250.20 SYSTEM GROUNDING (B)

[bobby ocampo]

250.20 (B) Alternating Current Systems of 50Volts to less than 1000 volts. Alternating-cutrrent systems of 50vollts to less than 1000 volts that supply premisses wiring and premises wiring systems shall be grounded under any of the following conditions:

(1) Where the system is 1-phase and can be grounded so that the maximum voltage to ground on the ungrounded conductors does not exceed 150 volts

(2) Where the system is 3phase, 4 wire, wye connected in which the neutral is used as a circuit conductor.

(3) Where the system is 3phase, 4-wire, delta connected in which the midpoint of one phase winding is used as a circuit conductor.

QUESTIONS:

FOR 250.20(B) (1)

a) Is the system allowed to be UNGROUNDED if the system is 3phase?

b) What are the incidence where a system cannot be grounded?

c) If the system is single phase line to line and 120 volts only, will the Code allow it to be ungrounded?


FOR 250.20(B)(2)

a) If the system is 3phase, 3-wire, wye connected in which the NEUTRAL is NOT USED as a circuit conductor, will the Code allow the system grounding to be UNGROUNDED?

FOR 250.20(B)(3)

a)If the system is 3-phase, 3-wire, delta connected in which the midpoint is NOT USED as a circuit conductor, will the Code allow the system grounding to be UNGROUNDED?

 

[Smart $]

...

FOR 250.20(B) (1)

a) Is the system allowed to be UNGROUNDED if the system is 3phase?
3? has no bearing on the requirement. If any conductor can be grounded and the other system conductors are less than 150V to ground, the system is require to be grounded. The only leniency per se is stated in 250.21.

b) What are the incidence where a system cannot be grounded?
I don't believe there are any. Where a system is ungrounded, it has to be permitted [240.21] and even then it is a design decision... and other requirements kick in.

c) If the system is single phase line to line and 120 volts only, will the Code allow it to be ungrounded?
No. Grounding one conductor will make the other conductor less than 150V to ground.


FOR 250.20(B)(2)

a) If the system is 3phase, 3-wire, wye connected in which the NEUTRAL is NOT USED as a circuit conductor, will the Code allow the system grounding to be UNGROUNDED?
Yes, if grounding the neutral supply terminal would result in an phase voltage to ground greater than 150V.

FOR 250.20(B)(3)

a)If the system is 3-phase, 3-wire, delta connected in which the midpoint is NOT USED as a circuit conductor, will the Code allow the system grounding to be UNGROUNDED?
Essentially the same as immediately above. However, it can be debated regarding say a 240/120 3? system because only two or the three phase conductors would be less than 150V.

Replies in blue...

 

[bobby ocampo]

3? has no bearing on the requirement. If any conductor can be grounded and the other system conductors are less than 150V to ground, the system is require to be grounded. The only leniency per se is stated in 250.21.

If the voltage is 120 volts line-to-line, will the Code allow it to be ungrounded because it is less than 150 volts to ground?

250.20(B) is not referred to the requirement of 250.21. Does it mean that UNGROUNDED will be allowed even if it does not comply with 250.21 requirement?

 

[bobby ocampo]

250.2(B)(2) Did not say that voltage to ground shall be less than 150 volts. The 150volts to ground limit is only in 250.2(B)(1), Will this mean that the Code allows 480Y/277 volts ungrounded?

 

[bobby ocampo]

250.2(B)(3) Did not say that 150 volts to ground as a limit. If the system is 480 volts delta, does it mean that the code allows it ungrounded?

 

[infinity]

250.2(B)(2) Did not say that voltage to ground shall be less than 150 volts. The 150volts to ground limit is only in 250.2(B)(1), Will this mean that the Code allows 480Y/277 volts ungrounded?

Are you using the neutral conductor?

250.20(B) Alternating-Current Systems of 50 Volts to 1000
Volts. Alternating-current systems of 50 volts to 1000 volts
that supply premises wiring and premises wiring systems
shall be grounded under any of the following conditions:
(1) Where the system can be grounded so that the maxi-
mum voltage to ground on the ungrounded conductors
does not exceed 150 volts
(2) Where the system is 3-phase, 4-wire, wye connected in
which the neutral conductor is used as a circuit conductor
(3) Where the system is 3-phase, 4-wire, delta connected in
which the midpoint of one phase winding is used as a
circuit conductor

250.2(B)(3) Did not say that 150 volts to ground as a limit. If the system is 480 volts delta, does it mean that the code allows it ungrounded?

Is it 3?, 4 Wire using the midpoint conductor?

(3) Where the system is 3-phase, 4-wire, delta connected in
which the midpoint of one phase winding is used as a
circuit conductor

 

[Smart $]

If the voltage is 120 volts line-to-line, will the Code allow it to be ungrounded because it is less than 150 volts to ground?

No. The requirement is it has to be grounded if (after grounding) the voltage does not exceed 150V.

250.20(B) is not referred to the requirement of 250.21. Does it mean that UNGROUNDED will be allowed even if it does not comply with 250.21 requirement?

250.20(B) is mentioned in 250.21(A)(4), included by reference in 250.21(B)(1). The accumulative result is any ungrounded system (permitted, exempted, or otherwise not required) operating at a nominal voltage of 120V to 1000V must have ground detectors.

