30 amps imbalance

[Frank DuVal]

Actually, 70 AMPS!

QO170

Editing to try and remove need to download page to read..... hold on....

 

[Frank DuVal]

 

[Frank DuVal]

Why would a transformer care if only one side of a center tap was fully loaded? The power rating would still be higher than the load on it. A center tap winding is just two secondary windings that happen to be tied together inside the transformer.

A transformer with several secondary windings would not care if some of the windings are unused.

The load is still single phase on the primary side, typically one phase to neutral, although some power companies use two phases and no neutral for the primary winding (have not seen those in these parts in 50 years!)

 

[wwhitney]

Why would a transformer care if only one side of a center tap was fully loaded?

That's true for a single phase secondary, but with a center-tapped delta secondary, imbalance can lead to transformer overload even when all the load currents are less than the transformer rated current, due to additional circulating current. See https://forums.mikeholt.com/threads...-of-3-phase-transformer.2582774/#post-2941681 for the details.

Cheers, Wayne

 

[OldBroadcastTech]

"That's true for a single phase secondary,"

which is where this discussion started.......no doubt there are 'concerns' that "with a center-tapped delta secondary, imbalance can lead to transformer overload " but we have strayed from the original question of whether or not 30 amps of neutral current is necessarily a Bad Thing

 

[ptonsparky]

IMO you would need to work at getting a sustained imbalance of 30 amps.

 

[Elect117]

"That's true for a single phase secondary,"

which is where this discussion started.......no doubt there are 'concerns' that "with a center-tapped delta secondary, imbalance can lead to transformer overload " but we have strayed from the original question of whether or not 30 amps of neutral current is necessarily a Bad Thing

As long as everything is done properly. No.

Neutral current is to be understood in context and reduced when able. Imbalance is not ideal.

Most replies are including context. For example, is the sky blue? Well, yes. But not always. Like at dusk or dawn. Or if it is cloudy. etc.

 

[chorty55]

IMO you would need to work at getting a sustained imbalance of 30 amps.

Based on what? Poor circuit placement in the panel? Who's to say later the homeowner or for whatever reason someone needs to juggle a repair, moves them and some how all 3 kitchen 20s, 20amp the outlets in the basement and the 20amp feeding the garage are all riding the same phase? And its January, running an electric heater in each, the toaster, and the microwave?

If its not a big deal to be out of balance and dumping amperage down the neutral, then why the efforts to stay as close to balance?

Further more, on both flip sides, there's a chance there's 2, 3, or 4 houses pulling from the same transformer. There very well could be a chance the neighbor has the same chaos going on in their panel, just on the other side of the phase, now there's a cancelation assuming both pull off the transformer at the same time.

The other flip, both houses pulling off the same conductor, even further adding more amperage on the neutral, at the transformer of course, combined return.

Is it a big concern or not? At this point its only concerning if the neutral isn't sized correctly, as the home I live in, from the 70s, has #3 copper mains and an aluminum neutral, so granted that panel should be kept as (pun intended) neutral as possible with breaker/load imbalance.

 

[Speedskater]

Not many loads maintain a constant current draw, so balance may change minute to minute.

 

[Birken Vogt]

It seems like every time I amp clamp a house or office, there is always 40 amps on one leg and 5 amps on the other, or something like that. But it seldom makes any difference. All the important loads are 240 volt anyway.

 

[winnie]

If its not a big deal to be out of balance and dumping amperage down the neutral, then why the efforts to stay as close to balance?

Don't say 'as close as possible'. Say 'as close as practicable'. It is a desirable goal but not a critical issue.

Minimizing neutral current is desirable because the balanced system is more efficient; you lose less energy in the wires.

Whenever current flows through a wire you get some 'voltage drop'. This is simply caused by the resistance of the wire. Double the current, double the voltage drop.

