3-phase distro and 2-phase service

LiWhit

Member
Hello all,

The venue I’m loading into soon has a 200amp 2-phase. My distro, as you’d assume, has 3 phases. Is it as simple as only using the phases I hook up? For example if I plug in phase “x” and “y”, I just can’t use the circuits devoted to phase “z”? No need for 208v by the way.

Thanks.
 
Yes, it'll do exactly that. As long as you aren't looking for 208 you'll be fine. Just watch your loads as usual.
 
Just for background, the NEC addresses this issue precisely in section 520.53(B):

520.53(B) Neutral Terminal.
In portable switchboard equipment designed for use with 3-phase, 4-wire with ground supply, the current rating of the supply neutral terminal, and the ampacity of its associated busbar or wiring, or both, shall have an ampacity equal to at least twice the ampacity of the largest ungrounded supply terminal.

Exception: Where portable switchboard equipment is specifically constructed and identified to be internally converted in the field, in an approved manner, from use with a balanced 3-phase, 4-wire with ground supply to a balanced single-phase, 3-wire with ground supply, the supply neutral terminal and its associated busbar, wiring, or both, shall have an ampacity equal to at least that of the largest ungrounded single-phase supply terminal.

The assumption of 520.53(B) is that when a road electrician with a 3-phase dimmer rack is presented with a split-phase service ( 2 hots and one neutral), they are going to connect their two most lightly loaded phases to L1, and their most heavily loaded phase to L2. Therefore, the neutral terminal of a portable switchboard must be able to carry the full current of two phases of the connected load.

ST
 
Hello all,

The venue I’m loading into soon has a 200amp 2-phase. My distro, as you’d assume, has 3 phases. Is it as simple as only using the phases I hook up? For example if I plug in phase “x” and “y”, I just can’t use the circuits devoted to phase “z”? No need for 208v by the way.

Thanks.
Well YOU DO why do venues do this? I swapped out and rewired some dimmerpacks fitted with a recessed twistlock plug receptacle from 3- phase to single phase. FYI Unless you're in Philidelphia "actual 2- phases" is extremely rare." "Single phase inplies 2-legs of a Std. "Pole pig" but 2-phases is 2 of any 3 of the actual available allotment to your venue from a real 3/phase, 120/208" v-volt utility distribution system, such as in NYC All power is 3-phase. You just only get 2 "phases" @ your address, next door look for that 3rd phase.

So when Dimmer channels 1 + 2 were on PH1, @ 20a total, 3 & 4 were on PH2, also 20a total... and 5 & 6 were on PH3... The third 20a plug) I Changed them to 1 + 2 + 3 would be on LINE 1, @ 30 amps ... and dimmer channels 4 + 5 + 6 on LINE 2.... now this is what you gotta do unless you can live w/o 1/3 of all your available dimmer channels.
 
Look here's a short electrical lesson.
2-phase is a misnomer. But if you got a 3-phase corner grounded delta utility (some of them do this because "thrifty" and all anyone really NEED 3-phase tesla power for in 1 place is heavy motor use) Look UP at the transformers, if ONE is small and the others are big then you got a bastard leg of use to no one except motors don't care. You'll find this a lot in the Northeast old original pre-war towns where commercial power use was intermingled with residential, since they could provide the best of both worlds to all in keeping with the rural electrification act and grab that cheap federal $$$. Utilities still participate to this day so legacy systems are simply "Maintained..." If you can call "LEAVE IT UNTIL IT FAILS" a "maintenance" policy.

Source: ( I know... ) https://en.wikipedia.org/wiki/Rural_Electrification_Act
 
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"2 phase" is, I think officially, a misnomer. The common term these days for 180 degree 1ph 3w 120/240VAC service seems to be "split-phase", though I don't think that's official either.

I worked at a place back in the late noughties that had a wild-leg open delta service, and that high leg freaked me out until I figured out what was going on. They did come in and close the delta at some poing, and all my gear was much happier then.
 
It's not 180°. Phase is a time-based phenomena; tell me how time was changed.

That leaves polarity. Tell me how a single, center-tapped transformer winding can change polarity.

The answers are neither time nor polarity, but contained within the Laws of Ohm. Maybe Kirchoff, too.

The YT video I link shows a DC source, with center tap, and all the batteries are in the same polarity, yet the same things happen with AC or DC.

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It's not 180°. Phase is a time-based phenomena; tell me how time was changed.

That leaves polarity. Tell me how a single, center-tapped transformer winding can change polarity.

The answers are neither time nor polarity, but contained within the Laws of Ohm. Maybe Kirchoff, too.

The YT video I link shows a DC source, with center tap, and all the batteries are in the same polarity, yet the same things happen with AC or DC.

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Technically correct -- the best kind of correct -- but in fact, the 180 degree differential in phase between the two legs of a 120/240 service could be an artifact of either phase or polarity: it's impossible to distinguish or measure that on the wires.

As for centertaps, its because *the neutral is derived from the center tap*.

The distribution transformer is taking one leg of the 3ph distribution, at 1200VAC to ground or so, usually, and transforming it down to 240 (5:1), and then peeling a neutral off the centertap of that secondary. When one leg is at full, the other leg is at zero.

It's correct to say that's not a "phase" artifact, but in most cases, it's not terribly important.
 
Technically correct -- the best kind of correct -- but in fact, the 180 degree differential in phase between the two legs of a 120/240 service could be an artifact of either phase or polarity: it's impossible to distinguish or measure that on the wires.

