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I have three legs entering with no neutral, from fuses. The voltages on the legs read 120, 208, and 118. When I put a light on the first breaker it will go out within a minute. No matter where I try to hook it up except the high leg the voltage either goes down or up. Is this panel bad or am I doing it wrong?

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  • sounds like you are measuring wrong... wrt gnd on a 3 phase floating delta Mar 11 '20 at 22:54
  • I understood you are measuring between ground and any of the three live wires. If that gives you 120V, 208V, 118V, the live wires are ok. Your problem is most likely a missing or bad ground connection. Either at your place or at the pole transformer or both.
    – Janka
    Mar 11 '20 at 22:54
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You have wild-leg delta. But without neutral, you can only drive 240V loads.

Since your service does not contain a neutral wire, you are not allowed to hook up any loads between hot and ground. The only loads you can serve are 240V loads.

The 240V loads must be connected on a 2-pole breaker (or 3-pole) with hot1 to one throw and hot2 to the other throw. You cannot do anything with neutrals, except re-mark them with tape to designate them as a hot wire.

If you have a desire to power 120V loads, you need to contact your power company about getting a 4-wire service with neutral.

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You get two phases opposite 90 degrees and one neutral which is the grounding at the pole transformer.

What you should measure is RMS 120, 120, 240. Or 110, 110, 220.

The 120, 208, 118 is worrying. It indicates that (at the time of measurement) you have a high power consumer which is probably on the 118V line, is a one-phase motor and generates lots of reactive power. Could be a pump, a fridge, a fan, a transformer. Please beware, it could also be a ground loop off an improper grounding.

Any circuit has the neutral and the ground tied in the panel. These two lines should never be tied at the other end. The ground is supposed to unload any short made between phase and external casing or even the local ground and panel ground. The neutral is only supposed to ensure neutral reference for voltage. When both ground and neutral come tied at the user end, then two distinct paths to panel ground are made available from user end to main panel. Is impossible for predict which path a short will take to panel ground.

To check a ground fault is not an easy task. So I will not deal with it now. What I would do is shutdown the entire panel (pull off all breakers) and measure the input voltage lines. I expect to see 120-120-240. If I don’t see that I would call the power company.

If the voltage is fine while my circuit breakers are off, then I would start troubleshooting each circuit, one at a time.

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Figure 1. Star or wye connection versus split-phase delta-connected. Image source: PlugSocketMuseum.

In Europe 3-phase star or wye connected supply would be most common in industrial supplies and in domestic in some countries.

In North America the split-phase or high leg delta type is popular. As shown in Figure 1, a ground connection is made to the centre-tap of one of the 240 V delta windings. This then leaves L1 and L2 with 120 V when referenced to ground or neutral while still leaving 240 V between each of the three phase pairs. Your readings of 120 V and 118 V are close enough. The 2 V difference may be due to imbalance at the transformer centre-tap or a higher loading on the '118 V' line which would cause the voltage to sag.

Note that L2 now has 208 V relative to neutral (and hence the term "high leg"). This can be verified by Pythagoras' Theorem taking the base L1-N = 120 V, the hypotenuse L1-L2 = 240 V and then calculating \$ V_{L2-N} = \sqrt {240^2 - 120^2} = 208 \ \text V \$.

So, if your measurements are taken with respect to ground or neutral those readings are to be expected. If you clarify how you connected your bulb - which wires to which phase - we may be able to help further.

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  • Well, high leg delta was popular :P most new 3ph in NA is wye Mar 26 '20 at 1:05

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