Consider a set of six 15A loads on six 20A circuits. If they are all on one leg of the service, you have 90A running through one of the service 'hots' and 90A running through the neutral. Say the length and gauge of the service conductors are such that you get 0.9V of drop on the 'hot', and probably 1.5V of drop on the 'neutral'. The specific amount of voltage drop depends on the length, gauge, and material of the conductors. I am assigning a greater voltage drop on the neutral because it is likely smaller in size or made from a less conductive material than the 'hots'.

So you lose a total of 2.4V of the 120V supply. Not great, not bad. If the neutral is properly sized to carry 90A, then you don't have a problem.

Now re-arrange the circuits so that you have three circuits on each leg of the service. You have 45A running on each of the service 'hots', and thus 0.45V of drop on each. You now have a total of 0.9V drop, and this is from the composite 240V supply. Instead of losing 2% of your power in the service conductors, you lose 0.4%.

In both cases the system _functions_ correctly, but the second scenario saves some energy.

-Jonathan

 

[chorty55]

Actually, 70 AMPS!

QO170

Editing to try and remove need to download page to read..... hold on....

Wow. Name one practical real world application that's hungry for a single leg of 70 amps @ 120v? I cannot think of one.

Actually let's call it 56amps with the 80% rule. Still, even with inrush, its still alot.

 

[ptonsparky]

Wow. Name one practical real world application that's hungry for a single leg of 70 amps @ 120v? I cannot think of one.

Actually let's call it 56amps with the 80% rule. Still, even with inrush, its still alot.

Ok.
You only have one space left in your SE panel. You need 50 amps of 120 volt lighting for your Holiday display. Five 15 amp circuit breakers arranged in a newly installed six circuit MLO panel.

 

[Birken Vogt]

Wow. Name one practical real world application that's hungry for a single leg of 70 amps @ 120v? I cannot think of one.

Actually let's call it 56amps with the 80% rule. Still, even with inrush, its still alot.

In the off grid world, we used to have a lot of 120 volt only inverter systems, that had several inverters in parallel. Sometimes if you needed a larger breaker, you would use one side of a double pole of some kind, or something like this QO170.

 

[OldBroadcastTech]

Ok.
You only have one space left in your SE panel. You need 50 amps of 120 volt lighting for your Holiday display. Five 15 amp circuit breakers arranged in a newly installed six circuit MLO panel.

That's how I would do it.

C. Griswold

 

[kwired]

Wow. Name one practical real world application that's hungry for a single leg of 70 amps @ 120v? I cannot think of one.

Actually let's call it 56amps with the 80% rule. Still, even with inrush, its still alot.

120 volt branch circuit may not be so likely, feeder more of a possibility, particularly if only 120 volts is available for whatever reason.

 

[jaggedben]

Based on what? Poor circuit placement in the panel? Who's to say later the homeowner or for whatever reason someone needs to juggle a repair, moves them and some how all 3 kitchen 20s, 20amp the outlets in the basement and the 20amp feeding the garage are all riding the same phase? And its January, running an electric heater in each, the toaster, and the microwave?

With a random 50/50 coin flip for each circuit, the chances that five circuits end up on the same leg are 1/32 to begin with. Then multiply that by the likelihood someone loads them all up at the same time as you describe, with no load on the other leg, and the chances of this happening by random luck are a small fraction of 1%. Most houses would never use 3.6kW in 120V loads at all at once anyway.

If its not a big deal to be out of balance and dumping amperage down the neutral, then why the efforts to stay as close to balance?

What effort? The vast majority of residential electricians make no such effort whatsoever.

Is it a big concern or not?

In residential single phase, really it's not. Just fill the panel from the top and things will be balanced enough.

 

[chorty55]

Can one per code install a subpanel with 1x50 amp single pole 120v conductor? I thought 2 open spaces were required for future install?

 

[ptonsparky]

Can one per code install a subpanel with 1x50 amp single pole 120v conductor? I thought 2 open spaces were required for future install?

Yes, why not?
Not required.