Sizing feeders (or, doesn't a branch circuit OCPD also protect its feeders?)

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Jon456

Senior Member
Location
Colorado
Ok, I confess... I've really confused myself now. :-?

For example... say we have 3-phase, 4-wire service with a 200A OCPD serving as a main cut-off. From there we have feeder conductors to three separate subpanels: one subpanel services a 3-phase 100A load, one subpanel services a 3-phase 60A load, and one subpanel services several (no more than 6) small single phase loads (less than 40A total). So for the 100A circuit, we use 1AWG for the branch. And for the 60A circuit, we use 4AWG for the branch. And for the single-phase subpanel, an assortment of 12AWB and 10AWG branches.

But what governs the size of the feeder to each subpanel? Is it the size of the branch load (100A, 60A, and 40A)? Or is it the size of the 200A OCPD that serves as the service disconnect? I suspect it's the latter. But don't the OCPD's in each of the subpanels also protect the "upstream" feeders (as well as the "downstream" branches)? For example, the 100A subpanel should never be able to draw more than 100A (because of the 100A OCPD inside), so why would that panel need a feeder larger than 1AWG? Of course there's still the possibility of a short in the feeders, but a short would trip the 200A OCPD.

If in fact it is the 200A OCPD that governs the size of the feeder, then that would mean running three 3/0 conductors from the cut-off to each of the three panels? And how should the neutral conductor to the single-phase subpanel be sized? Obviously, running nine hot 3/0 conductors plus a neutral and EGC's out of the cut-off would not be possible (let alone practical). So if the code requires 3/0 for these feeders, how would you wire this?

(Btw, why is it that feeders and branches must be larger than SEC's? For example, for 200A, the size for SEC's is 2/0, but for feeders and branches, it's 3/0.)
 

ibew441dc

Senior Member
Ok, I confess... I've really confused myself now. :-?


But what governs the size of the feeder to each subpanel?


215.2 Minimum Rating and Size.
(A) Feeders Not More Than 600 Volts.
(1) General. Feeder conductors shall have an ampacity not less than required to supply the load as calculated in Parts III, IV, and V of Article 220. The minimum feeder-circuit conductor size, before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load.

You got a lot jump'n in your post so here's my two cents on this one....... :smile: Keep it simple.
 
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Smart $

Esteemed Member
Location
Ohio
If you are going to run the feeders with source-end overcurrent protection greater than the feeder's ampacity, your feeders will have to comply with the feeder tap requirements of 240.21(B).
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Ok, I confess... I've really confused myself now. :-?

For example... say we have 3-phase, 4-wire service with a 200A OCPD serving as a main cut-off. From there we have feeder conductors to three separate subpanels: one subpanel services a 3-phase 100A load, one subpanel services a 3-phase 60A load, and one subpanel services several (no more than 6) small single phase loads (less than 40A total). So for the 100A circuit, we use 1AWG for the branch. And for the 60A circuit, we use 4AWG for the branch. And for the single-phase subpanel, an assortment of 12AWB and 10AWG branches.

But what governs the size of the feeder to each subpanel? Is it the size of the branch load (100A, 60A, and 40A)? Or is it the size of the 200A OCPD that serves as the service disconnect? I suspect it's the latter. But don't the OCPD's in each of the subpanels also protect the "upstream" feeders (as well as the "downstream" branches)? For example, the 100A subpanel should never be able to draw more than 100A (because of the 100A OCPD inside), so why would that panel need a feeder larger than 1AWG? Of course there's still the possibility of a short in the feeders, but a short would trip the 200A OCPD.
Your logic follows NEC. The 200 will ptrotect against shorts, within reason, and the panel OCP will protect from overload, however, as Smart$ states, you must follow the rules in 240.21. The wire sizes and lengths are regulated as well as where (how) they terminate.
If in fact it is the 200A OCPD that governs the size of the feeder, then that would mean running three 3/0 conductors from the cut-off to each of the three panels? And how should the neutral conductor to the single-phase subpanel be sized? Obviously, running nine hot 3/0 conductors plus a neutral and EGC's out of the cut-off would not be possible (let alone practical). So if the code requires 3/0 for these feeders, how would you wire this?
Neutrals are sized by Art 220 keeping 215.2 in mind.
(Btw, why is it that feeders and branches must be larger than SEC's? For example, for 200A, the size for SEC's is 2/0, but for feeders and branches, it's 3/0.)
When you mention 3 phase 4 wire, that is not the case. The use of a 2/0 for 200 amp SEC is very restrictive, see the header for 310.15(B)(6)
 

Jon456

Senior Member
Location
Colorado
Thanks to all above for getting me back on track. This makes much more sense now!

So allow me to revise my scenario a bit...

A. Three-phase, 4-wire 230VAC/200A service into a 200A main breaker
B. Subpanel #1 services a 3-phase 40HP motor rated 100A (FLA). This subpanel is between 10 and 25 feed from the main (although I'll probably relocate it to within 10 ft).
C. Subpanel #2 services a single 3-phase 60A load. This subpanel is less than 10 feet from the main.
D. Subpanel #3 services no more than six single-phase branch loads. The total load is less than 40A, but the breakers in this subpanel sum to 110A (as currently configured... I may reduce some of these breakers after I evaluate the loads on the branch circuits).

Additional questions:

1. Since none of my Subpanels require 200A, do I need to run 3/0 to any one of the panels? Or can all three subpanels be serviced by feeder taps? In other words, must the end of the source feeder be formally terminated (e.g., attached to lugs in one of my subpanels)? Or may I (and, if so, should I) just bring the 3/0 into my covered wireway and then take the three taps off it -- including one tap off the end of the 3/0 source feeder?

