Working space between equipment

[Grouch]

Hi all,
I have questions regarding Article 110.26 of the 2008 NEC (should apply to other versions). Let's say the incoming service into my electrical room is 277/480 volts. If i have 277/480 volt equipment facing each other, I need 4 feet clearance between equipment based on condition 3 (table 110.26(A)(1)).
My questions are:

1) there's step down transformers, and now i have 120/208 volt equipment in that same room as well. If I have 120/208 volt equipment facing each other, can i apply 3 feet clearance between this equipment, or does the 4 feet rule apply throughout the entire room?

2) What if my 277/480 volt equipment is facing 120/208 volt equipment? do i use 3 or 4 feet clearance between them?

3) What if my 277/480 volt equipment is facing 12 volt DC UPS batteries? how many feet do I need between them? How do you treat it when you have DC facing AC?

4) Table 110.26(A)(1) says these distances are based on nominal voltage to ground. so let's say my equipment is 480 volts (without the neutral, so we don't have 277 volts)... if i have 480 volt equipment facing each other, does condition 3 apply?.. that would give us 4 feet between the equipment... but since there's no 277 volts, does this table now apply?

Thanks for the help!

 

[Smart $]

1) 3ft
2) 4ft
3) 4ft
4) ???

 

[infinity]

Just because you have a 3Ø, 3W system doesn't mean that it's ungrounded but even if it were then you need to refer to Article 100:

Voltage to Ground. For grounded circuits, the voltage be-
tween the given conductor and that point or conductor of
the circuit that is grounded; for ungrounded circuits, the
greatest voltage between the given conductor and any other
conductor of the circuit.

 

[Carultch]

Just because you have a 3Ø, 3W system doesn't mean that it's ungrounded but even if it were then you need to refer to Article 100:

That's a useful clarification. So even with ungrounded systems that "float" symmetrically relative to neutral, the voltage to ground for this application, doesn't really exist. Therefore, a line-to-line voltage needs to suffice.

I often find this in ungrounded PV systems on the DC side, when considering the 250V to ground and bonding bushing requirements. The system is 400V plus to minus, and it is normally 200V to ground

 

[Carultch]

Hi all,
I have questions regarding Article 110.26 of the 2008 NEC (should apply to other versions). Let's say the incoming service into my electrical room is 277/480 volts. If i have 277/480 volt equipment facing each other, I need 4 feet clearance between equipment based on condition 3 (table 110.26(A)(1)).
My questions are:

1) there's step down transformers, and now i have 120/208 volt equipment in that same room as well. If I have 120/208 volt equipment facing each other, can i apply 3 feet clearance between this equipment, or does the 4 feet rule apply throughout the entire room?

2) What if my 277/480 volt equipment is facing 120/208 volt equipment? do i use 3 or 4 feet clearance between them?

3) What if my 277/480 volt equipment is facing 12 volt DC UPS batteries? how many feet do I need between them? How do you treat it when you have DC facing AC?

4) Table 110.26(A)(1) says these distances are based on nominal voltage to ground. so let's say my equipment is 480 volts (without the neutral, so we don't have 277 volts)... if i have 480 volt equipment facing each other, does condition 3 apply?.. that would give us 4 feet between the equipment... but since there's no 277 volts, does this table now apply?

Thanks for the help!

1. It only depends on the voltages of equipment that "share a common aisle" for their working space. Equipment with unrelated working space doesn't apply. Therefore it is 3 ft.
2. The larger voltage shall govern. Therefore 4 ft.
3. The larger voltage shall govern. DC vs AC doesn't matter for working space. Therefore 4 ft.
4. Base it off the 480V value.

 

[Smart $]

...

I often find this in ungrounded PV systems on the DC side, when considering the 250V to ground and bonding bushing requirements. The system is 400V plus to minus, and it is normally 200V to ground

I believe the rationale is that during a one-line-to-ground fault(s), which does not trip any OCPD, the voltage to ground is the nominal line-to-line voltage.

 

[kwired]

Hi all,
I have questions regarding Article 110.26 of the 2008 NEC (should apply to other versions). Let's say the incoming service into my electrical room is 277/480 volts. If i have 277/480 volt equipment facing each other, I need 4 feet clearance between equipment based on condition 3 (table 110.26(A)(1)).
My questions are:

1) there's step down transformers, and now i have 120/208 volt equipment in that same room as well. If I have 120/208 volt equipment facing each other, can i apply 3 feet clearance between this equipment, or does the 4 feet rule apply throughout the entire room?