As for centertaps, its because *the neutral is derived from the center tap*.

The distribution transformer is taking one leg of the 3ph distribution, at 1200VAC to ground or so, usually, and transforming it down to 240 (5:1), and then peeling a neutral off the centertap of that secondary. When one leg is at full, the other leg is at zero.

It's correct to say that's not a "phase" artifact, but in most cases, it's not terribly important.

Right, if it's a 240/120v transformer, then they're not called "Phase" you'd call the 2 incoming hot lines exactly just that, LINES. Hence: L1 &/ L2 like on any residential panel. The confusion comes in at say a large shopping center or mall, or apartment house with a 3-phase true 120/208v Y-service is you just send 2 of thoose "phases" to each panel, trying to keep the service in balance.
Yes, the oven takes longer to pre-heat. And an electric clothes dryer as well but since those heating elements running a little cooler at 208 instead of 240, so, they'll cycle less.
For a true 3-phase service, 3 primaries are required. Unless that "bastard leg" AKA "Stinger" which we have no use for unless you got the world's biggest glitter balll motor? BTW if you're not in a USA - voltage designed system, please disregard. Straight 240 would make everything so much easier. And ring main circuits = far less copper/ aluminum conductor is needed.
Helps if you're rebuilding your country after a "supply-is limited" World War.
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Section 210.71, apparently a fairly recent addition has a clause about meeting rooms larger than 1000 square feet. The section is supposed to apply to rooms where electronic devices will be brought and used but explicitly excludes auditoria, coffee shops, and schoolrooms. At least, that’s what the internet says.
 
Section 210.71, apparently a fairly recent addition has a clause about meeting rooms larger than 1000 square feet. The section is supposed to apply to rooms where electronic devices will be brought and used but explicitly excludes auditoria, coffee shops, and schoolrooms. At least, that’s what the internet says.
In the 2023 NEC, there is no section 210.71.

ST
 
Technically correct -- the best kind of correct -- but in fact, the 180 degree differential in phase between the two legs of a 120/240 service could be an artifact of either phase or polarity: it's impossible to distinguish or measure that on the wires.

As for centertaps, its because *the neutral is derived from the center tap*.

The distribution transformer is taking one leg of the 3ph distribution, at 1200VAC to ground or so, usually, and transforming it down to 240 (5:1), and then peeling a neutral off the centertap of that secondary. When one leg is at full, the other leg is at zero.

It's correct to say that's not a "phase" artifact, but in most cases, it's not terribly important.
Other than "because a neutral is derived from a center tap", there's nothing here, Jay. That's why I posted the video. The DC example behaves exactly like an AC circuit, but there is neither phase nor polarity at work in the example. And that, by itself, dispels most of the common assumptions about what is actually happening.
 
In the 2023 NEC, there is no section 210.71.

ST
Maybe it was in the 2020 edition? What would I know? I’m Canadian.
 
Section 520 sidebar: I've got a new house with a main auditorium of 350 retractable seats, and it has zero utility receptacles around the walls. Is this the "over 1000 sq ft" exemption, or something else, would you think?

Technically correct -- the best kind of correct -- but in fact, the 180 degree differential in phase between the two legs of a 120/240 service could be an artifact of either phase or polarity: it's impossible to distinguish or measure that on the wires.

As for centertaps, its because *the neutral is derived from the center tap*.

The distribution transformer is taking one leg of the 3ph distribution, at 1200VAC to ground or so, usually, and transforming it down to 240 (5:1), and then peeling a neutral off the centertap of that secondary. When one leg is at full, the other leg is at zero.

It's correct to say that's not a "phase" artifact, but in most cases, it's not terribly important.

I do not know of any "over 1000 sq ft" exemption in the NEC to which you refer.

ST
sk8ersdad just posted the exemptioon I believe is being referred to. The wording of NEC used to be "Put in whatever is needed after the minimum requirements of a building are determined." But nothing like residential code where receptacles must be within 6' of any wall space or permanent fixed glass whereas any lamp with a 6' cord (which is pretty standard) can be utilized. Further improvements came in the late 30's with the REA requiring a minimum standard 60 amp fuse box for a stove, TWO 20a kitchen/ cooking circuits, (used to be one prior to 1967) and countertop outlet spacing hence 3' cords on most modern kitchen appliances. Butheck in some commercial establishments even receptacles are not required except in shop windows.
 
Maybe it was in the 2020 edition? What would I know? I’m Canadian.
210.71, covering meeting rooms, went in to the 2017 NEC, but was moved in the 2020 edition to 210.65.

However, 210.65 does not apply to Jay's example:

"Section 520 sidebar: I've got a new house with a main auditorium of 350 retractable seats, and it has zero utility receptacles around the walls. Is this the "over 1000 sq ft" exemption, or something else, would you think?"

because of this informational note:

Informational Note No. 2: Examples of rooms that are not meeting rooms include auditoriums, schoolrooms, and coffee shops.

ST
 
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210.71, covering meeting rooms, went in to the 2017 NEC, but was moved in the 2020 edition to 210.65.

However, 210.65 does not apply to Jay's example:

"Section 520 sidebar: I've got a new house with a main auditorium of 350 retractable seats, and it has zero utility receptacles around the walls. Is this the "over 1000 sq ft" exemption, or something else, would you think?"

because of this informational note:

Informational Note No. 2: Examples of rooms that are not meeting rooms include auditoriums, schoolrooms, and coffee shops.

ST
Yes, I know it doesn’t apply and said so in my original post. That was sort of the point of the post.
 

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