Panelboard Breaker Sizing

[BenjiF23]

All,

Scope of project - adding (3) new HVAC units, totaling in 190A FLA.
Existing electrical - 800A, 120/208V main switchboard with (1) space.

I am planning on adding (1) new breaker to the existing SWBD to feed (1) new panel.

I am trying to size the breaker feeding the panel.

Should the breaker serving the panel be 250A? (per 210.20(A)), which would give 125%x190A = 237.5A minimum)

Or since I am sizing each breaker on the panelboard with at least 125% (in my case 175% for a c/b) , I can use a 225A breaker to feed the panel since there is only 190A of load?

I think I am thinking too much on this and starting to go back and forth...

Any help appreciated! Thanks

 

[PD1972]

I think the proper place to look in your case for the panelboard feeder is 215.2(A)(1) and not 210.20 because it is a feeder and not a branch circuit. You need to do an article 220 calculation to determine required load as noted in 215.2(A)(1). Additionally, you do not need a 125% factor. Motor loads are not continuous loads and should not be treated as such. Refer to examples in annex D for how to do an article 220 calculation with motor loads; take specific note of how example D(3)(a) handles motors.

How did you come to the conclusion that this is 190A FLA? Is this 190A of actual FLA that you obtained from fields that are noted as FLA on the mechanical data sheets or is that a sum of the MCAs?

If this is 190A of actual FLA and not the sum of the MCAs, you need to find the largest motor in all of your HVAC units and add 25% of that FLC to get your minimum required ampacity.

If this is 190A that is obtained from the sum of listed HVAC unit MCAs, the 25% factor for the largest motor load is already taken into account (multiple times already) so you don't need to add another 25% factor on top of it. Without trying to determine the actual minimum ampacity by getting the FLAs instead of MCAs, you can use 190A as required ampacity of the feeder. The next standard size circuit breaker would be 200A.

Having said all that, this is the minimum size and may not necessarily be a good idea. If this is 190A of motor load, even if allowable per code, it would not be a good idea to size to the code minimum due to the potential for inrush currents of simultaneous motors starting up which would trip the breaker. Your final breaker size should probably be between the minimum required per above and the maximum allowable for motor feeders per 430.62(A). The NEC only defines what the minimum and maximum sizes are but not necessarily what a good design would use.

 

[ASG]

Once I'm over 225A, I'd just do a 400A feed to a 400A panel. But that's more design practice than Code.

Also a reminder that if you're adding 25% of the service size to the existing electrical, might be a good idea to confirm with the utility that it can support that load.

 

[BenjiF23]

Thank you for the replies.

I am going to show my calcs based on responses and going back to the Code. Couple Side notes - the 190A initial load is not correct, for that I just summed the FLAs for each motor and actually used the MCA for one by mistake. Another is - we have confirmed there is enough capacity for the additional load. Thank you again

Now for the numbers...

Motor 1: 15.2 FLA
Motor 2: 59.19 FLA
Motor 3: 37.6 FLA
Motor 4: 66.2 FLA (largest)

Using inverse time breaker,
66.2A*175% = 115.85 - Next size UP (240.4) --> 125A breaker
(430.62 and Table 430.52)

OCPD shall be sized not greater (430.62) than the 125A breaker plus the sum of FLCs for rest of motors, giving:
125A+15.2A+59.19A+37.6A = 237A. Next size DOWN = 225A breaker.

Another question - if i have other loads on this panel (receptacles/non motor loads), I am assuming I use 125% of load for continuous loads and 100% of load for non-cont. (Art 220)?

Really hoping I got this or I am at least close. Next size up and down can be tricky. Thank you in advance

 

[augie47]

Keep in mind you are feeding a new panel so your individual equipment OCP will be in that new panel therefore the feeder to the new panel is not limited to you load calculations. There is nothing to prohibit the new feeder from being 250,, 300, 400 amps are larger if you wanted to consider future loads.
The numbers you developed give you a minimum and should be taken into account on the service size as ASG mentioned.

 

[BenjiF23]

Keep in mind you are feeding a new panel so your individual equipment OCP will be in that new panel therefore the feeder to the new panel is not limited to you load calculations. There is nothing to prohibit the new feeder from being 250,, 300, 400 amps are larger if you wanted to consider future loads.
The numbers you developed give you a minimum and should be taken into account on the service size as ASG mentioned.

Makes sense, so right now, I am planning on adding a new 225A OPD in the swbd, which would feed my new 225A MLO panel and I should consider increasing the wires up to 400A for future loads? The problem with that is actually my original concern/problem - the swbd cannot physically fit a breaker larger than 250A, which is why I wanted to make sure the calcs were correct.

Am I understanding correctly? Will keep this in mind for future designs.

Thank you all