Quick questions on wire size and associated greenfield size

[olytdi]

Quick questions for the electricians in the group:

Wiring a 12 KV electric unit heater. I understand that I need #4 copper from 70amp 2p breaker. What is the wire gauge needed for the associated ground? Can I use #8?

Will I need 1 inch greenfield for that run of wires or can I fit it in 3/4 inch?

Thanks up front -- I appreciate the shared knowledge!

 

[wyliesdiesels]

Quick questions for the electricians in the group:

Wiring a 12KV electric unit heater. I understand that I need #4 copper from 70amp 2p breaker. What is the wire gauge needed for the associated ground? Can I use #8?

Will I need 1 inch greenfield for that run of wires or can I fit it in 3/4 inch?

Thanks up front -- I appreciate the shared knowledge!

I assume you meant 12KW... you need 62.5a rated wire.... so #6 THWN will work (good for 65a).... ground wire would need to be #8...

might be kinda tight fitting that in 3/4"... i would go with 1"

 

[olytdi]

Thanks Wylie! Yes! I meant KW! LOL.

I thought it was #4 -- #6 just means more savings! Thanks for confirming the #8 ground and the greenfield size!

 

[pattenp]

3/4" greenfield is fine. NEC limit is four #6 thhn in 3/4". I assume this is a short whip?

 

[olytdi]

Yep. Short run about 15 - 20 ft including inside panel.

 

[olytdi]

3/4" greenfield is fine. NEC limit is four #6 thhn in 3/4". I assume this is a short whip?

So the manufacturer's website states #4 for this heater and a 70 amp 2p breaker. Any idea why that differs from the recommendation here? I already have the breaker -- 70 amp would require the #4 wire correct? I think that they're accommodating the 80% rule(?).

 

[wyliesdiesels]

So the manufacturer's website states #4 for this heater and a 70 amp 2p breaker. Any idea why that differs from the recommendation here? I already have the breaker -- 70 amp would require the #4 wire correct? I think that they're accommodating the 80% rule(?).

12KW / 240v is 50a. 50a x 125% = 62.5a wire.... #6 is good for 65a but the manufacturer is basing their wire size off breaker size.... 65a breaker is not a standard size and code allows you to use the next standard size as long as the load doesnt exceed the wire rating, which it wont....

 

[mm08822]

So the manufacturer's website states #4 for this heater and a 70 amp 2p breaker. Any idea why that differs from the recommendation here? I already have the breaker -- 70 amp would require the #4 wire correct? I think that they're accommodating the 80% rule(?).

The conductor ampacity needs to be 50a x 125% = 65A.
Mfr probably assumed worst-case use of 60C rated wire (NM-B) and #4 copper is good for 70A.
If you use THHN/THWN, rated for 75C (in this case) #6 copper is good for 65A.
The 70A cb can be used in either case.

 

[olytdi]

The conductor ampacity needs to be 50a x 125% = 65A.
Mfr probably assumed worst-case use of 60C rated wire (NM-B) and #4 copper is good for 70A.
If you use THHN/THWN, rated for 75C (in this case) #6 copper is good for 65A.
The 70A cb can be used in either case.

Okay, thanks! What I hear you saying is that for 12000 watts, I can use #6 THHN/THWN and use a 70 amp breaker.

 

[olytdi]

12KW / 240v is 50a. 50a x 125% = 62.5a wire.... #6 is good for 65a but the manufacturer is basing their wire size off breaker size.... 65a breaker is not a standard size and code allows you to use the next standard size as long as the load doesnt exceed the wire rating, which it wont....

Okay, thanks! What I hear you saying is that for 12000 watts, I can use #6 THHN/THWN and use a 70 amp breaker.

 

[alfredeneuman]

Okay, thanks! What I hear you saying is that for 12000 watts, I can use #6 THHN/THWN and use a 70 amp breaker.

Yes you can because the load is equal or less then the wire's amp rating.

 

[olytdi]