2) What if my 277/480 volt equipment is facing 120/208 volt equipment? do i use 3 or 4 feet clearance between them?

3) What if my 277/480 volt equipment is facing 12 volt DC UPS batteries? how many feet do I need between them? How do you treat it when you have DC facing AC?

4) Table 110.26(A)(1) says these distances are based on nominal voltage to ground. so let's say my equipment is 480 volts (without the neutral, so we don't have 277 volts)... if i have 480 volt equipment facing each other, does condition 3 apply?.. that would give us 4 feet between the equipment... but since there's no 277 volts, does this table now apply?

Thanks for the help!

1) nothing applies to entire room - rules apply to each piece of equipment that requires clearance
2) The 120/208 equipment needs 3 feet of clearance, the 277/48 equipment needs 4 feet of clearance - final answer is 4 feet.
3) assuming exposed live parts on the batteries - probably 4 feet - max voltage to ground is above 151. Min voltage to ground is still between 0 and 150, so I don't see the fact it is DC disqualifying anything.
4) 480 volt corner ground and 480/277 both fit into the 151-600 volts to ground category. The one common system that is possibly questionable here is a 240/120 delta with a 208 volt high leg.

Add: per infinity's quote - an ungrounded system is considered max line to line voltage to ground - which it is at a time when a phase is faulted to ground.

 

[Grouch]

Thanks everyone for your feedback. The voltage to ground definition in Article 100 actually helped clear up my 4th question. Regarding my 3rd question, I'm currently dealing with UPS batteries in front of my 277/480 volt equipment, and as usual, the space is so tight that i'm being questioned if these rules apply when DC is present... and it looks like it does.

Another issue i've come across is when you ceiling mount a new 480:208/120 delta-wye transformer in an existing electrical closet. I've had people tell me you need 3 feet clearance in front of the transformer. When you ceiling mount it, there's always existing conduits and light fixtures that would have to be relocated / rerouted so you have 3 feet clearance (which i try to avoid to do, since this is a ton of work and also causes disruptions). However, wouldn't the space that is required actually be 3-1/2 feet since we have 480 volts in the transformer?... based on condition 2 of the table?

 

[Grouch]

Actually, re-reading it again, it looks like the ceiling mounted transformer would apply to condition 3, not 2, of the table. so now you would need 4 feet of clearance in front of the transformer. Is this correct?

 

[kwired]

Actually, re-reading it again, it looks like the ceiling mounted transformer would apply to condition 3, not 2, of the table. so now you would need 4 feet of clearance in front of the transformer. Is this correct?

is the transformer (from 110.26(A) "likely to require examination, adjustment, servicing, or maintenance while energized"?

I think most AHJ will say it is not. Doesn't mean you would never do any work on it while energized either - just is not considered a place that is all that common to do so.

 

[Grouch]

i know! the problem is on some jobs i've been hit with this (i'm in NYC), where they'll say the transformer requires 3 feet in front of it... and since you can still do work / maintenance on the transformer while its energized, you can't fight back on this. So it seems like you have to follow table 110.26(A)(1). I just think that because there's 480 volts on the primary, and if any equipment is facing the transformer, you actually need 4 feet in between based on condition 3 of the table.

 

[kwired]

i know! the problem is on some jobs i've been hit with this (i'm in NYC), where they'll say the transformer requires 3 feet in front of it... and since you can still do work / maintenance on the transformer while its energized, you can't fight back on this. So it seems like you have to follow table 110.26(A)(1). I just think that because there's 480 volts on the primary, and if any equipment is facing the transformer, you actually need 4 feet in between based on condition 3 of the table.

It is totally up to the interpretation of the AHJ IMO as to whether or not the transformer is likely to require examination, adjustment, etc. while energized, as it is for most any piece of equipment. You do see differences from one AHJ to another in what they want working space in front of.

Also stands to reason if they do consider the transformer needing work space then if it has over 250 volts in it - the proper row/column's needs to be applied for work space. If they don't consider the transformer needing work spade then it still makes sense it is at least considered to be a grounded surface.