mc ampacity

[tommyrice]

I'm running a 100 amp feeder in mc cable with alumunum conductors.I called the store to ask how much 100 amp 3 wire alum. mc cable he had in stock.He said he had a full reel of #2 alum. 3 conductor mc cable.I explained that because this was a commercial job I could not use #2 because it only has an ampacity in these circumstances of 90 amps.He said commercial or not, everyone uses this for 100 amp feeders. I think if you used #2 alum. mc, you would have to put it on a 90 amp breaker if it was a lighting panel feeder,irregardless of a smaller load.I figure I'm right. Do you guys agree with me? Keep in mind I'm going to use a 100 amp sq-d nema3 enclosure and breaker as the service equipment that protects this feeder.

 

[kwired]

You are correct. You would need 1AWG aluminum if you wish to protect it with a 100 amp breaker.

You also need an equipment grounding conductor with the feeder - or is the cable you are using listed to use the sheath as an EGC?

 

[infinity]

My general rule never take advice from the guys working in the supply house regardless of how convincing they may be.

You were correct, #2 Al is only good for 90 amps.

 

[kwired]

My general rule never take advice from the guys working in the supply house regardless of how convincing they may be.

You were correct, #2 Al is only good for 90 amps.

There is a reason they work in the supply house and not as an electrician:happyyes:

 

[Ponchik]

Here is the way that i understand it.
The ampacity of #2 aluminum is 90A which means it will carry continuous load for 3 hours or more. A 100A breaker is good for 80A continuous load, which means #2 Aluminum will never see any continuous load higher than 80A.

please help me understand.

 

[jumper]

Here is the way that i understand it.
The ampacity of #2 aluminum is 90A which means it will carry continuous load for 3 hours or more. A 100A breaker is good for 80A continuous load, which means #2 Aluminum will never see any continuous load higher than 80A.

please help me understand.

Feeder ampacity is sized according to:

215.2 Minimum Rating and Size.
(A) Feeders Not More Than 600 Volts.
(1) General. Feeder conductors shall have an ampacity not
less than required to supply the load as calculated in Parts
III, IV, and V of Article 220. The minimum feeder-circuit
conductor size, before the application of any adjustment or
correction factors, shall have an allowable ampacity not
less than the noncontinuous load plus 125 percent of the
continuous load.

In general, size the feeder and than select your OCPD.

If you have 100 amps of non continuous load, then a 100 amp OCPD and wire is fine.

If you have 100 amps of continuous load, then multiply by 1.25, 125 amp OCPD and wire is needed.

Most feeder loads are a mixture, ie: 40 amps of continuous and 45 amps of non continuous,

(40 x 1.25=50) + (45 x 1.00=45) = 95 amps.

 

[kwired]

Feeder ampacity is sized according to:

215.2 Minimum Rating and Size.
(A) Feeders Not More Than 600 Volts.
(1) General. Feeder conductors shall have an ampacity not
less than required to supply the load as calculated in Parts
III, IV, and V of Article 220. The minimum feeder-circuit
conductor size, before the application of any adjustment or
correction factors, shall have an allowable ampacity not
less than the noncontinuous load plus 125 percent of the
continuous load.

In general, size the feeder and than select your OCPD.

If you have 100 amps of non continuous load, then a 100 amp OCPD and wire is fine.

If you have 100 amps of continuous load, then multiply by 1.25, 125 amp OCPD and wire is needed.

Most feeder loads are a mixture, ie: 40 amps of continuous and 45 amps of non continuous,

(40 x 1.25=50) + (45 x 1.00=45) = 95 amps.

And in general - if the op did a load calculation and came up with anything less than 90 amps he is good to go with a 90 amp breaker, but not a 100 amp breaker.

If the load is 90 amps before 125% continuous loading is factored in then minimum ampacity becomes 112.5 amps.

A continuous load of 72 amps would require a 90 amp conductor.

 

[Cow]

Asking your supply house for wire based on ampacity rather than size is what diy'ers do. YOU tell them the size, after YOU pick it out from the tables.