Single Phase Motor Fusing

I am curious as to how many of you fuse L1 and L2 before a motor starter to a single phase 120VAC motor. We have some people here that have said that it is required, but I don't see why you could not just fuse L1.

Its not required to fuse a neutral. Fuses are for overcurrent, technically there should be no current on the neutral.

Sorry rsdoran, there should be the same amount of current on L1 and L2, there should be no voltage on L2 in a typical 120VAC system.

Utilities love to do this stuff. I have seen single-phase L1 & L2 as well as DC+ & DC- fusing required at commercial power plants of all types (hydro/fossil/nuke) from TVA to Korea Electric Power. Since this way of thinking is so widespread throughout the power industry, it must be written down somewhere.

Story on single phase motor control.

My wife's a lawyer...(Never marry a lawyer, you'll never win an argument).

Anyway she had a case where a large printing plant had a lot of problems with buring up vent motors on rooftop HVAC units. The owner finally got Pi$$ed off and whent after the manufacture and contractor. After or during discovery she got a set of electrical drawings and I noticed that the single phase 220 blower motors were controled by a single contact on the HOT side of the motor. My thought was that the other side was not on a nutural as intended. Turns out she hired another contractor to take a look. He opened up one of the units and found the motor turning slowly with the PB relay open. All the motors where then placed on TWO pole relays....Walla!!! No more burned up motors... (the nutrual was also fixed). Origional Contractor paid for all past repairs. Case closed out of court.

Lesson: its not a perfect world. the enviroment (damp) caused a partial run to ground thus causing the motors to run when not on. Check that nutrual!!!

mgvol said:

... , it must be written down somewhere.

Click to expand...

AFAIK, Utilities (POCO's) don't have to follow the [local adapoted] NEC or any regulations. They make their own. So don't go by that.

Ah, see NEC (2002) 240.22 for fuses in the Grounded Conductor.

And rsdoran is correct for one specific case. Hint: Most every home has this.

zmanvortex: Is your neutral grounded or floating ?

Fused Neutral

Hello,

I do not think it is a matter of personal preference.

In the USA, the NEC is very strict about not fusing the grounded conductor. Refer to Article 240.22 Grounded Conductor in the 2002 National Electrical Code.

I can think of only one situation, where L1 and L2 would be fused. And that would be in a floating 120V system, where both L1 and L2 are ungrounded.

European equipment would have a similar situation with 380/220 wye. The potential of L1, L2, or L3 is 220 to ground. Small, single-phase motors are often fused on the line side and the neutral is grounded.

I hope you find this information helpful.

NJG

We are an OEM, so we are assuming ( I know, bad word ) that the customers have grounded their neutral at their main panel that feeds our machine. We require a separate grounding conductor and grounded conductor to be run to our machines.

Please tell me where L1 and L2 current on a 120VAC system would not have similar readings. This would trip ground fault circuit protection in a circuit.

that dang neutral

ASSUME
1. In USA where NEC applies.
2. Edison 3 wire OR 4 conductor 3 phase wye.
3. Neutral is grounded.

In case of Edison you have L1 and L2 and N.
240 is between L1 and 2 and 110 is between L1 (or 2) and N
on three phase wye you have L1 L2 L3 and N 208 is between L1 and 2 or 3. 120 is between L1 (or 2 or 3) and N.

The neutral is NEVER switched except in two cases (to my knowledge): transfer switches connected to the utility and in gasoline pump circuit breakers -- special breaker is required.

IF you insist on switching a neutral the switch must switch all conductors simultaneously. DO NOT switch only the neutral.

The neutral is not fused.

IF you do put in overcurrent protection on a neutral then it must open all conductors to the circuit simultaneously. this is why you don't fuse a neutral.

Dan Bentler

Thanks guys, after reading NEC 240.22 I see that it is not a matter of preference. In our case L2 (Neutral) should not be fused. I appreciate the help.

"And rsdoran is correct for one specific case. Hint: Most every home has this."

Can someone tell me what device most houses have where L1 and Neutral would not have similar current readings? It is killing me!!

Please tell me where L1 and L2 current on a 120VAC system would not have similar readings. This would trip ground fault circuit protection in a circuit.

Click to expand...

Ground fault should only occur if there is a fault to ground. A neutral is not ground, it is a grounded conductor.

As I mentioned there is no requirement to fuse the neutral but as I left out (because I couldnt locate the right Article) there are requirements in the US NOT to fuse a neutral.

Neutral has 2 definitions:
The first meaning refers to a core that acts as a common for several phase cores. The second refers to a current-carrying core that is tied to a grounding point. The NEC only defines neutral as the first of theese but the IEE use both in thier defintion and even americans use the second definition when reffering to say the neutral side of a 120V receptacle.

I am glad this question was asked. It made me realize that my incoming service still doesnt have an earth ground connected. The wiring was done in 1960 and I am rewiring and need to put in a ground rod.

grounded conductors

Hi Zman,

There is a huge difference in the panel wiring of a floating system. All devices would have to have both L1 and L2 fused. All relay contacts to solenoids, etc. would have to have an identical L2 contact. So, it is not just a matter of diconnecting the equipment grounding conductor. Think of 240V circuitry, such as an electric range. Two circuit breakers, two ungrounded conductors, two poles on the switch, etc.

Yes. The grounded conductor of a 120V motor is current carrying.
If you were to isolate the L2 wire from ground, you could also measure a potental 120V.

NJG

NEUTRAL CURRENT

Where you have just one phase conductor (L1) and the neutral supplying a load current will always be equal.

Where you have more than one phase conductor (L1 and L2) and the neutral supplying two or more loads the current in the neutral will be the difference in currents of L1 and L2. IF loads are balanced then neut current is zero.

Same goes for 3 phase wye.

Dan Bentler

Right on, Leitmotif!

I would like to thank all who responed, but I am noticing several references to single phase with two ungrounded conductors, such as a 220 volt device. I would like to point out that in my original and following posts I specified that it was a 120VAC system. This would mean that I had L1 and a Neutral line.

I find it an interesting subject the difference between fusing and not fusing the Neutral depending on if it is a floating circuit or not. The NEC 240.22 would not apply here because it specifically states that a grounded conductor should not be fused. On a floating circuit the Neutral would not be considered a grounded conductor.

Rsdoran, a ground fault circuit interrupter will trip if the current level between L1 and Neutral are not equal.