2. By my calculations, the feeder for the 40HP motor should be 1AWG: 125% of 104A = 130A (430.22 & Table 430.250). So I would take a 1 AWG feeder tap off the 3/0 source feeder and run it to my fusible disconnect (between 10 & 25 ft tap length). Even though the NEC permits TD fuses up to 175% of the FLC, I should use TD fuses no greater than 130A to protect the feeder taps, correct? (Note: The system has been running ~2 years on 125A TD fuses.)

3. The branch circuit for the 40HP motor, from the fusible disconnect to the motor itself should also be 1AWG, correct? (The system was installed with 4AWG feeder and branch conductors.)

4. I'm not sure how to size the feeder tap to subpanel #3. If non-continuous load + 125% of continuous load, then I have the following:

- One 7A continuous lighting load (dusk to dawn)
- One 2A lighting load on switches. Not on very much, but I suppose someone could forget to turn off the lights. So should this be calculated as a continuous load?
- One 20A utility receptacle (with no continuously connected load). Is this considered a non-continuous load?

So my I think my total load is 8.75A + 2.5A + 20A = 31.25A. So my minimum feeder tap would be 8AWG, correct? (If so, I'd probably use 6AWG to allow for any future load changes.) Or do I size the feeder tap to the sum of the OCPDs inside the panel (110A)?

5. What connectors do you recommend for making the feeder taps? I like the idea of a set of 4 port Polaris connectors placed at the end of the 3/0 source feeder in the wireway (one port for the source and three ports for the taps). Opinions?

Thanks again!
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Thanks to all above for getting me back on track. This makes much more sense now!

So allow me to revise my scenario a bit...

A. Three-phase, 4-wire 230VAC/200A service into a 200A main breaker
B. Subpanel #1 services a 3-phase 40HP motor rated 100A (FLA). This subpanel is between 10 and 25 feed from the main (although I'll probably relocate it to within 10 ft).
C. Subpanel #2 services a single 3-phase 60A load. This subpanel is less than 10 feet from the main.
D. Subpanel #3 services no more than six single-phase branch loads. The total load is less than 40A, but the breakers in this subpanel sum to 110A (as currently configured... I may reduce some of these breakers after I evaluate the loads on the branch circuits).

Additional questions:

1. Since none of my Subpanels require 200A, do I need to run 3/0 to any one of the panels? Or can all three subpanels be serviced by feeder taps? In other words, must the end of the source feeder be formally terminated (e.g., attached to lugs in one of my subpanels)? Or may I (and, if so, should I) just bring the 3/0 into my covered wireway and then take the three taps off it -- including one tap off the end of the 3/0 source feeder?
many options. most common is probably to tap off 3/0 in wireway (if you are tapping..most common might be one panel feeding loads)

2. By my calculations, the feeder for the 40HP motor should be 1AWG: 125% of 104A = 130A (430.22 & Table 430.250). So I would take a 1 AWG feeder tap off the 3/0 source feeder and run it to my fusible disconnect (between 10 & 25 ft tap length). Even though the NEC permits TD fuses up to 175% of the FLC, I should use TD fuses no greater than 130A to protect the feeder taps, correct? (Note: The system has been running ~2 years on 125A TD fuses.)!

will work although I'd go a bit bigger

3. The branch circuit for the 40HP motor, from the fusible disconnect to the motor itself should also be 1AWG, correct? (The system was installed with 4AWG feeder and branch conductors.)

ignoring adjustment factors, #1 will work

4. I'm not sure how to size the feeder tap to subpanel #3. If non-continuous load + 125% of continuous load, then I have the following:

- One 7A continuous lighting load (dusk to dawn)
- One 2A lighting load on switches. Not on very much, but I suppose someone could forget to turn off the lights. So should this be calculated as a continuous load?
- One 20A utility receptacle (with no continuously connected load). Is this considered a non-continuous load?

So my I think my total load is 8.75A + 2.5A + 20A = 31.25A. So my minimum feeder tap would be 8AWG, correct? (If so, I'd probably use 6AWG to allow for any future load changes.) Or do I size the feeder tap to the sum of the OCPDs inside the panel (110A)?

25 ft tap rule. minimum tap is 66 amps..#4 .....plus it must terminate in a main.
can increase both, of course, as long as you don';t exceed panel rating
5. What connectors do you recommend for making the feeder taps? I like the idea of a set of 4 port Polaris connectors placed at the end of the 3/0 source feeder in the wireway (one port for the source and three ports for the taps). Opinions?

Thanks again!
Many options. + I'm probably doing more "design" than Forum calls for.
 
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Smart $

Esteemed Member
Location
Ohio
Additional questions:
...

1) No; Yes; No; Yes.

2) Using 125A TD fuses is fine as long as the ampacity of the feeder tap conductors is greater than the fuse ratings. 240.21(B)(1)(1)(b)

3) Yes.

4) Correct; #8 minimum.

5) Yes, the multi-port tap connector approach is as good as it gets.

Additionally... if these are panelboards they cannot be MLO (main lug only). Tap conductors must terminate at a single overcurrent device.
 
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Jon456

Senior Member
Location
Colorado
Good catch if the panel is more than 10 ft away. For some reason I was assuming Panel #3 was closer than #1 and #2. Yet rereading posts he did not mention distance to #3.

Sorry, I forgot to mention that. Subpanel #3 is less than 10ft. And since I have to remove a lot of 4AWG from the original incorrect installation, I'll probably just use a 4AWG feeder tap to subpanel #3.

I had previously overlooked the requirement for a main breaker in this subpanel. So I guess I'll have to replace the current main-lug only panel that's currently installed, with a 70A main breaker subpanel.
 
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