Calculating Maximum Wire Size in Modular Metering

[Zeus32]

I am an electrical contractor who specializes in mult-family projects. Recently, we have been wiring mixed-use and high-density apartment projects. These projects, unlike your typical garden style apartments, have significantly longer SER/subfeeder runs to the unit panels due to meterbase locations being more spread out. We typically use Square D EZM Modular Metering rated at 125 Amps, example, a Square D EZMR115125 (125 Amp-rated 5 Gang). Within this meterbase we install unit main breakers ranging up to 125 Amps, example, a Square D QO2125-VH (125 Amp 2 Pole, 22K AIC, Unit Main). These "QO" breakers have a maximum wire size of 2/0 AWG. Voltage Drop on long subfeeder runs sometimes require the use of conductors larger than 2/0. We have been told that 125 Amp-rated meterbases are not UL approved for any conductors larger than 2/0 AWG. However, we can find no documentation through Square D engineers or the UL White Book that proves or disproves this statement.

I understand that there are certain products such as "MAC ADAPTS" and Ilsco reducing lugs that would reduce the wire size that terminates into the unit breaker. These would be installed in the wireway section of the meterbase and would allow a 4/0 AWG SER cable to be reduced to a 2/0 AWG and terminated under the unit breaker. Another option would be to increase the meterbase size to a 225 Amp that would require breakers that accept up to 300 KCMIL conductors. However, this set-up is very costly.

Question: Is there any restrictions on the maximum size of SER cable that can be installed in these meterbases that are rated 125 Amps?

We are looking for the least cost-effective design that meets the National Electrical Code.

 

[Dennis Alwon]

The info you look for should be on the meter can itself. I suspect that a 125 amp meter base would not take a 4/0 wire-- not suprising.

Also I would look at 250.122(B) The ser cable may not work unless the egc is full size.

 

[Zeus32]

The meterbase itself does not list a maximum wire size. The breakers have a maximum wire size listed, but you could possibly avoid this with a reducing lug or "mac adapt". Is there a way to prove or disprove the maximum wire sizes using NEC calculations? According to 376.22 (A) you are only allowed 20% IF this is considered a wireway.....? The dimensions of this wireway is 4.59" x 6".

 

[hurk27]

you can also use an insulated crimp to reduce the last few inch's of wire just before hitting the breaker lugs, there is no code against running larger conductors for voltage drop as long as the smaller conductor is protect by the proper breaker, I have reduced the wire by using the butt splice crimp in the wire way, this makes it easier to get the wires into the meter base. some use Polaris connectors also.

 

[LarryFine]

Question: Is there any restrictions on the maximum size of SER cable that can be installed in these meterbases that are rated 125 Amps?

We are looking for the least cost-effective design that meets the National Electrical Code.

If the cables and conductors don't exceed wireway capacities, they do or can be made to physically fit, and you can find no manufacturer's info or documentation to the contrary, the last hurdle would be the inspection department.

It (almost) never hurts to ask ahead of time, and costs way less than doing it twice.