Three 100 amp panels
- Thread starter mlnk
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- Lockport, IL
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- Semi-Retired Electrical Engineer
If I ordered two boxes of bank checks instead of just one, would that mean I can spend more money?
The ratings of the panels and the rating of the service are not relevant to your question. What matters is the calculated load. If it is lower than 200 amps, then you can feed as many panels as you wish.
The ratings of the panels and the rating of the service are not relevant to your question. What matters is the calculated load. If it is lower than 200 amps, then you can feed as many panels as you wish.
PaulMmn
Senior Member
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- Union, KY, USA
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- EIT - Engineer in Training, Lafayette College
"I can't be out of money... I still have checks!"
Truly is that simple , the checks are a good analogy. load calcs. 101
electrofelon
Senior Member
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- Cherry Valley NY, Seattle, WA
Only other comment I have is your meters are cold sequenced (after the service disconnect) which is odd to do with under 7 meters. Check with your POCO if u haven't.
Carultch
Senior Member
- Location
- Massachusetts
The idea is that not all loads will draw their full amps simultaneously. And neither will all 3x 100A panelboards, all draw 100A at once. As long as all loads among the 3 subpanels "add up" to 200A according to the NEC's prescribed load calculation, you feed the three subpanels from the same 200A panelboard.
The load calculations are based on somewhat conservative models of user behavior, taking account for the fact that only a fraction of the circuits will run at once in practice. So load diversity factors apply that allow us to take a "weighted sum" of the loads, for a practical approach to sizing the distribution equipment.
In residential I have fed three 100 amp panels from a 125 amp service. Of course the diversified demand was below 125 amps. I just wondered if I could do the same in commercial wiring. My idea is to have one 200 amp disconnect rated at 22k AIC and then have the 100 amp breakers and the smaller load center breakers rated at 10k AIC. POCO wants one main disconnect with test terminals
Think about a large commercial building with a 1600 Amp service, a school for example, odds are that if you added up all the sub panels, disconnects, MCC's etc... that are fed from the 1600 Amp main, they would far exceed that number.
Roger
Roger
kwired
Electron manager
- Location
- NE Nebraska
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- EC
NEC doesn't even require the disconnect on the supply side in that situation and when there is more than one service disconnecting means the supply conductors only have to be sized to the load calculation and not the total overcurrent devices.
But yes if these are POCO meters, most will not want potential customer access to unmetered conductors. Meter centers that have a main ahead of the meters are typically designed so that there is no easy access to unmetered conductors.
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
I have two comments, one about this quote and one more general. The more general one: Make sure you meter/mains are not "suitable for use ONLY as service equipment. That means you cannot separate the neuters and grounds which would be required (there is 250.142(B) exception #2, but I dont think that can be applied as you have a meter/panelboard combo not just a meter socket, and further, not sure if they can all be "immediately adjacent to the service disconnect).
Regarding the AIC ratings: You may be able to use a series rating for the 100 amp breakers. However, your branch breakers ALSO need to have an AIC equal to or greater than the available fault current. You can only also series rate those if there is a TRIPLE SERIES rating for the three breakers. The more likely scenario is you would fully rate the 200 and the 100's, and series rate the branches with one of those.
kwired
Electron manager
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- NE Nebraska
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- EC
What do you have. Is pretty common to see main breaker with 22k rating or even 25k and branch breakers that are 10k but series rated for use with the main.
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
Yes but he has three breakers in series so one has to be careful in these situations. A very common mis-application of series ratings is with 3 OCPD's in series, A-B-C, to series rate A-B, and then series rate C with the series rating of B. That is not allowed. IT is allowed to insert a fully rated OCPD in between or ahead of a series rated pair and that should be no problem for the OP: He fully rates the 200, fully rates the 100's, and series rates the branches to the 100's. Another option is to rate A-B and A-C as long as A-C has the same (or higher) interrupting level as A-B.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
Couple things - unless all three are in close proximity there is often good chance conductor impedance by the time you get to final device reduces the available current at that point to a level where you won't need to look at series ratings.
some cases you might have a MDP device that is series rated with the final device - if so does it really matter what is in between? Again distance may reduce current at the end anyway. If all in same vicinity, somewhat unnecessary to have say MDP feeder breaker - main breaker in panel - branch circuit breaker when you already have a panel disconnect within sight of the panel. Of course that panel main still needs rated for available current at it's location or be series rated with the one ahead of it.
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
Certainly taking into account conductor impedance can help avoiding having to use higher or series rated equipment. The AFC in the OP is pretty low. But take a service that has 65K at the service disconnect, its going to take quite a bit of length to get down to 10k.
The main thing you cant do, which many do, is the following: Three devices A-B-C, main distribution and branch respectively. Say AIC of A is 65k, B is 22k, C is 10k. AFC at line side of A is 65k, AFC at line side of B is 50k, AFC at line side of C is 20k. A and B series rate. B and C series rate. Many find it acceptable to use these three in a series but it is not acceptable unless A and C rate at the same level as A and B.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
Yes, I played around a little with fault calculator spreadsheet, starting at 65k is difficult to get level down to below 22k without at least 100 foot run and still needed even more length for 200 amp conductors.
starting at 42k seemed to be able to come up with some scenarios where I could get a 100 or 200 amp feeder that isn't real long and have under 22k at the load end.
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
You can actually typically get a series rating from 65k all the way down to 10k miniatures, but the distribution has to be 225 or smaller frame size. I think square D has a couple that will go 400 frame @ 65k all the way down to 10k but nothing higher than 400.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
I can't recall ever needing to do real world calculation that involved 65K available, have done some that get into ~40k range a time or two.
High available current levels is usually more common for stuff I work on if a larger capacity 208 volt system. Same KVA at 480 volts is normally much less available current. Lot of what I commonly do involves motors, if you need larger KVA supply, 480 makes the most sense most the time.
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