1P/3P Panels

[mstrlucky74]

I was under the impression that no matter what a single phase panel would have 2 hots,neutral and ground and a three phase panel would have a 3 hots, neutral and ground? But actually no that I think about it many times there are feeders to panels that don't contain a neutral. So the loads a panel is serving change the wiring configuration and possibly eliminate the neutral? The quantity of "hots" always remain the same for a 1P or 3P panel? Or it depends if you have 208v loads? Thanks.

 

[adamscb]

It depends on the loads that are fed from panels. For example if all loads coming out of a 480v panel are three-phase, then technically you don't need a neutral. The moment you're feeding a single-phase 277v load, then you need a neutral. In the 120v/240v world, if the panel feeds all 240v loads, then technically you don't need a neutral. However, if there are 120v loads mixed with 240v loads, then you need a neutral. Otherwise the current in 120v loads would have no return path.

With that being said, it's best practice to run the neutrals anyway, which gives you future flexibility.

 

[mstrlucky74]

It depends on the loads that are fed from panels. For example if all loads coming out of a 480v panel are three-phase, then technically you don't need a neutral. The moment you're feeding a single-phase 277v load, then you need a neutral. In the 120v/240v world, if the panel feeds all 240v loads, then technically you don't need a neutral. However, if there are 120v loads mixed with 240v loads, then you need a neutral. Otherwise the current in 120v loads would have no return path.

With that being said, it's best practice to run the neutrals anyway, which gives you future flexibility.

If anyone has a link or reference on the that explains wiring 1P/3P loads please share. There is a lot of info on Google but not much explains the electrical/field wiring and what's required depending on the loads...kind of like was explained above? Thanks.

 

[electrofelon]

As you mentioned, it is fairly common to not send a neutral to a panelboard. There are a few other scenarios. A corner grounded system will look just like a single phase panel with "2 hots and a neutral" but it is in fact three phases (one of which is grounded). You could also have a three phase panel only fed with single phase (or two phases of a 3 phases system), or a single phase panel fed with only one phase (maybe common to both busses) . Some of these could potentially be a 110.3 violation - listing/manufacturer instructions.

Note that it is very common to supply a single phase panel with two phases of a three phase system.

 

[mstrlucky74]

As you mentioned, it is fairly common to not send a neutral to a panelboard. There are a few other scenarios. A corner grounded system will look just like a single phase panel with "2 hots and a neutral" but it is in fact three phases (one of which is grounded). You could also have a three phase panel only fed with single phase (or two phases of a 3 phases system), or a single phase panel fed with only one phase (maybe common to both busses) . Some of these could potentially be a 110.3 violation - listing/manufacturer instructions.

Note that it is very common to supply a single phase panel with two phases of a three phase system.

Complicated it even more LOL

 

[JFletcher]

I was under the impression that no matter what a single phase panel would have 2 hots,neutral and ground and a three phase panel would have a 3 hots, neutral and ground? But actually no that I think about it many times there are feeders to panels that don't contain a neutral. So the loads a panel is serving change the wiring configuration and possibly eliminate the neutral? The quantity of "hots" always remain the same for a 1P or 3P panel? Or it depends if you have 208v loads? Thanks.

Generally true, tho not absolutely true. The system or equipment voltage can also dictate whether or not you need a neutral. Example: 277V lighting can only come from a 480Y/277V system (4 wire plus EGC, which could be a 5th wire or the raceway), tho the panel they're fed from might be 1, 2, or 3 phases +neutral (most times they would be 3+). 480V lighting is line to line, so that lighting panel wouldn't need a neutral wire on its feeder.

Then there's some panels like a spa subpanel that need a neutral feeder even tho the eqpt may not need it, so that the GFCI breaker can operate correctly.

 

[adamscb]

If anyone has a link or reference on the that explains wiring 1P/3P loads please share. There is a lot of info on Google but not much explains the electrical/field wiring and what's required depending on the loads...kind of like was explained above? Thanks.

The picture above shows a 120v/240v system. What you see are two coils that supply 120v, but one coil is electrically 180 degrees out of phase from the other one (this isn't shown in the picture). Think of it as one being "positive" 120v, and the other one being "negative" 120v (I use parenthesis, because this is still AC electricity so there really is no "positive" and "negative", but it helps make my point). When you measure the potential difference across the two, you get 240v.

Now how do you wire a 120v load up to this source? You'll use one of the hot legs (120v) and the middle wire, which is the neutral (you'd also need a ground wire). Current comes out of the top or bottom coils, through the load, and returns on the neutral wire to GND. This brings up the issue of load balancing. It's not good practice to put all 120v loads on one of the 120v coils and not the other. What you should try to do is to balance the 120v loads as much as possible between the top and bottom coils. Both 120v sources share the same neutral. Balancing the loads reduces the current going through the neutral wire. In effect, the current coming from the top and bottom 120v coils "cancel" each other out while flowing on the neutral wire. Best case scenario, if all 120v loads are perfectly balanced, then the neutral will be carrying very little current at that point. 120v loads inside of a panel like this use what are called "single-pole" circuit breakers, because they're only connected to one of the hot legs.

Wiring up 240v loads is just as easy, but instead of connecting the load to one of the hot legs and the neutral, you connect it between both of the 120v legs. This in turn supplies 240v to the load. You don't need to use the neutral in this case. The issue of load balancing does not apply here, because you're already using two out of the two hot legs, there's no other way to connect 240v loads. 240v loads connected inside of a panel like this use what are called "two-pole" breakers, because they're connected to two of the hot legs.

