. Maybe there's no code violation in running an 80A feeder to a 100A panel with a 60A main. Perhaps this is another example of why the NEC shouldn't be used as a design guide.

Article 220 covers load calculation of the NEC... nothing else. However, as you stated, per other articles and sections, these calculation results set the minimum allowable circuit ampacity... and also minimum ocpd in many instances.


I'm uncertain how you perceive Article 220 calculations, but over the years I've witnessed many a tradeperson going to extremes trying to avoid doing a load calculation... expending more time and energy than they would have if they just did the calculations... and in they end, more often than not, they end up doing the calculations anyway. Not saying this is you, but it is my advice that you not let it be you. If you need help doing the calculations, we are here to help!

Now for another tidbit...

I do not know of any instance offhand that a load is not supplied by an outlet. The outlet does not have to be wires an outlet box. It can simply be the wires landing in a motor controller (i.e. starter) enclosure or utilization equipment termination section. See the definition in Article 100.

 

[petersonra]

The calculated load of that panel, once performed as proscribed under Article 220, will provide you with the best and only start. Return to ask questions related to that article if need be.

I agree. go do the calculation and see what it comes up with.

 

[SteamDonkey]

Article 220 covers load calculation of the NEC... nothing else. However, as you stated, per other articles and sections, these calculation results set the minimum allowable circuit ampacity... and also minimum ocpd in many instances.

I'd be interested to know where 220 specifies minimum OCPD. There are many instances?

 

[kwired]

Overcurrent protection is outside the scope of art 220, 220 is all about determining what the load is for whatever the application is.

That said, there is not much in 220 that directly applies to motors. I only found three things in 220 that directly involve motors and all three of them send you to art 430.

220.3 tells us there are items in other articles that are in addition to or modifications of what is in 220 - motors and art 430 are in the list.

220.14(C) (in section on branch circuits) sends us to art 430

220.50 (in section on feeders) sends us to art 430.



Determining load calculations for motors is covered in art 430. You possibly take what value you end up with and plug it in with other data determined from art 220 for feeder and service calculations though.

 

[Smart $]

I'd be interested to know where 220 specifies minimum OCPD. There are many instances?

Take 210.20(A) as an example...

"Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the rating of the overcurrent device shall not be less than the noncontinuous load plus 125 percent of the continuous load."

Where is it you calculate the branch circuit load?


Review 215.3 in the same manner.

 

[Cow]

One other method I like to use to determine feeder or service loading is using a power logger. We use a Fluke 1735 set to a 15 minute sample rate for at least 30 days. It takes the guess work out of it, especially if not everything runs at once. I use it A LOT on MCC's, especially ones with VFD's, etc when it's hard to know exactly how much load the gear is pulling at any given time.

 

[kwired]

One other method I like to use to determine feeder or service loading is using a power logger. We use a Fluke 1735 set to a 15 minute sample rate for at least 30 days. It takes the guess work out of it, especially if not everything runs at once. I use it A LOT on MCC's, especially ones with VFD's, etc when it's hard to know exactly how much load the gear is pulling at any given time.

IMO that is the only way to get a realistic idea of the load on a feeder - especially when there is a lot of motor loads, unless all your loads are pretty fixed and seldom vary. Otherwise NEC basically requires you to use full load of all the motors plus another 25% of the largest and sort of assumes they can/will be loaded to that value but most of the time they will not, some never run when others are running, some are lightly loaded at some times and up to full load at others. Every application has it's own set of demand data.

 

[Smart $]

IMO that is the only way to get a realistic idea of the load on a feeder - especially when there is a lot of motor loads, unless all your loads are pretty fixed and seldom vary. Otherwise NEC basically requires you to use full load of all the motors plus another 25% of the largest and sort of assumes they can/will be loaded to that value but most of the time they will not, some never run when others are running, some are lightly loaded at some times and up to full load at others. Every application has it's own set of demand data.

Don't forget the 30-day logging method must be adjusted for periodic or seasonal loading.

 

[SteamDonkey]

Take 210.20(A) as an example...

"Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the rating of the overcurrent device shall not be less than the noncontinuous load plus 125 percent of the continuous load."

Where is it you calculate the branch circuit load?


Review 215.3 in the same manner.

I see what you're getting at, but technically the OCPD requirement comes from other articles, not 220. Calculated load is not interchangeable with minimum OCPD rating.

For what it's worth I think I identified the source of my confusion... A motor that runs continuously is not a continuous load as defined by the NEC.

 

[Smart $]

I see what you're getting at, but technically the OCPD requirement comes from other articles, not 220. Calculated load is not interchangeable with minimum OCPD rating.

Which is what I said.

For what it's worth I think I identified the source of my confusion... A motor that runs continuously is not a continuous load as defined by the NEC.

That is correct. Motor loads are neither noncontinuous or continuous... but most will lump them into the noncontinuous category of load calculations. After all, other articles such as 210.20 and 215.3 only state noncontinuous plus 125% continuous. If you took that literally, you could leave motor loads out.

 

[augie47]

Proving once again the NEC is so straightforward and simple.

 

[kwired]

I think the NEC recognizes most motors as continuous loads. Unless it is identified as intermittent duty we generally treat it like we do most continuous loads. Minimum conductor size is 125% of FLA, overload protection is allowed to be up to 125% FLA for most applications. It is the higher settings allowed for short circuit and ground fault protection on both branch circuits and feeders supplying motors that can confuse people at times.

 

[Smart $]

I think the NEC recognizes most motors as continuous loads. Unless it is identified as intermittent duty we generally treat it like we do most continuous loads. Minimum conductor size is 125% of FLA, overload protection is allowed to be up to 125% FLA for most applications. It is the higher settings allowed for short circuit and ground fault protection on both branch circuits and feeders supplying motors that can confuse people at times.

The NEC may treat motors as continuous loads... but all the treatment is done through and in Article 430 only and nowhere else. We need to be clear on this so as not to confuse others.