ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals
ECN Shout Chat
ShoutChat
Recent Posts
Al Hildenbrand - Minneapolis - obituary
by bcpappas - 02/02/23 08:01 PM
Violation?
by renosteinke - 01/27/23 09:52 PM
Does NEC 551.71 (F) apply to dwellings?
by BigB - 01/20/23 10:46 AM
Power submeter connections
by HotLine1 - 01/19/23 09:09 AM
New in the Gallery:
Burger King crown sillyness
Burger King crown sillyness
by wa2ise, December 11
240/208 to a house
240/208 to a house
by wa2ise, October 9
Who's Online Now
0 members (), 120 guests, and 9 robots.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate Thread
Page 2 of 2 1 2
Joined: Nov 2007
Posts: 763
K
Member
One thing I find that often gets overlooked when using the equipment manufactures minimum branch circuit rating is voltage drop due to the length of conductors. With things like rooftop units, you often end up with hundreds of feet of wire which could lead to under voltage issues and hard starting for motor/compressors.

Joined: Oct 2002
Posts: 830
S
Member
Scott, this raises the question in my mind; how did they come up with 65.68 MCA and 110 MOP using the figures you stated. They just made a mistake on the MCA or the MOP?
(65.68 x 175% = 114.94 amps)Should they have went up to a 125 amp MOP instead of 110? If I'm understanding you correctly, we have to go by the manufacture's requirements even though the code may allow us to not to have a disconnect if the unit has a marked "off/on" switch? I should have insisted on seeing the manual for the unit, which of course I will if I get the job. Hope I've got enough figured in to cover the unexpected:(
Thanks again for the help.

Joined: Jul 2004
Posts: 9,788
Likes: 14
G
Member
The engineers at the manufacturers shop come up with these numbers in accordance with the U/L marking guide for HVAC condensers.
They want a breaker big enough to reliably start the unit.
The 175% is just a general rule for motor loads.


Greg Fretwell
Joined: Oct 2002
Posts: 830
S
Member
KJay, thanks for the reminder. This unit I may be wiring in going on a possible 250ft. run. The #4 I'm figuring is good for 85 amps(75 degree column). Hope that will be sufficient. Very good thing to remember though. Can't always go by just what's on the nameplate, and I'm finding out on industrial wiring and commercial, a person can go bankrupt real quick if you don't do the right math:(

Joined: Jul 2004
Posts: 9,788
Likes: 14
G
Member
If you assume a 80% load, based on min circuit amps of 65 you get 52a normal running current and that translates to a 8v drop over 250' of #4. (under 2% at 480v)


Greg Fretwell
Joined: Mar 2011
Posts: 98
V
Member
Originally Posted by Scott35
the Branch Circuit Breaker may be sized to be Maximum 250% the FLA.


This is not true if the end use equipment is not a single motor and most 'packaged' units are not. 450.52 only applies to single motor circuits, I only mention this because I've seen this section applied to the wrong equipment too many times.

To further complicate things some engineers and AHJ's will view the circuit from the disconnect to the mechanical unit as a 'motor' feeder, but not the circuit from the circuit breaker to the disconnect and they will size or require the circuit between the breaker and switch to be sized per the breaker size, 110A (#2's) or 150A (1/0's) in the event it's a single motor device).

If you use an un-fused switch at the equipment then the breaker can be no larger than 110A (per the equipment label) and the feeder between the breaker can remain #4's (for a single motor).


Page 2 of 2 1 2

Link Copied to Clipboard
Featured:

Tools for Electricians
Tools for Electricians
 

* * * * * * *
2023 National Electrical Code (NEC)
2023 NEC Now Available!
 
* * * * * * *

2020 Master Electrician Exam Preparation Combos
2020 NEC Electrician
Exam Prep Combos:
Master / Journeyman

 

Member Spotlight
The Watt Doctor
The Watt Doctor
Humble, TX
Posts: 440
Joined: December 2001
Top Posters(30 Days)
BigB 4
triple 3
Popular Topics(Views)
303,015 Are you busy
232,124 Re: Forum
216,777 Need opinion
Powered by UBB.threads™ PHP Forum Software 7.7.5