5% RULE

[rswolff001]

I've been asked to duplicate an existing system. When I looked at the schematics I believe there is a problem as follows;
15kva transformer 3ph, 480vac delta primary, 240vac delta secondary with a center tap. The center tap is grounded and being used as a neutral for 120vac. I summed the loads on each leg and they look to be within limits, and reasonably balanced. However the third leg (one of the low voltage) is being used for both 220vac and all the 120vac. The neutral has a circuit breaker @30a. There is about 24a being used for the 120vac devices. Doesn't this exceed the 5% rule for the center tap?? Thanks, bob

 

[kwired]

A 15kVA full delta transformer would have a 5kVA coil for each phase.

If you are putting single phase load across any side you are sort of limited to 5 kVA per side though you can get away with more than that if other sides are actually idle at the time.

If you are putting all the 120 volt load on one half of the phase with the neutral in it, that segment of secondary alone is only rated 2.5 kVA which would be about 20.8 amps @ 120 volts.

You do have to factor in what other load is being supplied by the rest of the transformer as some of that load may be utilizing the same segment of the winding.

 

[ptonsparky]

The neutral, or grounded conductor should not have a circuit breaker. There are exceptions but not what you describe.

 

[rswolff001]

everything is within limits of the windings... I was concerned that the ~20a exceeded the 5% rule.. that would have limited the 120vac to about 750va, or only 6a. And yes, I was also thought the cb on the neutral was a bit bizarre. I believe the fellow who originally designed it thought that was an acceptable method to limit the 120vac. Just what everyone expects.. a tripped breaker with hot devices. Sounds like fun. Thanks, Bob

 

[winnie]

The 5% limit is supposedly based on circulating currents from the unbalanced load, not just the load current flowing through the connected coil.

Is there a 5% limit for the single phase loading on Delta-Delta connected Transformers with 240/120 Center Tapped secondaries? | Schneider Electric USA

 

[rswolff001]

thanks.. I was aware of the rule but had never needed to apply it myself in the past.. when I started looking into it more closely, it was not apparent how to apply it. Thanks everyone!!!! Have a great holiday!!! or weekend... or both!!!

 

[kwired]

The 5% limit is supposedly based on circulating currents from the unbalanced load, not just the load current flowing through the connected coil.

Is there a 5% limit for the single phase loading on Delta-Delta connected Transformers with 240/120 Center Tapped secondaries? | Schneider Electric USA

If you think about what they say in your link, you really shouldn't unbalance a delta -delta transformer by very much period even though they kind of focus only on loads connected to neutral in their explanation.

 

[rswolff001]

ok... If I add up all the total loads for each leg, regardless of voltage, they are all around ~30a. So the total loads appear to be fairly balanced. Should I consider the leg with both 220v and 120v differently or should I consider all the 120v differently? Apparently the 5% rule doesn't apply if the loads are balanced. And yes, a really unbalanced delta could be an issue. I could use a WYE secondary for the new project but I've had a problem finding a 220vac WYE secondary... all I find is 208.. which is the outside limit of some of the equipment. Thanks, Bob

 

[ptonsparky]

Many delta-wye step downs have taps. Choose a tap that gets you closer to the voltage you want. A good share seem to run closer to 212. Close enough?

 

[winnie]

A 220v wye has 127V L-N, a touch high for nominal 120V loads. A 240V wye has 139V L-N, much too high for 120V loads.

 

[rswolff001]

yes and yes... yes, 212 would work... and yes, 127 and 139 are high and way too high. I'll take a look again... I didn't see any that had those taps but I must have missed. I'll take another look and make sure I've consumed the required dose of caffeine beforehand. Thanks, Bob

 

[kwired]

yes and yes... yes, 212 would work... and yes, 127 and 139 are high and way too high. I'll take a look again... I didn't see any that had those taps but I must have missed. I'll take another look and make sure I've consumed the required dose of caffeine beforehand. Thanks, Bob

It is a mathematical thing. With a Wye system line to line voltage will always be 1.732 times the line to neutral voltage. Every outer end is same distance from the center point and all equally distributed 120 degrees apart. So if you build it with three 139 volt coils you get 240 volts line to line.

Similar with a delta system and mid point ground. line to neutral is half the line to line on the grounded neutral side. High leg to neutral is always 1.732 times the line to ground voltage seen on the other two corners.

That ratio in either example above never changes regardless what voltage you design it for