MC cable when there is voltage drop

[Tainted]

We are supposed to upsize the equipment grounding conductor if there is voltage drop.

MC cables have preinstalled equipment grounding conductors in them so the only way to get the correct equipment grounding conductor size is to get a significantly larger MC cable with larger phase conductors for the correct equipment grounding conductor size...

Is there a way to get custom MC cable with any equipment grounding conductor size?

for example cable with #2 phase conductors and with #2 equipment grounding conductor?

A typical #2 cable only comes with #6 equipment grounding conductor size but I need it to be #2

 

[ActionDave]

Yes. You can get custom MC you just have to buy enough of it. There may be some off the shelf that has upsized EGC.

 

[Tainted]

Yes. You can get custom MC you just have to buy enough of it. There may be some off the shelf that has upsized EGC.

Which manufacturer

 

[electrofelon]

A job i was on a while back they got custom tray cables with larger EGC. I dont know quantity or manufacturer. I would talk to a supply house and they can ask their wire vendors.

 

[infinity]

for example cable with #2 phase conductors and with #2 equipment grounding conductor?

Is this for a 20 amp circuit? You mentioned #2, for smaller cables MC-ap comes in up to #8 and doesn't have the EGC problem when up-sizing. Then there's always type AC if you can find it.

 

[letgomywago]

Is this for a 20 amp circuit? You mentioned #2, for smaller cables MC-ap comes in up to #8 and doesn't have the EGC problem when up-sizing. Then there's always type AC if you can find it.

The price on mc ap isn't all that bad when looking at the other cost savings it has. I'm sure it's cheaper than a couple rolls of upside egc mc.

Feeder mc also sometimes comes with larger than required 75 degree column egc in the larger sizes.

Last issue is you can look at re phasing it since it's over #4 if the ahj is on board it's a cable and you can even strip off the insulation completely where the formerly phased wire is exposed.

 

[curt swartz]

We are supposed to upsize the equipment grounding conductor if there is voltage drop.

MC cables have preinstalled equipment grounding conductors in them so the only way to get the correct equipment grounding conductor size is to get a significantly larger MC cable with larger phase conductors for the correct equipment grounding conductor size...

Is there a way to get custom MC cable with any equipment grounding conductor size?

for example cable with #2 phase conductors and with #2 equipment grounding conductor?

A typical #2 cable only comes with #6 equipment grounding conductor size but I need it to be #2

The NEC doesn't state that the EGC shall be increased in size for voltage drop. It states if the ungrounded conductors are increased the EGC shall be increased in size proportionately to the increase in circular mil area of the ungrounded conductors.

Moving up to a larger size MC will not help since you will also be increasing the size of the ungrounded conductors requiring the EGC to be even larger.

 

[hbiss]

MC cables have preinstalled equipment grounding conductors in them so the only way to get the correct equipment grounding conductor size is to get a significantly larger MC cable with larger phase conductors for the correct equipment grounding conductor size... A typical #2 cable only comes with #6 equipment grounding conductor.

So, all the MC manufacturers are making their cable wrong?

-Hal

 

[Tainted]

Is this for a 20 amp circuit? You mentioned #2, for smaller cables MC-ap comes in up to #8 and doesn't have the EGC problem when up-sizing. Then there's always type AC if you can find it.

with 125A OCP, I'm going from #2 phase conductors to 3/0 phase conductors, then I would need #2 ECG.

cannot find MC 3/0 Cable with #2 ECG.

I also came across the GEMI program to see if 3/0 cable with #6 ECG would be sufficient but I don't think it has MC cable in the library, should I just select "allowable circuit length without conduit"?. I'm trying to learn how to use this software and it's asking for operating voltage. Is the operating voltage the Line to ground voltage or line to line? is 40V a typical arc voltage parameter?

 

[roger]

For the price of custom cable you would probably be better off changing the OCPD to 200A and install a 125A enclosed breaker at the end.

