Single sub-panel from 3 phase panel and the main breaker size

Question

I have an old 3 phase panel next to a separate single phase 100 amp panel that is being used by one residence. This space has been used as a carpenter shop and an office in the past. Now it is being divided into two residences and I want to use the 3 phase panel for one of the residences by installing a single phase 120/240 subpanel. The place that I will need to install the subpanel is in the same building but it is about 60 ft away from the three phase panel. The 3 phase panel is powered by three #6 wires from the pole and there is no main breaker in this panel. I believe the system is Delta and two legs are 120 and one is 208. The questions are:

1. Since the wire coming from pole is #6, what is the maximum Amp that I put in the 3 phase panel in order to power the sub-panel. Can I use 70amp?
2. Can I use an entrance service cable ALU#4 inside the building above the existing suspended ceiling with or without conduit?
3. Is it better to connect the cable#4 to the 120 legs or 70Amp breaker?
4. I will use a 60Amp breaker in the sub-panel because I want the 60 amp breaker to trip before the 75 Amp in the main panel.
5. There is an existing three phase 60 amp breaker in the panel. Can I use the two connectors that give 120 Amps? Is it ok for me to put 60 Amps in the subpanel?

ThreePhaseEel has asked me a couple of questions. First, I want to thank him for the answer. As to the color of the 3 phase wires from the pole, all are green. I don't see any white either. The high leg is the first wire from left in the box that is 208v. The second and third are 120v. So I think instead of A-B-C I have B-A-C. In the box the color of Bus bar C has been painted red but the voltage of this bar is 120. The voltages that you are showing are correct.

What I don't understand is where the white wire from the pole is, although I see the white wire has entered the adjacent breaker box from the pole.

What should I do when I don’t see white neutral coming from pole to 3phase panel? Is it possible there is only one neutral for both single and 3 phase panels? Can I use the neutral in single phase to connect to the single-phase 60-amp panel that is from 3 phase panel? How can you distinguish #6 PE from XLPE?

What is this balancing issue warning that some people are talking about when you want to install a single phase sub-panel of a 3 phase panel? Is it OK to have 60 amp breaker as feeder and as main sub-panel? Which one will trip first if there is a short?

Finally a question about conduit. Can I use two 1.5” 90 degree elbows (like S shape) connected to each other?

Answer 1

First things first: do you see any orange wires, or wires with orange tape on them? If so, you definitely have what is known as a "high leg" or "wild leg" delta system -- based on your voltages, I believe you have this, which was used historically to supply both 3 phase 240VAC and 1 phase 240/120VAC to mixed occupancies, as in the illustration below (courtesy Wikipedia/Gargoyle888):

In this system, the secondary center tap forms the split-phase neutral, with the A and C phases as the normal 120V "hot" legs in the derived split-phase supply, and the "high" or "wild" leg, while normally the B phase (this is from 408.3(E)(1) in the NEC, by the way) sits unused as it has 208V to the neutral on it.

Now that that's explained, to answer your questions in turn:

1. Typical 6/4 service entrance quadruplex uses PE (XHHW) insulation rated to 75°C and is thus limited to 60A. If you can confirm that the service entrance uses XHHW-2 (XLPE) insulation, though, you can run it up to a 90°C rating, which gives you a 70A (some sources say 75A) max ampacity. The XHHW or XHHW-2 designation is part of the markings on the insulation, by the way.

2. Is the ALU#4 cable type SE(R) or type USE (also called SEU) cable? SE(R) cable can be used for feeders indoors provided that the bare conductor in the cable is used only for equipment grounding purposes, as per 338.10(B)(2), or if all wires in the cable are individually insulated, as per 338.10(B)(1). However, USE/SEU cable cannot be used for indoor feeders as per 338.12(B)(1), as its insulation is not flame retardant.

3. Connect the feeder cable to the feeder breaker (either 60A or 70A) in the three-phase panel; connecting a load directly to panel busbars is simply not cool.

4. While your thought of making it so the subpanel main breaker trips before the feeder breaker in the main panel is appreciated, selective coordination is a much more complex piece of work than simply using a smaller subpanel main breaker than the feeder breaker. Here's an article on the topic if you want a taste of the gory engineering details that you'll have to work out to do this. You can use 60A breakers for both the feeder and the subpanel main, by the way; however, there are no guarantees as to which breaker trips first into a bolted fault (hard short).

5. You can tap the A and C legs from the existing 60A three phase breaker in the main three phase panel and use them to feed the subpanel; this is the most cost effective approach, and doesn't require any inspection of the service entrance conductors.

Finally, keep in mind that 60A is a very limited amount of current for a single dwelling unit. It can be managed, though, if you are able to run the heavy single loads (dryer, range/stove, hot water, and HVAC) using whatever fuel gas supply is plumbed to the building instead of using electric heavy-load appliances, or if the heavy loads for that dwelling unit are run directly from the three-phase supply -- although in some high leg services, the B phase is limited to a small fraction of the total load, which can make this infeasible.

Is there a reason the utility won't simply replace the obsolete high leg delta service with either a 240/120V split phase or a 208Y/120V three phase wye service?

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Addressing the conduit problem, the neutral, and the balancing issue:

• I would use conduit bodies instead of elbows, unless elbows are the only thing that fits in the space. In any case, make sure you have no more than 360 degrees of bends between your pull points!

• The neutral coming from an overhead pole is on the bare wire in a triplex or quadruplex cable, just about always.

• Phase balance isn't typically worried about in high-leg deltas; it's a concern in a wye system due to unbalanced currents flowing through neutrals, which need to be sized appropriately to carry it.