50A service 208VAC 3P4W and 6AWG SOOW

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DubDub

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Boise
Hi All,
I'm trying to understand what AWG of SOOW cordage should be used for a 3 pole 4 wire 50A feed?
Scenario: A design is rated for 40A max continuous (3 pole 4 wire 208VAC), and therefore per NEC will have a 50A feed and a 50A (40A*1.25) internal breaker. Using NEC Table 400.5(A)(1) leads me to believe I should select 4AWG and not 6AWG. But the CS6365C 50A twist lock plug is only rated for #10-#6AWG and not the 4AWG I would expect. Beyond that it seems like I've seen instances of 6AWG cordage for 50A services. Do I need to stick with specifying 4AWG or can 6AWG be used and comply with NEC?
Thanks, Wayne
 
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zbang

Senior Member
Location
Roughly 5346 miles from Earls Court
Regardless of code, it's extremely common to protect 6/4 SOOW cables with a 2p 50a breaker. Do you really need to apply the 125% rule? To be scrupulously correct, you may need 4g and a 60a pin-and-sleeve connector.

What kind of load is this?
 
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DubDub

Member
Location
Boise
zbang said:
Regardless of code, it's extremely common to protect 6/4 SOOW cables with a 2p 50a breaker. Do you really need to apply the 125% rule? To be scrupulously correct, you may need 4g and a 60a pin-and-sleeve connector.

What kind of load is this?
Click to expand...

It is a server rack. We are designing a power distribution unit because of some special requirements.
Thanks, Wayne
 
Smart $

Smart $

Esteemed Member
Location
Ohio
zbang said:
Regardless of code, it's extremely common to protect 6/4 SOOW cables with a 2p 50a breaker. Do you really need to apply the 125% rule? To be scrupulously correct, you may need 4g and a 60a pin-and-sleeve connector.
Click to expand...
And that would be correct for only 2 current carrying conductors (ccc) under Column B of 400.5(A)(1). However, 3 ccc applications must use Column A. 4AWG is the minimum required size.
 
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zbang

Senior Member
Location
Roughly 5346 miles from Earls Court
Smart $ said:
And that would be correct for only 2 current carrying conductors (ccc) under Column B of 400.5(A)(1). However, 3 ccc applications must use Column A. 4AWG is the minimum required size.
Click to expand...

Yabut- most of the applications where I see this are 3p4w so the grounded conductor is considered one of the CCC. (I should have mentioned that.) Otherwise, I agree with you.


Isn't most bitcoin mining done in the GPUs now? Anyway, thatsa lotta power. And a lot of cooling. Some data centers require special cooling hoods on any cabinet with more than about 9-10kw in them.

Can you get an actual load measurement?
 
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DubDub

Member
Location
Boise
jumper said:
I guess one question is that if the loads are continuous is the 125% already factored into the name plate info; ie, like the MCA of a HVAC unit so the load is not getting factored twice?
Click to expand...

I have been assuming when, on similar products, I see "Maximum Input Current 50A" and then "Rated Input Current 40A" or "North America Derated to 40A" that means 40A continuous and the 50A is the 125% rule applied. Yet the cord cap is CS8365C (3P4W).

Here is an example (it does not list the 6AWG - I will have to dig to find an example.)

http://www.raritan.com/product-selector/pdu-detail/px3-5969u-v2

Thanks, Wayne
 
Smart $

Smart $

Esteemed Member
Location
Ohio
DubDub said:
I have been assuming when, on similar products, I see "Maximum Input Current 50A" and then "Rated Input Current 40A" or "North America Derated to 40A" that means 40A continuous and the 50A is the 125% rule applied. Yet the cord cap is CS8365C (3P4W).

Here is an example (it does not list the 6AWG - I will have to dig to find an example.)

http://www.raritan.com/product-selector/pdu-detail/px3-5969u-v2

Thanks, Wayne
Click to expand...
40A continuous amounts to the same as 50A noncontinuous in dealing with minimum conductor size ampacity. Take your pick. :D
 
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zbang

Senior Member
Location
Roughly 5346 miles from Earls Court
Ah, a couple of things-
Forget about the cord and plug that come with the PDU, it's a UL listed assembly.

Now, also forget about the max ratings of the PDU and look to the connected equipment. Those are the ratings that matter. The PDU itself doesn't consume anything (other than the slight burden for the monitoring electronics), so you don't apply the 125% to it, you apply that to the actual loads. You may find that to make a compliant installation you have to limit the connected load to <100% of the PDU's rating. (And yes, I've dealt with the "There must be power available; there are unused outlets." line of reasoning :D.) Or you may find that all of the equipment only adds up to 8kva.

Are you going to supply the PDU with a cord or fixed wiring? The matching receptacle (CS8365?) is also only rated for 6g.

If you really need 14kw in one cabinet, I'd really suggest multiple feeds and PDUs.
 
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DubDub

Member
Location
Boise
Smart $ said:
40A continuous amounts to the same as 50A noncontinuous in dealing with minimum conductor size ampacity. Take your pick. :D
Click to expand...

Which seems to make it okay to use 6/4 SOOW (45A) according to Column A of 400.5(A)(1)? But then if I have a 50A breaker internal it gets back to violating NEC 240.5(A) where I should have a 45A breaker to protect the 6/4?

Thanks, Wayne
 
Smart $

Smart $

Esteemed Member
Location
Ohio
DubDub said:
Which seems to make it okay to use 6/4 SOOW (45A) according to Column A of 400.5(A)(1)? But then if I have a 50A breaker internal it gets back to violating NEC 240.5(A) where I should have a 45A breaker to protect the 6/4?

Thanks, Wayne
Click to expand...
So you're picking 40A continuous?

It would be okay for a 40A continuous load, because minimum conductor sizing is per Table 310.15(B)(16).

If you have a 50A noncontinuous load, 6/4 SOOW* would not be compliant.

*I used your nomenclature. Technically, with cord the EGC is counted for the designation. For 3H 1N & 1GRD you'd need 6/5 SOOW.
 
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zbang

Senior Member
Location
Roughly 5346 miles from Earls Court
Smart $ said:
If you have a 50A noncontinuous load, 6/4 SOOW* would not be compliant.
*I used your nomenclature. Technically, with cord the EGC is counted for the designation. For 3H 1N & 1GRD you'd need 6/5 SOOW.
Click to expand...

I wondered about the neutral since the connector is 3p4w. That particular PDU is 3p + EGC, no neutral; so 6/4 would be correct. Line-line connections are getting to be common in data centers.
 
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