 

[winnie]

So, all the MC manufacturers are making their cable wrong?

-Hal

No. The cables are just fine for their most common use.

Say you want to run a 100A feeder. You select 3 AWG Cu for the phase conductors (75C terminations...) Per 250.122, you have to use a minimum #8 EGC. It looks like normal 3/3 MC has a #6 EGC, so there is no problem.

Now imagine you want to run a long 20A circuit, and because of voltage drop you need to use #3 AWG Cu conductors. Table 250.122 says you need a #12 EGC, but the increased in size rule pushes that EGC up. If you want to run a long 20A circuit in 3AWG MC, it needs to have a #3 EGC.

Jonathan

 

[hbiss]

Now imagine you want to run a long 20A circuit, and because of voltage drop you need to use #3 AWG Cu conductors. Table 250.122 says you need a #12 EGC, but the increased in size rule pushes that EGC up. If you want to run a long 20A circuit in 3AWG MC, it needs to have a #3 EGC.

I understand that. It's just another example of where the NEC doesn't make sense.

-Hal

 

[ptonsparky]

For the price of custom cable you would probably be better off changing the OCPD to 200A and install a 125A enclosed breaker at the end.

I've done a variation more than once in the past.

 

[winnie]

I understand that. It's just another example of where the NEC doesn't make sense.

-Hal

IMHO the general concept of 'if you increase the circuit conductor size for voltage drop you should increase the EGC size' makes good sense.

IMHO the way the code implements this general concept is problematic, and creates paradoxical situations that I absolutely agree do not make sense.

-Jonathan

 

[tom baker]

what if smart mc cable was used (EGC is cable sheath with al bond strip)?

 

[tortuga]

with 125A OCP, I'm going from #2 phase conductors to 3/0 phase conductors, then I would need #2 ECG.

cannot find MC 3/0 Cable with #2 ECG.

I also came across the GEMI program to see if 3/0 cable with #6 ECG would be sufficient but I don't think it has MC cable in the library, should I just select "allowable circuit length without conduit"?. I'm trying to learn how to use this software and it's asking for operating voltage. Is the operating voltage the Line to ground voltage or line to line? is 40V a typical arc voltage parameter?


View attachment 2579267

In your GEMI screen shot under system parameters > Operating voltage you set 120 volts? I would think that should be your L-L voltage for voltage drop on a feeder?

 

[hbiss]

what if smart mc cable was used (EGC is cable sheath with al bond strip)?

Or AC as somebody mentioned. That's my point. You get to use what the cable is made with.

Where does the MC cable manufacturer get that they can use a #6 with 3/0? (I'm too lazy to look for it.) If #6 is good for 125A I see no reason there would be a problem with 20 unless you make the cable so long that the VD on the #6 won't trip the breaker.

-Hal

 

[infinity]

what if smart mc cable was used (EGC is cable sheath with al bond strip)?

#8 is the largest size for MC-ap. At least from Southwire. Just got with what Roger stated in post #10.

 

[ActionDave]

Where does the MC cable manufacturer get that they can use a #6 with 3/0? (I'm too lazy to look for it.)

They get it from table 250.122

If #6 is good for 125A I see no reason there would be a problem with 20 unless you make the cable so long that the VD on the #6 won't trip the breaker.

#6 is good enough for 125A no matter how long the feeder is unless you go down a breaker size then you have to comply with 250.122(B) and increase the size of your EGC, but the MC makers don't have to worry about that, only the electrician.

 

[don_resqcapt19]

In your GEMI screen shot under system parameters > Operating voltage you set 120 volts? I would think that should be your L-L voltage for voltage drop on a feeder?

This is about the voltage drop and the fault clearing properties of the EGC...it uses the line to ground voltage and not the line to line voltage. However using 5x the OCPD rating for the ground fault current seems very low. I would expect for many systems using 3/0 the available line to ground fault current would far exceed 625 amps.