Double derating

Status
Not open for further replies.
N

Nietz001

Member
Location
St Paul MN
I have a load issue on a large stadium project. The receptacle is approximately 150? from the panel. I pulled in 6 #10?s, (three hots and three neutrals) plus a ground though a ? ? EMT and connected them to a 20 amp breaker. At around 14 amps the breakers trip. The Owner is blaming me but honestly I?m not sure what I could have done differently. Any Ideas?
Would this installation require me to double de-rate? Also, when counting conductors to de-rate does anyone know where in the NEC it tells you to count the neutral as a current carrying conductor?
Any help would be most appreciated.
 
It will have nothing to do with derating.

If a 20 amp breaker is tripping with only 14 amps RMS current at the breaker than the breaker is bad.
 
If the load is 14 amps and it is tripping a 20 amp breaker I would change the breaker. If that doesn't work then something is shrting out after the circuit has been energized. Are all circuits tripping or just one?
 
Not sure what you mean by double derate? Your circuits are code compliant as far as derating. 6 CCC's would require a derating of 80%. For #10 THHN that would be, 40 amps * 80 = 32 amps adjusted conductor ampacity. The neutrals would all count since you have two wire circuits. That information is in 310.15(B)(4).
 
I'd go with changing the breakers first. I actually got a box of breakers once that were all bad.
 
By the VD calculator #10 at 150ft with a 14amp load would give you a voltage drop of about 5.4 volts. For this load it shows to be good for only 100ft to stay with in the 3% VD range.
 
480sparky said:
I would assume he means he's upsizing the conductors for two reasons..... CCCs and VD. Throw a high ambient temp into the mix, and you're triple derating.
Click to expand...

I see your point about double derating but I wouldn't call up sizing for VD derating.
 
A couple other issues to consider. I measured the 14 amps at the conected load, not at the panel. It's possible their were other loads I was not aware of on the same ckt. Also when I checked voltage at the rec with no load I had 120 volts, load connected it dropped to 106 volts. Has anyone experienced this?
Jeff
 
Nietz001 said:
A couple other issues to consider. I measured the 14 amps at the conected load, not at the panel. It's possible their were other loads I was not aware of on the same ckt. Also when I checked voltage at the rec with no load I had 120 volts, load connected it dropped to 106 volts. Has anyone experienced this?
Jeff
Click to expand...
Yes that is what we call voltage drop. This may be cause by a bad connection. I thought you ran new circuits so you need to check the amperage at the breaker.
 
I did run new ckts. When I was called to the site the breaker was already tripped. I checked the only load connected at the time. There may have been other loads removed before I got there. This just seems a bit excessive for voltage drop. Could it have anything to do with the type of rating on the transformer?
 
Nietz001 said:
I did run new ckts. When I was called to the site the breaker was already tripped. I checked the only load connected at the time. There may have been other loads removed before I got there. This just seems a bit excessive for voltage drop. Could it have anything to do with the type of rating on the transformer?
Click to expand...
106V is excessive if you started with 120V-- that's about 13% drop.
 
Nietz001 said:
I pulled in 6 #10’s, (three hots and three neutrals) plus a ground though a ? “ EMT and connected them to a 20 amp breaker.
Click to expand...
Are these parallel conductors, or is it three circuits?

If three, do all three trip at 14a?
(25 to go)​
 
Nietz001 said:
Also when I checked voltage at the rec with no load I had 120 volts, load connected it dropped to 106 volts.
Jeff
Click to expand...

If you have good connections and the breaker is good, that VD works out to about a 35 amp load.

IMO, you need to follow the circuit and check every connection. Then if they are all good, with the breaker off, tie the conductors together at the load and megger them from the panel side to make sure the conductors are not compromised.
 
George Stolz said:
There you go expecting us to read past the third sentence again. :D
Click to expand...

As posted by RETRAINDAILY..Needs some re-reading

Not that it makes a difference but

Based on #10 AWG 150' at 14 amps for 120 VAC

Estimated Drop 5.1 Volts
Load At Circuit End 114.9 Volts
Voltage Drop 4.2% Percentage
 
Last edited:
Status
Not open for further replies.
Top