 

[bobby ocampo]

No. The requirement is it has to be grounded if (after grounding) the voltage does not exceed 150V.

120Volts line to line will not exceed 150 volts line to ground. Does it mean that it is allowed UNGROUNDED?

250.20(B) is mentioned in 250.21(A)(4), included by reference in 250.21(B)(1). The accumulative result is any ungrounded system (permitted, exempted, or otherwise not required) operating at a nominal voltage of 120V to 1000V must have ground detectors.

Yes, I stand to be corrected. Based on this provision, 250.20(B)(2) & (3), system is allowed to be ungrounded if neutral will not be used? The requirement of the code is to have ground detectors.

 

[don_resqcapt19]

120Volts line to line will not exceed 150 volts line to ground. Does it mean that it is allowed UNGROUNDED? ...

No, a 120 volt system is required to be a grounded system. Any system that can be grounded so as to have 150 volts or less from the other conductor(s) to ground, must be grounded.

 

[kwired]

120Volts line to line will not exceed 150 volts line to ground. Does it mean that it is allowed UNGROUNDED?

I'm not seeing anything that would prohibit it to be ungrounded.

Maybe look a little further into requirements of 250.21 - It will require ground detectors from (B) and marking from (C).


What is a good application for such a system? It is good for supplying a process that needs to shut down in an orderly fashion like where there may be more hazards presented by sudden shut down because of opening an overcurrent device during a ground fault. With an ungrounded system you have indication of the ground fault with time to shut down in an orderly fashion. The system actually becomes grounded during a ground fault, and if there should be a second ground fault on another conductor - you get immediate opening of overcurrent protection anyway, so allowing it to run longer then necessary when a fault is indicated kind of eliminates the whole idea of why the system was ungrounded in the first place, but it does happen.

 

[Smart $]

...Yes, I stand to be corrected. Based on this provision, 250.20(B)(2) & (3), system is allowed to be ungrounded if neutral will not be used? The requirement of the code is to have ground detectors.

Yes. Ground detectors are required on any ungrounded system with a nominal voltage from 120V to 1000V.

 

[meternerd]

Glad it says Alternating Current. Substation 125VDC systems are ALWAYS ungrounded with ground detectors. That's so that a single ground won't disable a protective system by blowing a fuse. Nota bene'....Navy ships and likely many other seagoing vessels use ungrounded AC systems throughout (yes, they have ground detectors). The thinking is that a torpedo or other such unfriendly device that causes a ground fault may not disable the electrical system. Just had to say that so you guys would think I was ReeLLY smart!

 

[GoldDigger]

If you start with any number of conductors in any phase relationship, and grounding some wires will cause one or more conductors to be more than 150 volts above ground, BUT there is one wire which when grounded will cause all of the others to be less than 150V from ground, then you must ground that system and must choose to ground that wire.
For example, in a 120/240 single phase three wire system, you must ground the neutral. I do not think that you are allowed to ground one leg instead, producing a single ended grounded/120/240 system. But the section need not be read that way unambiguously. It may require the system to be grounded, but not actually require that the ground scheme chosen be the one that limits the voltage to ground to less than 150V?

 

[don_resqcapt19]

120Volts line to line will not exceed 150 volts line to ground. Does it mean that it is allowed UNGROUNDED?

I'm not seeing anything that would prohibit it to be ungrounded. ...

How does 250.20(B)(1) not require a two wire 120 volt system to be a grounded system?

 

[K8MHZ]

How does 250.20(B)(1) not require a two wire 120 volt system to be a grounded system?

The way I read it is that if it's less than 150 volts it has to be grounded. Not the other way around.

 

[K8MHZ]

If you start with any number of conductors in any phase relationship, and grounding some wires will cause one or more conductors to be more than 150 volts above ground, BUT there is one wire which when grounded will cause all of the others to be less than 150V from ground, then you must ground that system and must choose to ground that wire.
For example, in a 120/240 single phase three wire system, you must ground the neutral. I do not think that you are allowed to ground one leg instead, producing a single ended grounded/120/240 system. But the section need not be read that way unambiguously. It may require the system to be grounded, but not actually require that the ground scheme chosen be the one that limits the voltage to ground to less than 150V?

250.26(2) requires the neutral on single phase 3 wire to be grounded.

I really like 250.26(4). Multi-phase systems where one phase is grounded - one phase conductor.

So, the NEC requires that a multi-phase system that has one phase grounded must have one phase grounded. No kidding. Where have I heard that before???

 

[Smart $]

...
So, the NEC requires that a multi-phase system that has one phase grounded must have one phase grounded. No kidding. Where have I heard that before???

Can't recall who, but I believe one of the members here quotes it in his signature.

 

[K8MHZ]

Can't recall who, but I believe one of the members here quotes it in his signature.

It used to be in mine.

It was an OSHA rule, IIRC. I took it off a while back.

 

[don_resqcapt19]

The way I read it is that if it's less than 150 volts it has to be grounded. Not the other way around.

That is correct.

I guess I read your post wrong. I was reading you post as saying a 120 volt system is not required to be a grounded system. Sorry

 

[Smart $]

That is correct.

I guess I read your post wrong. I was reading you post as saying a 120 volt system is not required to be a grounded system. Sorry

It wasn't his post that you quoted.... :?