404.8 (b)

[mbrooke]

What is the intention of 404.8 (b)? Does this literally mean I can't have two switches from different circuits in the same box?

 

[david luchini]

What is the intention of 404.8 (b)? Does this literally mean I can't have two switches from different circuits in the same box?

No. It means voltage between the two circuits can't exceed 300V.

Two switches on 120V circuits of different phases would not exceed 300V between them.

Two switches on 277V circuits of different phases would exceed 300V between them.

 

[mbrooke]

No. It means voltage between the two circuits can't exceed 300V.

Two switches on 120V circuits of different phases would not exceed 300V between them.

Two switches on 277V circuits of different phases would exceed 300V between them.

So I literally can not have a large gang box with a 277/480 MWBC and lights on differing phases? Are you serious?

Also, why wouldn't it apply to 120 volts? 120+277= 397v

 

[david luchini]

So I literally can not have a large gang box with a 277/480 MWBC and lights on differing phases? Are you serious?

Yes you can. You can install barriers between the adjacent devices.

Also, why wouldn't it apply to 120 volts? 120+277= 397v

I think it would or wouldn't depending on the phases.

 

[mbrooke]

Yes you can. You can install barriers between the adjacent devices.



I think it would or wouldn't depending on the phases.

Do you have link to the barriers?

 

[mbrooke]

Yes you can. You can install barriers between the adjacent devices.



I think it would or wouldn't depending on the phases.

Any idea on the code's reasoning?

 

[infinity]

As David stated it would depend on the phasing in the box to end up with more than 300 volts when supplying devices from two different systems.

 

[mbrooke]

As David stated it would depend on the phasing in the box to end up with more than 300 volts when supplying devices from two different systems.

How is he getting 381 volts vs 397? just curious on how the math works.

But being honest, I can't see a reason for this rule.

 

[infinity]

How is he getting 381 volts vs 397? just curious on how the math works.

But being honest, I can't see a reason for this rule.

I'm guessing that some engineer worked out the numbers. As for the usefulness of this rule that's open to debate.

 

[mbrooke]

I'm guessing that some engineer worked out the numbers. As for the usefulness of this rule that's open to debate.

IMO, the rule is superfluous once the box, yoke and plate is in place grounded.

 

[david luchini]

How is he getting 381 volts vs 397? just curious on how the math works.

The phases are separated by 30deg from the 277V side to the 120V side ( standard phase shift.)

So for a 277V A phase circuit to a 120V A phase circuit the voltage difference is 183V. No barrier would be required

From 277V A phase to 120V B phase the voltage difference is 385V.

From 277V A phase to 120V C phase the voltage difference is 301V.

 

[mbrooke]

The phases are separated by 30deg from the 277V side to the 120V side ( standard phase shift.)

So for a 277V A phase circuit to a 120V A phase circuit the voltage difference is 183V. No barrier would be required

From 277V A phase to 120V B phase the voltage difference is 385V.

From 277V A phase to 120V C phase the voltage difference is 301V.

Neat

 

[texie]

Do you have link to the barriers?

Barriers are available for 4SQ boxes from all the manufacturers. As an FYI, as best I can recall this requirement has been in the code since dirt was invented.

 

[Terminator5047]

How is he getting 381 volts vs 397? just curious on how the math works.

But being honest, I can't see a reason for this rule.

So someone that’s not you doesn’t send 480 to a light or series of lights when replacing switches creating a series of mini fire starting bombs?

 

[electrofelon]

So I literally can not have a large gang box with a 277/480 MWBC and lights on differing phases? Are you serious?

likely just an academic point, but the restriction is for "adjacent devices" not "all devices in a box". Not sure if there is some combination where, say for three devices, you would have less than 300V between A and B, and also B and C, but more than 300V between A and C.

 

[mbrooke]

So someone that’s not you doesn’t send 480 to a light or series of lights when replacing switches creating a series of mini fire starting bombs?

In that case code would not allow MWBC, and OSHA would be ok with live work.

 

[mbrooke]

Barriers are available for 4SQ boxes from all the manufacturers. As an FYI, as best I can recall this requirement has been in the code since dirt was invented.

Any idea what year?

 

[electrofelon]

Any idea what year?

Dirt was invented about 4.5 billion years ago.....

 

[texie]

Any idea what year?

I knew you would make me look! The oldest code book I have is 1975 and it is there as Article 380.8. There was no indication it was new in that edition. Maybe some else with older books can look to see how much further it goes back.

 

[mbrooke]

I knew you would make me look! The oldest code book I have is 1975 and it is there as Article 380.8. There was no indication it was new in that edition. Maybe some else with older books can look to see how much further it goes back.

An older rule makes it less likely to be relevant today IMO.