Keep in mind I'm just talking about a 120v/240v system. There's other systems out there such as three-phase three wire or three-phase four wire, and a corner-grounded system in which a previous poster mentioned.

 

[adamscb]

The above picture shows a "three-phase, four-wire system", with three-phase meaning that there's three hot wires, that are each separated by 120 degrees electrically (this is shown in the drawing by that angle-looking symbol). Four-wire refers to the fact that there's four total wires (three hots and the neutral). This particular system is ungrounded, which poses safety hazards, but that's another topic.

Let's say you're trying to connect a single-phase, 120v load to this system. You would connect the load to one of the hot wires, and the center neutral wire. If you want to connect multiple 120v loads to this system, you would use the other hot 120v legs as well, once again trying to balance the loads as much as possible. The same concept here applies to the 120v/240v system I was talking about above; the more you are able to balance the loads across the three hot legs, the less current will flow through the neutral.

You can also feed single-phase, 208v loads from this system. You may ask how I got the number 208v. The thing with three-phase systems like this, is that the line-to-line voltage is 1.732 (or square root of three) times larger than the line-to-neutral voltage. So in the previous paragraph I mentioned wiring up a single-phase 120v load. The 120v is the line-to-neutral voltage. Also, the diagram shows the line-to-neutral voltage by listing the voltage on each individual coil. Therefore, the line-to-line voltage is 120v x 1.732, or 208v. In wiring up a single-phase 208v load, you would use any two hot legs. Load balancing is still an issue here, even if the neutral isn't being used, because you want to keep the loading on all three coils nearly the same. Also if the loading across the three coils becomes too imbalanced, you could start tripping overcurrent protection devices that have current-imbalance tripping enabled.

And finally, you can feed three-phase 208v loads from here. You would use all three hot 120v wires. On three-phase loads, the rated voltage is given in line-to-line volts (from my experience). One might ask, why wouldn't I need the neutral? It turns out, that if the load is a perfectly balanced three-phase load, the summation of all three currents travelling across the three wires is zero. If you were to take an ammeter and clamp it around all three hot phase wires feeding a perfectly balanced three phase load you would and should get a reading of zero amps. If you don't, then something is wrong (I've used this method before when hunting for ground faults). This seems kind of counter-intuitive, because you have three current going to the same point and then cancelling each other out in a sense. The magic is in the math, and that 120 degree phase difference I mentioned earlier. There are three-phase systems like this one, in which the line-to-neutral voltage is 120v and the line-to-line is 208v. The most common three-phase systems I see in the industry in which I work in are 277v line-to-neutral and 480v line-to-line (again, the square root of three multiplier shows up).

A three-phase, three-wire system is almost exactly like the picture shown above, except that there is no neutral wire. Therefore, you cannot feed line-to-neutral loads from a three-phase, three-wire system, because there's no neutral. You can still feed single-phase, line-to-line loads however.

Hope this all helps.

 

[infinity]

I was under the impression that no matter what a single phase panel would have 2 hots,neutral and ground and a three phase panel would have a 3 hots, neutral and ground? But actually no that I think about it many times there are feeders to panels that don't contain a neutral. So the loads a panel is serving change the wiring configuration and possibly eliminate the neutral? The quantity of "hots" always remain the same for a 1P or 3P panel? Or it depends if you have 208v loads? Thanks.

I'll go out on a limb and say that you will only see 208Y/120 and 480Y/277 volt 3Ø systems. In either of those 3Ø systems you can use only 3 hot conductors which would give you straight 208 volts or 480 volts. If you had a panel, DB or switchboard serving only 3Ø loads then you would not need a neutral conductor.

 

[mstrlucky74]

View attachment 18576
The picture above shows a 120v/240v system. What you see are two coils that supply 120v, but one coil is electrically 180 degrees out of phase from the other one (this isn't shown in the picture). Think of it as one being "positive" 120v, and the other one being "negative" 120v (I use parenthesis, because this is still AC electricity so there really is no "positive" and "negative", but it helps make my point). When you measure the potential difference across the two, you get 240v.

Now how do you wire a 120v load up to this source? You'll use one of the hot legs (120v) and the middle wire, which is the neutral (you'd also need a ground wire). Current comes out of the top or bottom coils, through the load, and returns on the neutral wire to GND. This brings up the issue of load balancing. It's not good practice to put all 120v loads on one of the 120v coils and not the other. What you should try to do is to balance the 120v loads as much as possible between the top and bottom coils. Both 120v sources share the same neutral. Balancing the loads reduces the current going through the neutral wire. In effect, the current coming from the top and bottom 120v coils "cancel" each other out while flowing on the neutral wire. Best case scenario, if all 120v loads are perfectly balanced, then the neutral will be carrying very little current at that point. 120v loads inside of a panel like this use what are called "single-pole" circuit breakers, because they're only connected to one of the hot legs.

Wiring up 240v loads is just as easy, but instead of connecting the load to one of the hot legs and the neutral, you connect it between both of the 120v legs. This in turn supplies 240v to the load. You don't need to use the neutral in this case. The issue of load balancing does not apply here, because you're already using two out of the two hot legs, there's no other way to connect 240v loads. 240v loads connected inside of a panel like this use what are called "two-pole" breakers, because they're connected to two of the hot legs.

Keep in mind I'm just talking about a 120v/240v system. There's other systems out there such as three-phase three wire or three-phase four wire, and a corner-grounded system in which a previous poster mentioned.

Appreciate that.