Feed wire sizing to my garage

Don't completely disagree with you abrace. I am making some assumptions of the load and not balancing it. I have seen that before. All 120 breakers on one side, 240v on the other. "Looks" pretty. The 240 volts does indicate #8 as an option. This was more to indicate that there is more to wire sizing than current carrying capacity. Myself, I ran 170 feet of #4 to my barn. Breaker size is 60 amp. Could go to 70 amps. More than enough power to run everything I need. Not even considering harmonics.
 
Wondering what you mean by split phase service? I agree with you if this is 3 phase but I'm pretty sure I read single phase in the question being presented along with the need for the VFD.

Split phase is your traditional residential electric service in the USA. Basically it is a single phase 240V transformer out at the pole with a center tap. That center tap is grounded and becomes your neutral. Any circuit you run that uses a single hot and the neutral only gets half the windings at the transformer and develops around 120V RMS. A circuit that uses both hots gives you the entire winding width and develops the full 240V.

In a properly balanced panel the neutral sits there and does nothing.

You can see this working by using an amp clamp in your panel. Clamp one hot and take the reading. Clamp the other hot, take that reading. Then clamp the neutral and take a reading. You will see that the neutral is exactly whatever the difference between the two hots are.

Most panels balance out pretty well on their own as long as you fill the panel in order with a 120/240V split phase system.
 
Don't completely disagree with you abrace. I am making some assumptions of the load and not balancing it. I have seen that before. All 120 breakers on one side, 240v on the other. "Looks" pretty. The 240 volts does indicate #8 as an option. This was more to indicate that there is more to wire sizing than current carrying capacity. Myself, I ran 170 feet of #4 to my barn. Breaker size is 60 amp. Could go to 70 amps. More than enough power to run everything I need. Not even considering harmonics.

Even in that model with 240's on one side and 120's on the other you should end up in pretty good shape. Where people get into trouble is using only odds or evens, or hook up multi wire branch circuits but have both breakers on the same pole...which is a violation of course, but what happens is the neutral ends up melting.

170 feet is a long run, you can go all the way to 90A (next size up rule) with #4 if you run THWN2 in pipe, but that is a long way and voltage drop would be a killer. Seems like a smart approach bumping up a size. A little bit bigger wire doesn't cost much more.
 
Split phase is your traditional residential electric service in the USA. Basically it is a single phase 240V transformer out at the pole with a center tap. That center tap is grounded and becomes your neutral. Any circuit you run that uses a single hot and the neutral only gets half the windings at the transformer and develops around 120V RMS. A circuit that uses both hots gives you the entire winding width and develops the full 240V.

In a properly balanced panel the neutral sits there and does nothing.

You can see this working by using an amp clamp in your panel. Clamp one hot and take the reading. Clamp the other hot, take that reading. Then clamp the neutral and take a reading. You will see that the neutral is exactly whatever the difference between the two hots are.

Most panels balance out pretty well on their own as long as you fill the panel in order with a 120/240V split phase system.

I understand the transformer arrangement, whether it's single of three phase. I'm just not sure how three phase applies to the original question asked. A balance single phase electrical panel seems more like an electricians myth than reality. Even if you have a single row of breakers down the middle or a double row of breakers in the panel the 220/240 breakers pull from the buss bar on the 110/120 side. It doesn't matter how you stack them in because the only difference between 220/240 and 110/120 breakers is the pin that trips both on the 220 breaker, and ohms law hasn't change.

If a panel has two rows then it's logical to even up the sides, top down, but the buss bars are rated much higher than the panel rating. If you want balance because it looks pretty, I'll give it a thumbs up.
 
Balancing the load is no myth. As long as your neutral is fully sized it doesn't matter.

However, my point was that voltage drop calculations were being done at 120V. Voltage drop calculations should be done for 240V when looking at a split single phase feeder, and not for 120V. I was simply trying to explain why, from a feeder's perspective, you don't need to do drop calculations at 120V.
 
Balancing the load is no myth. As long as your neutral is fully sized it doesn't matter.

However, my point was that voltage drop calculations were being done at 120V. Voltage drop calculations should be done for 240V when looking at a split single phase feeder, and not for 120V. I was simply trying to explain why, from a feeder's perspective, you don't need to do drop calculations at 120V.

If the feed was going to a sub-panel, yes you would add a neutral and the voltage drop would be calculated on the 240 feed. If the 240 was on a breaker inside the residence then there would only be two hot legs and a ground sized per NEC. Most wire size charts for amperage are based on 100' so voltage drop is already factored in. The reality of a need to balance a single phase panel is a myth. In reality the sub panel or main circuit breaker panel has been designed to compensate. You just can't unbalance a panel with one row of breakers and a person would need to work really hard to unbalance a panel with two rows by only using one one row and then if you look at the buss bars.............nah, can't be done.
 
If the feed was going to a sub-panel, yes you would add a neutral and the voltage drop would be calculated on the 240 feed.

Yes, a feeder to a outbuilding is what is being discussed.

The reality of a need to balance a single phase panel is a myth. In reality the sub panel or main circuit breaker panel has been designed to compensate.

