Separately derived systems in the same raceway 300.3(C)

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brycenesbitt

Senior Member
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United States
San Francisco California has a local amendment to the NEC / CBC as follows:
San Francisco Electrical Code
300.3(C) Conductors of Different Systems.
(1) 1000 Volts, Nominal or Less. Conductors of ac and dc circuits, [←] from separately derived systems, from separate services, or from separate utility meters shall not be permitted to occupy the same equipment wiring enclosure, cable or raceway with conductors from other systems, services, or meters.
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Anyone has any insight into the motivation for this change, or the harm it is seen as preventing? I heard it's a solar thing, but not quite sure what. Many existing buildings mix AC conductors from tenant meters: have there been incidents?
 
It's not a solar thing.

This requirement probably goes back nearly a century to when San Francisco had its own electrical code. They have a few ammendments like this that they stubbornly hold on to.
 
brycenesbitt said:
San Francisco California has a local amendment to the NEC / CBC as follows:


Anyone has any insight into the motivation for this change, or the harm it is seen as preventing? I heard it's a solar thing, but not quite sure what. Many existing buildings mix AC conductors from tenant meters: have there been incidents?
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If I had to guess, I would say it's just another one of these "it seems like a good idea" or "what if" things someone who has too much free time and an enlarged ego came up with.
 
My impression was always that in a city with a lot of multi-unit apartment buildings they didn't want, for example, troubleshooting for one tenant to affect other tenants. But looking again it just seems like pure overkill. I never really noticed before that it applies to separately derived systems. So my theory that it was about separating tenant and ownership holds less water with that there. Why should a commerical building with ne meter have to separate SDSs?

My new specilative theory is that there was some head inspector in 1920s who always did it that way because he just never thought of pulling multiple circuits in a raceway when he was an electrician and that newfangled rigid metal conduit and TW wire came out, so he got it put into the code to make everyone do it and save face. Or he had a stake in the biggest local supply house for conduit. Or perhaps there was an actual incident, but everybody who knows about it has been dead a long time.
 
I have a pretty clear recollection of the first time I encountered this and standing on the street with the inspector who gave the correction. Pretty sure I asked why they have that rule and his answer was "It's just in our local code."
 
jaggedben said:
I have a pretty clear recollection of the first time I encountered this and standing on the street with the inspector who gave the correction. Pretty sure I asked why they have that rule and his answer was "It's just in our local code."
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And there you have it. Local codes in different AJJs' territories when it comes to (for example) PV systems can and do vary significantly, and the people who wrote these rules may or may not have understood how electricity works. The solar company I most recently worked for maintained a large spreadsheet document to try to keep track of it all.
 
ggunn said:
And there you have it. Local codes in different AJJs' territories when it comes to (for example) PV systems can and do vary significantly, and the people who wrote these rules may or may not have understood how electricity works. The solar company I most recently worked for maintained a large spreadsheet document to try to keep track of it all.
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In California it's actually pretty unusual. San Francisco is a bit of a standout with respect to the number of weird old ammendments they have.
 
I was involved in a school installation where emergency lighting supplied by a battery backup system suppled 120 volts to only emergency lighting. When the contractor installed it he used common conduits. The problem was that the neutrals between the building power and the emergency power were not identified as separate systems. The neutrals were crossed between systems creating a situation where you could turn off a circuit breaker and still have a load on the neutral no matter which system was operational. The possibility of shock by getting in series between neutrals.
This code is very effective in situation like this.
 
Rjryan said:
I was involved in a school installation where emergency lighting supplied by a battery backup system suppled 120 volts to only emergency lighting. When the contractor installed it he used common conduits. The problem was that the neutrals between the building power and the emergency power were not identified as separate systems. The neutrals were crossed between systems creating a situation where you could turn off a circuit breaker and still have a load on the neutral no matter which system was operational. The possibility of shock by getting in series between neutrals.
This code is very effective in situation like this.
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It is my opinion that the code should not try to make codes that prevent a problem due installation mistakes, incompetence, or unqualified people.
 
Rjryan said:
I was involved in a school installation where emergency lighting supplied by a battery backup system suppled 120 volts to only emergency lighting. When the contractor installed it he used common conduits. The problem was that the neutrals between the building power and the emergency power were not identified as separate systems. The neutrals were crossed between systems creating a situation where you could turn off a circuit breaker and still have a load on the neutral no matter which system was operational. The possibility of shock by getting in series between neutrals.
This code is very effective in situation like this.
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That is why the code requires grounded conductors of different systems to have different color insulation. But separate conduits is not necessary.
 
Rjryan said:
The neutrals were crossed between systems creating a situation where you could turn off a circuit breaker and still have a load on the neutral no matter which system was operational.
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I would not have a problem with a code rule that addressed an actual namable hazard like that.

But note for anything originating in a main panel, the neutrals are not switched, and they're all tied together with the grounds. It's hard to imagine how you could even create the named hazard through willful malice. Yet San Francisco prohibits it.
 
I don't think it's about neutrals per se. A multi-wire branch circuit with the two hot legs going back to different panels on different meters is the sort of scenario I think of when I think of some San Francisco houses that were (legally or illegally) subdivided into multiple units at some point. I kind of got them keeping this rule for that sort of thing. Not that there aren't other rules that cover that. Still don't understand the separately derived systems part though.
 
With that rule you should never be able to have a 24 volt contactor that controls a 240 volt load without redesigning the contactor so the control wiring is somehow in separate closure than the power wiring.
 
brycenesbitt said:
Many existing buildings mix AC conductors from tenant meters: have there been incidents?
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That doesn't violate this rule, most instances those AC conductors are normally derived from same source.

If the goal were to keep from mixing conductors that originate from different tenant meters - they need to rethink the whole wording of the rule.
 
kwired said:
That doesn't violate this rule, most instances those AC conductors are normally derived from same source.
If the goal were to keep from mixing conductors that originate from different tenant meters - they need to rethink the whole wording of the rule.
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Looks like that's exactly what they did:

San Francisco Electrical Code
300.3(C) Conductors of Different Systems.
....or from separate utility meters shall not be permitted to occupy the same equipment wiring enclosure, cable or raceway with conductors from other systems, services, or meters.
 
200.6 (D) NEC 2023 Grounded Conductors of Different Nominal Voltage Systems Identification of neutrals for systems different nominal voltages systems. The example I gave both systems had the same nominal voltage and used the same grounded conductors identification.
When you have a battery system supplying the 120 voltage the grounded conductor does not originate from the same neutral buss as building power.
 
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