Medium Voltage manholes

[Pizza_Guy]

Hey everyone, I've been lurking for years. Finally decided to join to ask about manholes.
I have a project where we will be installing underground duct bank for 2 different MV (12470V) distribution systems, and a low voltage (480V) system. 4 conduits each. I haven't done much manhole design, so looking for advice.

Can we use 1 large manhole for all 3 systems (say, 6'x6' or 8'x8')? Or should we separate each system into its own manhole?
The point of our multiple systems is redundancy, so we will only be able to deenergize one system at a time.

Thoughts? Thanks!

 

[drktmplr12]

It can be done, and 300.3(C)(2) lays out the exceptions. I would discourage using 1 large manhole for everything.

That said, working with LV and MV are completely different skills and mindsets. At a minimum suggest a separate raceway system each for 12.4 and 480V. If you want TRUE redundancy with the 12.4 systems, they ought to be in separate raceway systems as well. That can be cost-prohibitive.

 

[David Castor]

I'd recommend separate vaults - one for medium-voltage and one for low-voltage. Many years ago I did a design using a common vault for MV and LV cables - it got built, but I wouldn't do it again. You might also want to consider arc-proofing tape for the MV cables in the vault to avoid one fault taking out both systems.

 

[drktmplr12]

Most sites around here have MV, 480 and I&C manhole systems. Some combine 480 and I&C. I'm not a fan of that approach.

If it were me doing the design... I would do one vault for MV and one for LV and let the client tell me no. Make it clear in my design there are separate MV and LV vaults. If the budget is high and they want to value engineer, I wouldn't volunteer it either, because my opinion is that-yes it's optional, but it's also a terrible idea to combine them. If they want me to reduce the number of manholes after hearing that, I get it confirmed in writing. Manholes also aren't going to be a driver of budget.

Now if you have a space constraint, there's good reason to consider it and it should be discussed with the Owner.

 

[Pizza_Guy]

Thanks for the advice everyone. I think the approach we will use is to have each system in its own vault. The 480V system is likely to be upgraded to a 12470 in the future. Having separate vaults allows for safer, more reliable operation I think.