I don't know what the sub panel/main breaker panel has to do with it. If you end up with a feeder to an outbuilding where you have 100A over hot A, and 20A over hot B, you wasted money on the feeder. That second hot is doing very little. You ran #3 out the building, when you could have run #6 if it was properly balanced.

You just can't unbalance a panel with one row of breakers and a person would need to work really hard to unbalance a panel with two rows by only using one one row and then if you look at the buss bars.............nah, can't be done.

Yes, in this specific case discussing needing 2 circuits correct. My statements were just an intent to educate on voltage drop calculations for single phase services, and why they should be done at 240V. We really arent disagreeing, but it seems you think we are. You have to try to unbalance a single phase panel, hence why, again, my suggestion is to do voltage drop calculations at 240V. If you fully read my posts you will see I said:

abrace said:
In most cases the feeds to outbuildings end up carrying very little current over the neutral.

abrace said:
Most panels balance out pretty well on their own as long as you fill the panel in order with a 120/240V split phase system.

abrace said:
Where people get into trouble is using only odds or evens

See? We seem to actually agree.
 
1). Why are you referring to the commercial power source as a split phase?
2). If this is a feeder then it would be supplying a sub-panel in the outbuilding, protected at the main circuit breaker panel with a circuit breaker of fused disconnect box.
3). Voltage drop is a factor but a very small one.
4). As a feeder the conductors would be three of the same size.
5). If this was a branch circuit (240 v) you would only have to hot legs and both the same size, plus the ground.
6). How can you provide a 240 v feeder with two differnet size conductors?
 
1). Why are you referring to the commercial power source as a split phase?
Because that is what residential 120V/240V service is commonly referred to as by many, myself included. Call it whatever you like.

Rustrp said:
2). If this is a feeder then it would be supplying a sub-panel in the outbuilding, protected at the main circuit breaker panel with a circuit breaker of fused disconnect box.
Correct, and code requires that for any more than a single circuit run to an outbuilding. You can't run 2 circuits to an outbuilding without putting in a feeder, panel, and ground rod. It is the only way to do it legally.

Rustrp said:
3). Voltage drop is a factor but a very small one.
Agreed, I wasn't the one raising voltage drop as a concern.

Rustrp said:
4). As a feeder the conductors would be three of the same size.
Most likely yes, although depending on the install the neutral could be smaller than the line conductors as long as it is properly calculated and meets requirements. In some cases, the neutral actually has to be bigger.

Rustrp said:
5). If this was a branch circuit (240 v) you would only have to hot legs and both the same size, plus the ground.
Correct again, but I think you misunderstood my post. I was saying of 100A on leg A and 20A on leg B then I need to size the feeder for 100A, meaning I would use #3 assuming conductors in pipe. If I rebalance the load by moving single pole breakers to swing 40A of load from leg A to leg B, then I would have 60A for each leg and now I would only have needed a 6AWG feeder. These problems tend to happen a lot more in commercial settings than residential. Again, as I said, things generally balance themselves.

I have run into 120V inverter based solar systems were balancing can be a problem, but that is an edge case nowadays.

In commercial settings with UPSs, balancing the 120V loads is critical. I actually had to do that in January. A UPS was slipping into soft bypass because it was overloaded all because one of the 2 legs was much hotter than the other. I moved some of the loads to the other leg, and the UPS went from over 100% utilization (where it goes into bypass) down to the low 90's.

It may not mean much to some, but this allows my company to defer the UPS upgrade to next year.

Rustrp said:
6). How can you provide a 240 v feeder with two differnet size conductors?
Who suggested that and how did you read that into my post? That said, see comment above about neutral. However, my previous post was not advocating using 2 different size conductors for the feeder. Just that if my example 100A/20A load had been properly calculated at the start, and balanced effectively, then the feeder could have been installed as a 6AWG feeder to begin with. Balancing the load properly most efficiently uses both line feeder conductors.

At this point I am done at this discussion. I feel that you are intentionally trying to twist my words and interpret what I am saying differently to try to make me out to look like an idiot. For what reason I do not know, but I am done with it. This forum isn't about that.
 
In the South, a few decades ago, some machine tools that were very "affordable" hit the used market..... These were machines coming out of regions in the north that were designed for 2-phase power (voltage differing by 90°)... these were 4-wire systems, though some of them were fed with 3 wires, a large diameter "common" wire and two smaller "hots." Some were, I am given to understand, even powered by two different generators. Those systems were split phase, but not single phase.

So, I think "split phase," in this sense, is probably a regional use of the word. Technically, it is correct as our "single phase" (voltage differing by 180°) power is derived by split voltages from a center tapped transformer. Single phase implies that the two legs are in phase and, are thus, additive (120V + 120V = 240V). Split phase, on the other hand, doesn't necessarily carry that distinction. Single phase is probably a better descriptor for that tingly stuff hiding behind the outlets in our homes and garages.


Purchasers of the above affordable machines quickly discovered that split phase motors (2-phase) don't run on polyphase (3-phase) power or single phase power.....
 
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